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Journal of the Illuminating Engineering Society, 13, 1, pp. 174-190, 1983-10
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Behavioral responses to a flexible desk luminaire
Rea, M. S.
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BrnGBEHAVIORAL RESPONSES TO A FLEXIBLE DESK LUMlNAlRE
by M.S. Rea
ANALYZED
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L I B R A R Y
Reprinted from IES JournalVol. 13, No. 1, October 1983
p. 174- 190
DBR Paper No. 1231
Division of Building Research
Price $1.50
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Behavioral responses to
a
flexible desk luminaire
Behavioral responses
flexible
Mark
S. Rea
Introduction
luminaire
The recent concern for energy conserva- tion has prompted individuals interested in lighting to seek more efficient
methods of delivering luminous flux to tasks without compromising visual quali- ty. Task lighting hag been one method frequently proposed [l r r
, ,
61.
The proposed advantage of task lighting as a method for energy conservation rests upon the fact that, compared to ceiling fixtures, the same illuminances can be delivered to a task with lower lamp wat- tages because the lamp is placed closer to the task [71.
Despite the speculations thattask
lighting can be used as amethod for enexgy consexvation 2 , 3,
8j, it has been recently, and persuasive- ly, argued by ~~ielvugel[91 and Zackrison
that task lighting might
not
beas
attractive a solution as previously touted. They point out that the inter- action between the lighting system and the ventilating and air conditioning systems may actually result in larger energy consumption for buildings. Fur- ther, Spielvogel [
91
contends that the ergonomic aspects of lighting systems are poorly understood, and, again, tasklighting may actually cause an increase in energy consumption relative the more conventional ceiling mounted lighting systems.
These important energy related issues aside, however, the ultimate viability of task lighting does not rely so much upon its ability to provide high illumination levels but, more correctly, upon its
ability to deliver a suitable luminous environment for seeing tasks. Unfor- tunately, there have been no suitable direct tests of visual performance under
The author: National Research Council Canada, Ottawa, Canada. This paper was presented at the 1983 IES Annual Con- ference, August 7-11, Los Angeles, CA.
task lighting, although several studies have been published attempting to eval- uate the "quantity and quality" of the lighting environment under such systems.
Cuttle and slater[ll, for example, attempted to evaluate the visual charac- teristics of some task lighting installa- tions using photometric measurements and some mathematical expressions hypotheti- cally providing indices of lighting aua- lity. The utility and the validity of some of these expressions have recently come into question [12 r l3 141
.
Because these indices may not properly characte- rize visibility, it is still desirable to have some direct measurement of visual performance under task lighting.Another method for evaluating lighting quality has been through questionnaires probing subjective assessments of the luminous environment. Perhaps the best data of this type on task li hting came from a pilot study by Boyce [q51
.
Sub- jects' reactions to several desk lumin- aires were reported, and, for a variety of reasons, subjects were most favourable toward a flexible, fluorescent "angle- poise" desk luminaire. Subjective im- pressions of the suitability of the lu- minous conditions associated with the anglepoise lamp (and the accompanying ambient illumination) while performing, more or less, realistic tasks, were the major basis for the evaluations of desk luminaire suitability. While these sub- jective reports can be quite useful and informative in pilot studies, ~ e a has pointed out that subjective impressions like those used in Boyce's study and si- milar ones [l7, 181 may not always be in- dicative of visual performance. One can not be sure, without some calibration procedure, that such reports are valid assessments of the luminous conditions*Cu&tle and Slaterl discuss the impetus for task liqhting.
because some subjects may be uncoopera- tive or unperceptive. It is at least conceivable then that peoples' subjective preferences may not be synonymous with their optimal ability to see,
It should also be noted that Boyce[15] employed a search task (scanning a
matrix of Landolt rings for a specific gap orientation) in his study, but it was impossible to ascertain the effects the desk luminaire had on his subjects' visual performance, because there were no other luminous conditions to which the performance data could be satisfac- torily compared. The search times re- ported in his study were much longer than those obtained e rlier by Boyce using
19?
the same task [
.
Further, the realis-tic search task employed by Boyce was of high contrast, and, according to the high illuminances reported, was performed at relbtively high task luminances. Under these performance-saturated conditions it may be difficult to detect subtle effects on visibility produced by the luminous conditions without a carefully planned experimental design[201. There- fore, these performance data could not be satisfactorily used in the evaluation of a desk luminaire's ability to provide suitable visibility, although the longer search times might indicate that the luminous environment associated with the desk luminaire lowered task visibility.
In the study reported here, a direct and more sensitive analysis of visual performance was undertaken to evaluate subjects' ability to see under task lighting conditions produced by a flexi- ble anglepoise desk luminaire similar to that employed by Boyce. Analyses of subjective responses to the luminous environment and of the behavioral mani- pulations of the desk luminaire were also performed
.
Methods
Experimental testing rooms and apparatus TWO test rooms, a "black" and a "pink" room, were constructed to perform the tests. Both rooms were, approximately,
three meter cubes. The pink-room was
intended to be like a typical, small, interior office. It was carpeted and furnished with a desk, a chair, a filing cabinet, a book shelf and wall decora- tions Figure (1). The reflectances (r) of the carpet, walls and white acoustic ceiling tiles were .25, .60, . 0 4 , respec- tively*. A desk luminaire (described below) and four recessed ceiling lumina- ires provided illumination throughout the
experimental sessions. i3ach ceiling fixture contained one lamp (Westing- house F40CW) and a DSI "Phantom tube". The black room Figure (1) was intended to represent the worst possible environ- ment for an inhomogenous lighting distri- bution. It had no furnishings other than a desk and a chair. The walls were
draped with black felt (r = .02) and the ceiling covered with black construction paper (r = .05) and black adhesive tape (r = .06). Carpeting like that used in :he pink room covered the floor. A desk luminaire was the only source of illumi- nation.
Both rooms contained identical desks. Both desks were 152 cm wide, 76 cm deep and 73 cm high, and they were positioned slightly off-center in the room Figure
(1). The tops of the desks were covered
a h a sheet of thinly lined manilla-
white paper (r = .86). A chin rest, 30 cm high, was rigidly attached to the
front of each desk F.- Affixed
to the top left corner was a bracket to hold an anglepoise fluorescent desk luminaire.
One commerc~ally availabJe desk lumi-
naire (Luxo Magnifier) was used in both rooms Figure ( 2 )
.
This type of desk luminaire housed a circular magnifying lens and a surrounding cool white fluo- rescent lamp (Sylvania, FC819-CW-RS). It was attached to the desk bracket by an articulated metal arm. The top of the desk luminaire was modified for the ex- periment, however; through two sets of cross-hairs above the magnifying lens the experimenter could determine the lhp's position over the lined desk top to with- in one centimeter. A n opaque card was attached to the top of the sighting sys- tem aperture during test trials to pre- vent projection of an image of the ceil- ing onto the desk surface.Recording equipment was also employed in both rooms to monitor changes in the luminous output of the ceiling and desk luminaires during the experimental ses- sions. The monitoring illuminance probe can be seen on the desk top in Figure (2). Periodically the desk luminaire was
turned "off" in the pink room to measuze the illuminance on the desk top from the ceiling fixtures. The output of the desk lamp was monitored by positioning it at a specific location above the probe. These
*All reflectance measurements, unless otherwise noted, were made using a Hagner, 1 degree spot meter and a reflectance standard at lighting geometries avoiding specular reflections.
values were then used for relative cor- rection terms to adjust photometric mea- surements to the stimuli obtained after the experiment was completed (Section 11.6).
Subjects
Five females and seven males partici- pated as paid, naive volunteer subjects in the experiment. They ranged in age
from 18 to 26 years (M = 21). All were
or had recently been students, were right handed, and had excellent, uncorrected vision, as determined by a Keystone Oph- thalamic Telebinocular screening test administered prior to the experiment. Tasks
Subjects were asked to execute three tasks for this study. First, they were asked to move the desk luminaire from one of two pre-trial positions (top of the desk, giving veiling reflections, or the far left of the desk, giving no veiling reflections) to a position that they preferred to perform the task. The task was located at the centre of the desk. (Subjects were required to keep the bottom of the desk luminaire parallel to the desk surface.) Stimulus sheets like those to be presented in the numer- ical ver ification task* were given to subjects for the desk luminaire adjust- ments Figure (2). Second, they were asked to scan two number lists for dis- crepancies as "quickly and accurately" as possible. This is the numerical
verification task (NVT)
.
Third, theywere asked to evaluate the visual "dif- ficulty" of the stimuli by a ma nitude
estimation procedure [21, 22, 271.
Numerical values from "0" (lowest possi- ble visual difficulty) to "10" (sub- threshold visibility) were assigned to every stimulus condition after comple- tion of the NVT and before the lamp was returned to the next pre-trial position. Stimuli
Juxtaposed, 8 1/2
x
11 inch, referenceand response sheet like those used in an earlier study[281** were employed as
stimulus materials. ~riefly, a reference
sheet was a list of twenty, five-digit, numbers. A corresponding response sheet was a nearly identical list with zero to
six possible
,
one-digit, discrepanciesrandomly distributed throughout the list. White, matte reference sheets were
printed with four ink types; black matte, black gloss, gray matte and gray gloss. White, matte response sheets were printed
with a matte black ink, A calibration square was also printed on every sheet above the top center of the number list to provide convenient photometric mea- surements under the various lighting con- ditions.
Procedures
All subjects were individually tested. They were initially screened for their visual capabilities, age and handedness. A preliminary session in a large testing room was conducted to acclimate the sub- jects to the experimental procedures. Subjects were comfortably positioned at a chin rest and given 32 practice trials at the NVT and the magnitude estimations. The desk orientation was a 0' and 90' [20] so that they could experience the extreme conditions for veiling reflections.
Subjects were then brought to one of the test rooms for the first experimental session. Again, they were comfortably positioned at the chin rest. Four ex- perimental blocks of 16 trials each were completed by the subjects. On every trial, subjects completed, in order, adjustment of the lamp, the NVT, and the magnitude estimation. The experimenter recorded the subject's lamp position, time to complete the list in the NVT, and stated task difficulty, respectively, after each task was completed. Blocks of trials were performed in alternate test rooms; the orders of test room presenta- tion were counterbalanced and assigned randomly without replacement to each subject (Half the subjects performed the task with the test room order: black, pink, black, pink. Half performed them with the order: pink, black, pink, black). A short oral, debriefing ques- tionnaire was administered to each sub- ject after completion of the last block. Photometry
Photometric measurements for each trial were made after completion of the experi- ment. The ink calibratibn square on the reference sheet presented to the subject for a given trial was positioned at the point corresponding to the centre of the reference list during the experiment. The lampys position set by a subject for the trial was reestablished and luminance measurements obtained from the ink tali-
*
This numerical verificati~n task (NVT)has been described briefly below (Section 11.4) and in detail elsewhere f20]
.
**Complete details regarding the stimuli may be found in this reference.
bration square (Lt) and an adjacent area of the white paper (Lb). Stimulus sheet contrast ( (Lb-Lt)
/
Lb ) was determined from these measurements. Absolutevalues of background luminance were de- termined from the Lb values and the re- lative changes in the output of the desk and ceiling luminaires recorded periodi- cally during the experimental blocks
(Section 11.1)
.
Results*Task 1: Adjustment of the desk luminaire' There was a wide variation in subject's positioning of the desk luminaire, but on average subjects positioned the lumi- naire 325 mm above and 117 mrn to the
left of the task. The average contrasts for the four kinds of inked stimuli ob- tained from thbse at all lamp positions were consistent with those from a "typi- cal" sheet measured at this average position. Bocn sets of values gave con- trasts higher than those obtainable from the same material under hemisphere illu- mination (i.e., CRF >1.0)++. The con- trast frequency distributions for the four ink types in the two test rooms are shown in Figure (3) with the averages and the hemisphere contrast values. As with the contrast measurements, "typical" reference sheet background luminances measured at the average luminaire posi- tion were consistent with the average measured luminances. The average back- ground luminance for the reference sheets was 345 cd m-2. This average luminance value corresponds to a task
illuminance of about 1300 lux. The spread in background luminances for dif- ferent lamp distances from the centre of the task are shown in Figure (4) for the two test rooms.
Lamp positions in the two test rooms produced reference sheet background
luminances that were not statistically different even though the pink room il-
lumination was supplemented by ceiling fixtures. and minimum luminances were higher there Figure (4)
.
The average stimulus sheetcontrasts in the two rooms were, however, quite different; contrasts in the black room were stati- stically (p. 4.01) greater than in the pink room. Consistent with this obser- vation, the average lamp positions in the black room were more likely to be to the extreme left of the task than in the pink room where the lamp tended to be closer to and toward the top of the task.The gray ink and the black ink refer- ence sheets influenced the subject's ad- justments of the desk luminaire. Sub- jects were apparently more careful to avoid veiling reflections with the gray inks. Luminaire positions tended to be to the left and slightly toward the top of the task. Reference sheet background luminances, however, were nearly identi- cal for the two types of inked sheets. Also, subjects took significantly less time (p. 4.01) to adjust the desk lumi- naire with the black ink stimuli (about 8 sec) than with gray ink stimuli (about 10 sec). The specularity of the inks, gloss or matte, had no significant effect on the adjustment of the lamp by any dependent measure.
Starting position of the lamp in- fluenced the ultimate contrasL and lumi- nance of the stimuli but not the time to adjust the luminaire. The starting posi- tion at the top of the desk resulted in significantly lower contrasts but signi- ficantly higher luminances than the left starting position. Thus, subjects
tended to allow slightly more veiling reflections when the starting position was at the top. Still, subjects were generally careful to avoid veiling re- flections; CRFs were usually greater than 1.0.
Experimental blocks were significantly different by two measures, background luminance and time to adjust the lumi- naire. Background luminance increased and adjustment time decreased progres- sively across blocks.
Subjects were significantly different on all three dependent measures used to gauge performance on Task 1.
Task 2: Numerical verification task Ink pigment density, gray ink and black ink stimuli, had the largest effect on
*
See Appendix 1 for a discussion ot the analyses of variance (ANOVAs) used in this experiment to evaluate the various tasks.+
See Appendix 2 for a discussion of the dependent variables and the significant terms in the ANOVAs for Task 1.++Contrast Rendering Factor (CRF) is the ratio of the contrast measured under ac- tual luminous conditions to the contrast of the same stimulus measured under hemisphere illumination.
performance
.
[28cfore, based upon speed and accuracy,
was used to evaluate visual performance, and it was deter- mined predominately by response times to compare the lists. Gray ink stimuli re- sulted in highly significant, longer, response times, and thus lower scores, than the black ink stimuli (p. 4 . 01).
Specularity, gloss ink or matte ink stimuli, had no significant (p. a.05) ef- fect on performance, because subjects seemed particularly careful to avoid veiling reflections when positioning the desk luminaire.
The test rooms, black and pink, also resulted in significantly different times and scores (p.
<.
025.
The peformance in the black room was poorer than in the pink room, despite the higher contrasts, due to different desk luminaire positions(Section 111.1)
.
The simultaneous effects of ink pigment density and test room on performance
score are shown in Figure (5).
The starting position of the lamp also resulted in significantly different per- formance scores (p. 1.01)
.
The starting position at the top of the desk was asso- ciated with higher scores than the start- ing position at the left of the desk.The experimental blocks were signifi- cantly different. Initial and final blocks had higher scores than the middle two blocks. Subjects were also signifi- cantly different. No other main effects or interactions were significant.
Task 3: Magnitude estimations
In some ways the average magnitude estimations mirrored the performance scores. Rated task difficulty decreased when performance score increased. Gray
ink stimuli were rated as significantly more difficult than the black ink stimu-
li (p. <.01), and the black test room was rated as significantly more diffi- cult than the pink test room (p. 4.025)
.
Unlike the performance results, how- ever, the pigment density by room inter- action term was significant for the mag- nitude estimations (p. <.01). Task difficulty was higher for the gray inks
in the black room than for the gray inks in the pink room (as would be indicated by the performance data), but black ink stimuli were rated equally difficult in the two rooms (not indicated by the per- formance data). The difference between
the slopes of the two lines in Figure (6) connecting gray inks (dashed line)
and
the black inks (solid line) illus- trates this significant interaction.The subject main effect and the ink pigment density by subject interaction were the only other terms significant
in the magnitude estimation ANOVA. Task 4: Debriefing
Responses to a few questions were re- corded from subjects after the experi- ment was completed. All subjects said that they liked the ability to move the desk luminaire. Several spontaneously replied that it was advantageous to be able to tailor the lighting to the task.
Subjects felt that the pink room was similar to other small offices, but the black room was very atypical. While most subjects preferred to work in the pink room,others preferred to work in the black room; some of the latter group felt there would be fewer distractions. Al- though most subjects felt that they could do the task better in the pink room, four felt that they could do the task better in the black room. This was not the case; on the average, these four subjects performed better in the pink room.
Discussion
Contrast, luminance and luminous unifor- mity
-
It was clear from the lamp position data that subjects preferred high stimu- lus contrasts and task luminances. The CRFs were almost always in excess of 1.0. These data seem to be the first to clear- ly demonstrate that people avoid contrast reducing veiling reflections when given some flexibility in their lighting. Task luminances selected by subjects were in excess of those recommended, for example, by the Illuminating ~ngineering Society of North America for subjects of this age and tasks of this type [241
.
Thesevalues, however, would be in very good agreement with th i luminance values reported by Boyce 715f for subjects ad- justing a similar desk luminaire for a visual performance task.
It has been stated in other reports on adjustment of desk luminaires[l51 17, 181 that subjects prefer uniform luminous distributions. This did not seem to be as important to subjects in this study as high contrasts and luminances, based upon their lamp positions. Despite Boyce's interpretation that subjects
opted for uniformity "across the centre of the desk," it is not altogether clear that subjects in fact used a desk unifor- mity criterion to adjust the lamp for the visual performance task in his study. Although Boyce did not report the distan- ces between the lamp and the visual per- formance task, the marked similiarity be- tween the illuminances reported in his study and this one indicates that sub- jects placed the lamp close to the task to give high task luminances, thus sacri- ficing uniformity across the desk.
The physical dimensions of the other tasks used in Boyce's study were not reported, but from their descriptions it is likely that they covered a much larger portion of the desk than the visual per- formance search task. These larger tasks were associated with lower illuminances, clearly due to higher lamp positions. Probably subjects in Boyce's study were disregarding luminous uniformity on the desk, per se, in favor of uniformity over the perceived task area. Hypothetically
then, the larger the ~erceived task area,
the higher the lamp position, and the lower the task luminance. Therefore Boyce's data could be reinterpreted as indicating that people set lamp height for task uniformity rather than desk uni- formity as previously proposed.
This task uniformity criterion, hypo- thetically used by subjects to position the lamp, should not be inferred as the best strategy for visual performance. Several studies indicate that the lumin- ous distribution outside the task area influences the visibility of a stimulus
[25, 26, 27, 28, 29, 30, 31, 32, 33, 341. The performance data presented here can be interpreted as qualitatively rein- forcing these data. Performance dropped in the black room, relative to the pink room, for both the black and gray ink stimuli, even though lamp positions in the black room produced higher luminances and contrasts. By at least one conven- tional measure of "visibility" [351
,
per- formance should be slightly higher in the black room than in the pink room. [It is high1 unlikely that "transient adaptation" [351 361 reduced performance in the black room. Subjects did not make glances to the darker areas in the room during the test trials, and they had five to ten seconds to adapt to the sameluminance as the task background prior to the start of the trial. At the unbleach- ing luminances encountered in this study, complete recovery from adaptation to the dark surrounds should take place withir about one second.] Consistent with the
previous vision literature investigating nonuniform luminous distributions in the visual field, however, the reduced level of performance in the black room was ap- parently due to the poorer luminous dis- tribution outside the bright task area.
As noted above, the task luminances were high, yet the absolute performance level was not any higher in the pink room than that obtained from subjects in
simil r experiments at half the lumin-
ance
["I+.
This is not to say that per- formance does not continue to improve with luminance[20, 31, 37, 381, but ra- ther doubling the luminances at this level does not make a significant impact on performance. This being the case, then, it would likely have been a better strategy for the subjects to elevate the desk luminaire, thus sacrificing some task luminance, for a more unifoq lumi- nous distribution across the desk as existed in the studies report by ReaI2O]. Subjects could not have significantly affected the luminances of the black walls in this experiment, but thesesurfaces would be less effective in in- fluencing performance because of their angular distance from the task area. Luminous points just outside the task
area have the stron est effect on the
3
visibility of task[ 0, 39, 40, 41, 421. Therefore, improving the uniformity on the desk near the task would have like- ly improved performance. Indeed, some subjects may have done this to a small degree because the lamp positions were about two cm further away, on average, from the task in the black room than in the pink room. It is important to
note, then, that Boycels recommendation
for luminous uniformity across the desk is sound, despite, however, the behavi- our exhibited by the subjects in his experiment. As noted above, they were apparently positioning the desk lumi- naire for luminous uniformity on the task and not for the entire desk surface. Starting position as a significant effect
As noted in the results there was a significantly higher score associated
with the, top starting position than the
left starting position. Although CRFs associated with both starting positions were high and usually greater than 1.0,
+
See the data in the main experiment aswell as those in the appendix of that study.
*
See Appendix 3.contrasts were lower for the top starting position. This should have produced, if anything, lower scores. It is unlikely that glare was important. If anything glare would have been, again, worse for the top starting position because the final lamp positions tended to be slight- ly closer to the direction of view for the task. Luminances were higher for the top position, but at these levels such slight differences in luminance are in- significant for visual performance.
Importantly, ,however, the luminous dis-
tribution across the desk was more uni- form for the top starting position than for the left starting position. Here again then, this significant effect sup- ports the notion that luminous uniformity is important for optimum visual perfor- mance.
Paranthetically, subjects made no indi- cation through estimates of task diffi- culty that they were aware of any poten- tially deleterious effects from the dif- ferent luminous uniformities produced by the final lamp positions. Therefore, it seems that this significant effect was inadvertently produced by the subjects, and that the "ergonomic aspects" of the lamp starting positions can ultimately, but unconsciously, influence the visibi- lity of the task.
Subiective resDonses
In a previous report[161 a comparison was made between subjective scaling and visual performance. A different method of obtaining subjective reports, magni- tude estimation, was employed in this experiment to see if evaluations of task variables were more accurate with this method than with the seven noint semantic differential scales that were previously used. The results were, however, similar with both methods. In particular, sub- jective responses were ideosyncratic; for example, the ANOVA revealed a significant ink pigment density by subjects' interac- tion term (Section 111.3). On the other hand, visual performance, as measured by score and response time, was more homo- geneous for subjects; the subject term in ANOVA did not interact with any other experimental variable (Section 111.2). As indicated in the previous report [16], subjects differ in their ability to relate the influences of task variables on visibility through subjective scaling.
In the present experiment, all subjects registered a difference between the gray and the black ink stimuli by their magni- tude estimations of task difficulty. Gray ink stimuli were always rated more
difficult than black ink stimuli. In- deed, ink pigment density had the strongest influence on visual perfor- mance. Most subjects, however, did not reveal the more subtle influences on visibility by their magnitude estima- tions. For example, only five of twelve subjects registered an increase in task difficulty for the black ink stimuli in the black room even though performance dropped under these conditions. Averag- ing the subjective responses across sub: jects eliminated this increase in task difficulty (and actually reversed it slightly). Compare the solid lines in
Figures (5 and 6). Therefore, this in-
sentivity of the "average subject" to the more subtle influences on visibility can potentially distort inferences about task visibility. To echo the earlier report then, without some calibration procedure one does not know which subjects can accurately report changes in task visibi- lity, and, further, one does not know the relative influence that insensitive or inaccurate subjects have on the average response.
Adjustments of the desk luminaire can also be conceived of as a form of subjec- tive response. If people are cognizant of the effects that luminous conditions have on visibility then ideally a flexi- ble lighting system could be used to overcome any deleterious effects that the luminous environment might produce. In this experiment subjects did in fact overcome the detrimental influences of veiling reflections by consistently plac-
ing the desk luminaire to the side of the task. Like magnitude estimations of ink pigment density then, contrast was a strong determinant i nj the (subjective) adjustment of the desk luminaire.
The magnitude estimation data and the desk luminaire adjustment data were also similar with respect to the more subtle influences on visibility by the test rooms. As noted above, not all subjects indicated an increase in task difficulty for black ink stimuli in the black test room. This more subtle influence on visibility was not reduced by the average subjects' lamp position. The average distances from the desk lurninaire to the task were about the same in the two test rooms, yet, as noted in Section IV.1, subjects could likely have improved their visual performance by elevating the desk Luminaire to provide a more uniform desk luminance in the black room. Some sub- jects did in fact elevate the lamp more in the black room than in the pink room, but not enough to significantly alter the average background luminances. It is in- JOURNAL OF IES/OCTOBER 1983
teresting to note, then, that the insensi- tivity of subjective reports could lead the investigator to erroneous interpreta- tions about visibility as noted earlier
fIV.21; it can also lead to lower visual performance in a situation where people do not take complete advantage of light- ing flexibility to improve the luminous environment.
Implications for practice
The flexibility of the desk luminaire enabled subjects to produce high stimulus contrasts. This by itself makes desk lamps more useful than "hard wired" lighting installations that can produce veiling reflections at some occupant lo- cations. This same point was made in an
earlier However, one cannot
rely upon desk luminaires alone to ensure high levels of visual performance.
Other, more subtle influences on visual performance were ignored or misestimated by subjects. Specifically, they were somewhat insensitive to the deleterious effects of inhomogenous luminance distri- butions in the task and desk areas.
These findsing indicate a need for a division of labour between the occupant and the lighting-environment designer in- terested in improving visual performance. Currently, illuminating engineers inter- ested in assessing veiling reflections that reduce stimulus contrast make crude or inaccurate estimations of occupant head position and stimulus reflectances. The data presented in this study indica- ted that such questionable, and tedious, measurements and calculations are super- fluous because in practice occupants are likely to avoid luminous conditions that limit stimulus contrasts. Given some flexibility in the lighting geometry, then, it appears that subjects will largely solve the problems caused by veiling reflections.
Overcoming the more subtle variables reducing visual performance apparently cannot be left to the occupant. Subjects did not seem particularly sensitive to or efficient at removing luminous non- uniformities. Based upon a wide variety of visual experiments and supported by the data presented here, it seems that opportunities for inhomogenous luminous conditions, both brighter and darker than the task, should be reduced by the light- ing designer or engineer if it is impor- tant to optimize visual performance. More specifically, it can be inferred that nontask areas should have reflec- tances similar to the task, and surround
areas should be illuminated uniformly. This point is not new to illuminating engineering, it has been made by others
[51 7 1 43, 441. Further it is in com- plete agreement with the current recom- mendations for most illuminating engj- neering societies 124 I 45, 461
.
As noted in the introduction, energy conservation considerations have led to the proposition that task lighting is a viable method of reducing lighting loads and still ensuring high levels of task illuminance. Presumptively these high levels of illuminance will ensure high levels of visual performance. One
should be cautious, however, in oversim- plifying the advantages of task lighting for visual performance. It is true that the data presented in this report sup- port the notion that flexible lighting systems like desk luminaires can be ef- fectively used by people to produce high luminances and to avoid contrast reduc- ing veiling reflections. However, the lighting in nontask areas near the task can adversely affect one's ability to see, and these data do not support the notion that people can effectively avoid the deleterious influence of these
areas. Therefore one should always be careful to include both task and nontask luminous conditions in a discussion of lighting "quality and quantity."
Conelusion
The purpose of this report was to gain some insight'into the visual condi- tions associated with one type of task lighting, a flexible desk luminaire. By monitoring behavior on three tasks, positioning the desk luminaire, visual performance, and subjective estimations of task difficulty, the following con- clusions were drawn:
First, people seem to use the desk luminaire effectively to achieve high task contrasts. Contrast is a very im-
ortant variable for visual performance 7451, and one to which most subjects are conscious. This sensitivity to contrast was evidenced by the positioning of the
luminaire to the side of the task to avoid veiling reflections and by their subjective estimations of task diffi- culty for different ink pigment densi- ties.
Second, visual performance was appa- rently affected by luminous inhomogeni- ties. Subject's final lamp positions provided high luminances and contrasts, yet under essentially equivalent lumi- JOURNAL OF IES/OCTOBER 1983
nance and contrast conditions, perfor- mance was lower when there was poor lumi- nance uniformity across the desk. A wide variety of data from vision experiments
indicate that the lower performance was due to the darker, nontask areas within
the subjects' field'of view.
Third, most subjects may not be cogni- zant of the more subtle effects that al- ter visibility. Performance was lower for the black ink stimuli in the black room, but the "average subject" did not register an increase in task difficulty under these conditions. This insensiti- vity was further exhibited by the
"average subjects'" desk luminaire posi- tions. The position of the desk lumi- naire above the task did not always pro- duce the most uniform luminous distri- bution, and, as argued in the second con- clusion above, the lamp positions were not always in the best position for opti- mum visual performance.
Fourth, to provide better seeing con- ditions in practice, attention must be placed upon the luminous conditions in nontask areas as well as those at the task. Although subjects were apparently aware of the deleterious effects of lower task contrast, they were relatively in-
sensitive to the influence of inhomogen- eous luminous distributions near the task surface. Consequently it is important for the lighting designer-engineer to consider luminous uniformity within the occupants' visual field, particularly near the task, when optimal visual per- formance is desired.
Appendix 1
A cross-over, Latin square experimental
design [ 4 7 1 was used in this study.
Either of two estimates of error viabi- lity, within or between cells residuals, can be used to determine the significant levels of the terms in this design by an F-test. To minimize Type 1 errors (i.e.,
false positives) in estimating the signi- ficant levels of the terms in the ANOVAs, the larger error term was used as the denominator in the F-ratios. For perfor- mance score and task time, the dependent measures for Task 2, and for magnitude estimations, the dependent measure for Task 3, selection of the larger, more conservative, error term did not serious- ly affect the number of terms that ex- ceeded the significance criterion
(p. 4.05)
.
For score and task time onlyone less term (the ink pigment density by blocks interaction) did not reach the significance criterion when the more con-
servative error term was used. For mag- nitude estimations the tests were equi- vocal in the number of terms reaching the significance criterion. For Task 1, how- ever, biasing the F-test against Type 1 errors severely limited the number of terms reaching the significance criterion for every dependent measure; background luminance, contrast and time to adjust the desk luminaire. For Tasks 2 and 3 then, the Latin square model seems ap- propriate, but for Task 1 some interac- tion terms assumed to be nonexistant in
the Latin square model were in fact im-
portant. Therefore it should
be
realizedi
that the negative bias in the F-ration I for Task 1 may have eliminated
some
im- portant (significant) effects from dis- cussion.Appendix 2
I
Background luminance, stimulus con- trast, and time to move the desk lumi- naire to a final position were used as dependent variables to statistically evaluate subjects' responses on Task 1, adjustment of the desk luminaire. The ANOVA for background luminance revealed only three significant terms; subjects, blocks and initial orientation of the desk luminaire. Similarly, the ANOVA for the time to move the desk luminaire had three significant terms; subjects, blocks and ink pigment density (gray or black ink stimuli). On the other hand, the ANOVA using stimulus contrast as the evaluative criterion revealed the follow- ing significant terms: Subjects (S),
Room (R)
,
Lamp orientation (0),
Tnk pig-ment density ( C )
,
Ink specularity ( G ),
C x G, C x S, C x Blocks (B), C
x
0,G x S , G x B , G x R , G x O , G x G x B . Most of these were ignored in this report because they were not significant accord- ing to the dependent variables gauging the other two tasks, performance score and magnitud e timations. As pointed
out elswhere
tz0Y
performance score andcontrast are related by a marked non- linear function. Small changes of con- trast at low levels will have a strong effect on score whereas large changeg of contrast at high levels will have little or no effect on performance. Presumably magnitude estimations based upon the perception of contrast would give similar results. Therefore statistically impor- tant or unimportant effects associated with stimulus contrast that might be re- vealed in the ANOVA will not necessarily be indicative of the visually significant differences. This being the case, only those stimulus contrast terms simultane- ously significant according to these
JOURNAL OF IES/OCTOBER 1983
I
other, more important, dependent measures were discussed.
Appendix 3
Boyce (personal communication) has spe- culated that discomfort glare may have deterred people from placing the lamp at higher positions. Indeed, at lamp
heights between 350 and 400 nun above the desk, subjects looking at the task can perceive the bright fluorscent lamp under the edge of the desk luminaire in their extreme peripheral visual field. This may have been discomforting to some sub-
jects, and, thus, limited the height ad- justment of the desk luminaire. This hypothetical limitation was not consis- tent across subjects, however. Further the three best performing subjects re- peatedly placed the desk luminaire in this range. Three other subjects perio- dically placed the luminaire in this height range. Clearly the potential discomforting effect was not very strong or consistent and it did not seem to af- fect performance. Certainly, disability glare at these extreme angles would be small. Nevertheless, discomfort glare should be considered as a potential pro- blem in desk luminaire design. Elimina- tion of this potential problem could possible increase the likelihood of sub- jecfs setting more uniform luminous dis- triputions.
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blishment Digest, 232, 105, Dec. 1979b.
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7. Helms, R.N., Open Plan Off ice ~ i g h t -
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8. Carlton, J.W., Task Lighting--Don't Throw the Baby Away with the Bath Water!, Lighting ~ ~ u i p m e n t - ~ e w s , No. 183, 6-7, March 1982.
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and ~ n e r ~ ~ Use, ~lectrical contractor,
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259-270, 1979.12. Vos, J.J., Is Report CIE 19/2 A Use- ful Instrument for the Foundation of In- terior Lighting?, Paper Presented to the Public Works Canada Round Table Con- ference 25-27 January, 1982.
13. Padmos, P., Vos, J.J., The Validity of Interior Liqht Level ~ecommendations; Some ~ e ~ l e c t e d - ~ s ~ e c t s , Proceedings of
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14. Rea, M.S., The Validity of the Rela- tive Contrast Sensitivity Function for Modelling Threshold and Suprathreshold Responses, Symposium of Integration of Visual Performance Criteria to Illumina- tion Design, DPW, Ottawa, 25-27 Jan., 1982a.
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No. 3, 121-129, 1982b.17. McKennan, G.T., Parry, C.M., Tilic
M., An Investigation of Task Lighting
Installations, paper presented at the CIBS National Lighting Conference, 1980. 18. Bean, A.R., Hopkins, A.G., Task and Background Lighting, Lighting Research
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425-447, 1913.26. Cobb, P.W., The Effect on Foveal Vi- sion of Bright Surroundings, Psychologi- cal Review,
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30. Cobb, P.W., Moss, F.K., The Effect of Dark Surroundings Upon Vision, Journal of the Franklin Institute, 206, 827-840,
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32. Adrian, W., Eberbach, K., On the Re- lationship Between the Visual Threshold and the Size of the Surroundinq Field, Lighting Research and ~ e c h n o l o ~ ~ , - 1, 251- 254, 1969.
33. McCann, J.J., and Hall, J.A., Effect of average-luminance surrounds on the
visibility of sinewave gratings, J. Opti- cal Soc. of Amer., Vol. 70, No. 2, 212-
34. Wilson, A.J., Lit, A., Effects of Photopic Annulus Luminance Level on Reaction Time and on the Latency of Evoked Cortical Potential Responses to Target Flashes, Journal of the Optical Society of America, 71, No. 12, 1481- 1486, December, 1981,
35. Commission Internationale de 1'Eclai- rage (CIE). An Analytic Model for Des- cribing the Influence of Lighting Para- meters Upon Visual Performance; Vol. 1, Technical Foundations, Technical Commit- tee 3.1, CIE Publication No. 19/2.1 (TC-
3.1). 1981.
36. Boynton, R.M., and Miller, N.D., Visual Performance Under Conditions of - - -~ -Transient Adaptation, Illuminating Engi- neering, 541-550 August, 1963.
37. Blackwell, H.R., Contrast Thresholds of the Human Eye, Journal of the Optical Society of America,
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No. 11, 624-643, Nov. 1946,38. Smith, S.W., Is There An Optimum Light Level for Office Tasks? Journal of the Illuminating Engineering society, 255-258, July 1978.
39. Crawford, B.H., The Effect of Field Size and Pattern on the Change of Visual Sensitivity With Time, P x c . Royal Soc.
(London) B, Vol. 129, 94-106, 1940. 40. Ainsworth, G., Crouch, C.L., Dates, H.B., Kraehenbuehl, J.O., Spencer, D E., Tuck, D.N., Luckiesh, M., Brightness and ~riqhtness. Ratios, Illuminating Engineer- ing,
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713-730, 1944.41. Luckiesh, M., Brightness Engineering, Illumination Engineering,
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75-92, 1944.4 2 . Kalff. L.C., Environment in Seeing,
43. Chase V.D., Lighting and Energy Ef- ficiency, Lighting Design and ~pplica- tion, 14-17, Sept. 1977.
44. Anon., Supplementary ~ightinq From Local sources;- Light and ~ighting
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57, March, 75-78, 1964.45. Illuminatinq Enqineering Society of North America, ~ E S Lighting- Handbook, Re- ference Volume, J . E . Kaufman (ed.), 11111-
minating Engineering society of North America, New York, 1981.
46. Boyce, P.R., Human Factors in Light- ing, Applied Science Pubs., London, 1981. 47. Neter, J. and Wasserman, W., Applied Linear Statistical Models, Richard D. Irwin, Inc., Homewood Ill., 1974. Acknowledgements
The author would like to thank B. Guzzo, M. Ouellette and R. Jaekel for technical assistance and constructive criticism. Thanks also go to P. Boyce, K. Cuttle, D. Kambich and D. Stephenson for commenting on the manuscript.
Special thanks go to W. Evans who worked as a summer research student on the pro- ject.
DISCUSSION
Kit Cuttle (Victoria Univer
Wellington, ~ d h o o l of Archite - -
.
lington, New Zealand). This study de- monstrates effectively the sensitivity of Dr. Rea's experimental technique. We learn that subjects were largely obliv- ious of luminance contrasts outside the task area, and yet their performances are shown to have been affected. In fact, even the minority of subjects who pre- ferred the black room are found to have performed less well in that situation, and in this way the author has provided the first objective verification of
theoretical predictions which, as his re- ferences show, date back to the early years of illuminating engineering. Pre- sumably this finding would have been un- obtainable by subjective assessment.
However, the question must be asked whether the controlled conditions that enable these relationships to be identi- fied do not eliminate aspects of the real world that could be significant influences. Subjects were "comfortably positioned at the chin rest" and required to examine flat sheets of paper on an uncluttered desk. We learn that they adjusted the task light for high contrast rendering, but the average setting was only slightly to the left of the overhead position at an angle of incidence of 20'. This suggests that the tasks were of a type that gives a narrow cone of CRFU.0
(compare with RQQ5 data for the standard pencil task which indicates that an in- cident angle of more than 30° is required in this direction).
We must consider the extent to which the photometric data describe the experi- mental situation, and the extent to which the experimental situation can be genera- lized to real situations. The luminance measurements employed a precise and sta- tic geometry, and ah the variables that characterize the realities of people at work are added to this model, it must be seen that the practical "offending zone" is larger than that measured in these experimental conditions.
Furthermore, the light distribution of the luminaire must be influential. The luminaire employed appears to have been of the type that provides maximum illumi- nance directly beneath, and varying
across the desktop more or less as cos48. We can expect, then, that for the average setting the illuminance at the centre of the task was some 20 percent lower than directly beneath the luminare. The au- thor discusses the importance of evenness of task area illuminance upon subject's settings, and it seems reasonable to ask whether subject's settings in fact represent a compromise between elimina- ting poor contrast rendering from the entire task area and maintaining accept- able evenness of task area illuminance.
My own studies of office task lighting have led to the development of luminaires which mount over one end of a desktop and project a soft-edged band of light across the centre, defining a broad task area having sidelighting at angles of inci- dence ranging from 45' to' 70' [l]
.
This arrangement has been found to be highly practical as the illuminance distribution encourages users to locate their work such that good contrast rendering will be achieved for a very wide range of task types. These luminaires are not adjust- able, and have precise optical control to eliminate both direct and reflectedglare.
There have, of course, been many ingen- ious luminaires, and as Dr. Rea states in the introduction to his paper, several authors have claimed advantages of task lighting on the bases of energy savings, task visibility, and the satisfaction that comes from people being able to con- trol aspects of their own working en- vironment. On every count these claims have been contested, but in particular there seems to be a lack of conviction that if people are offered the opportuni- ty to apply lighting to good effect, that they will in fact do so. The important contribution of this paper is its recog- nition of the need to examine and to have JOUFPTAL OF IES/OCTOBER 19 83
evidence of these behavioural aspects. It is an important finding that subjects adjusted the luminaire for good contrast rendering even to the point that the ex- perimental variable of matte and glossy task detail became insiqnificant.
As we must be wary before translating laboratory findings to field applica- tions, it could be instructive to con- sider how observable behaviour of people in offices corresponds to this research finding. Is it reasonable to suppose that when they tilt their tasks, incline their heads or adopt postures that must be conducive todiscomfort, that they are adjusting those components of the viewing situation that they are able to control in order to achieve good contrast rendering? The scope for research into behavioural responses to lighting is ex- tensive, and this paper is to be welcomed not only for its useful findings but also as a demonstration of the potential of this form of inquiry.
Reference: 1. Cuttle, K., The Cemac Sidelighter Project, Lighting in Austra- lia, 3, No. 3, 19-22, 1983.
-
-
REBUTTAL
Author: First, I wish to thank Kit Cuttle for commenting on this paper. He was of the first to seriously investigate task lighting as an energy saving and visual quality alternative to conven- tional ceiling fixtures. For this rea- son, then, his comments are most welcome.
I thank him too for his kind comments on the experimental technique employed in this and earlier studies on visual per- formance. I too bleieve that these ex- perimental procedures and stimuli help us to characterize visual performance un- der simulated realistic conditions. In further agreement with Cuttle, I believe that this technique enables us to, at least partially, bridge the gap between basic visual sciences and lighting appli- cation.
Cuttle makes the recurrent point in his comments that it may not be possible to generalize the results of this study to real situations. This is, of course, true. But it is also true that every experiment and every field trial faces the same limitation. No single study can cover all of the important variables that influence the behavior of people performing a real task. Rather, we must rely upon the accumulated knowledge ob-
tained from many experiments and from practical experience. This study offers new information that will augment our ac- cumulated knowledge about proper light- ing.
Tn particular, this study points to two conclusions important for lighting prac- tice. First, it seems that subjects will avoid the 'offending zone' when given the opportunity. It seems reasonable and parsimonious to conclude too the poten- tially larger 'practical offending zone' mentioned by Cuttle would also be avoided by people doing work in an actual job. Second, this study indicates that unifor- mity in the work area is important, but
that this relatively small effect is generally unnoticed by subjects. In partial contradiction to this conclusion, Cuttle speculates that the lamp position set by subjects may represent a compro- mise between high contrast and luminous uniformity. It is probably more accurate to say that subjects made a compromise between high luminance and high contrast. They tended to place the luminaire close to the task, thus giving high luminance; if they had been interested in uniformi- ty, they would have elevated the lamp.
Although this study indicates thac er- gonomics and luminous uniformity in the task area are important to lighting prac- tice, one should be careful, as Cuttle points out, in extrapolating from this one study to lighting practice. This line of research should receive further attention so that lighting practice will be improved.
DESK LAMP
lu BRACKET
+
CENTRE O F 1 x 4 f t . (300 x 1200 mrn)FLUORESCENT TROFFER N
I
RESTBLACKROOM
'1
't,
PINK ROOM+
Figure 1 (Above)--Plan view of the two test rooms.
Figure 2 (Below)--Subject adjusting desk luminaire in the "pink" test room.
BLRCK GLOSS BLACK HRTTE PINK ROOM BLACK ROON CONTRRST CONTRFtST
GRAY GLOSS GRAY MRTTE
t. 0 Z W PINK
2
5 ROOM W K L m 0 W I t CONTRflST CONTRRSTFigure 3. Contrast frequency distributions for the four ink types in the two test rooms. Ascending arrows are the mean hemisphere contrasts for the different inked stimulus sheets. Descending arrows are the mean
observed contrasts produced by the different inked stimulus sheets in the two test rooms.
PINK ROOM
23 4 4 BLACKROOM
0 0f
a 0 0 200 400 600 800 1000 Q O O 0 200 400 600 800 1000 1200I.ALlI' 1)IS'IINCE (mm) ],AMP DISTANCE (mm)
Figure 4. Background luminance values observed in the experiment for the assoc- iated lamp distances from the centre of the task. The values in both test rooms are represented on the same scales. One point in the black test room exceeded the ordinate range; the arrow indicates the lamp distance, and the associated luminance is indicated by the adjacent number.
MEAN
SCORE
60 62 64 66
Figure 5. Visual performance (score) for the two types of stim- ulus ink pigment densities in the two test rooms. The solid line indicates per- formance for the black inked stimuli; the dashed line indicates performance for the gray inked stimuli. Vertical lines represent standard errors of the mean.
MEAN PERCEIVED DIFFICULTY
2 3 4 5
'i
-
I I 1 I Itd,
z
-
w
'-y
-
Figure 6. Subjective scaling (per-x
, ceived difficulty) for the!a
i
two types of stimulus ink0 \
0 two test rooms. The solid pigment densities in the
$td
line indicates perceivedt' ,
*
-
&
-
difficulty for the blackn
inked stimuli; the dashedFI
line indicates perceiveddifficulty for the gray inked stimuli. Vertical lines represent standard errors of the mean.
1 I I I I
JOURNAL OF IES/OCTOBER 1983
T h i s paper, w h i l e being d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It should n o t be reproduced i n whole o r i n p a r t w i t h o u t t h e p e d s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, O t t a w a , O n t a r i o ,
