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A method for predicting the economic consequences of changes to
visibility
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National Research
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Construction
construction
A Method for Predicting the
Economic ConsequenGs of
Changes in Visibfiity
by D.K. Tiller, M.J. Ouellette,and I.C. Pasini
ANALYZED
Reprinted from
Proceedings of the 22nd Session of the
Commission internationale de I'eclairage
Vol. 1, Division 1, pp. 71-72,1991
(IRC Paper No. 1883)
I
L I B R A R Y
A METHOD FOR PREDICTING
THE
ECONOMIC CONSEQUENCES OF CHANGES IN VISIBILITY DaleK.
Tiller, MJ.
Ouellette, & I.C. PminiThis paper describes a method for forecasting the economic impact of changes to task visibility. The method uses Rea and Ouellette's Relative Visual Performance (RVP) model1. The model predicts visual processing time, Tvis, as a function of object size, contrast, adaptation luminance, and observer age. We calculated Tvis for alphanumeric task samples measured at various corporate mail-sorting stations. These values were compared to TviS values for the same task samples measured under a reference condition of high-intensity uniform lighting. This comparison provided estimates of the penalty in visual processing time for each sample in the field condition, relative to the reference condition. The method outlined also has wider heuristic value. providing specific and testable hypotheses regarding the consequences of changes in visibility in other application areas where quick and accurate visual processing is critical (e.g., roadway lighting, production line quality control).
UNE
I&I'HoDE POURPRBDIRE
L'INCIDENCEI~CONOMIQUE DES MODIFICATIONS APPORTI~ES A LA V I S I B I L ~
Le
present article d&t une mithode pennettant de pdvoir l'incidence hnomique des modifications apporthB
l'klairage sw la visibiliti de la &he. La mithode utilise le modkle de performance visuelle relative mise au point par Rea et Ouellette (1988). Le modkle sertB
p&e le temps ntcessaire au syseme visuel pow traiter l'information, soit Tvis, en fonction de la taille de l'objet, du contraste, de l'adaptation lumineuse et de l'gge de l'observateur. On a calculi le Tvis requis pow l'exkution de Gches khantillons exigeant le traitement de d o m k s alphanumtiriquesB
divers endroits de tri de coumer. Les valeurs obtenues ont it6 cornparks au Tvis calculi pour des tkhes similaires exkutks cette fois dans des conditions de dfirence, soit un klairage uniforme de haute iptensiti. Cette comparaison a permis d'obtenir une estimation des coiits du Tvis pour chacun des khantillons observ6 sw le terrain, par rapport aux conditions de dfirence. La mithode prisentk posskde i g a l e m e une importante valew heuristique puisqu'elle fournit des hypothkses p k i s e s et vMables quant aux consQuences des changements apporttsP
l'klairage sur la visibilit6, dans des domaines d'application ob la rapiditi et la prkision de la &he visuelle sont primordiales (parexemple. l'klairage des routes, le contr6le de la qualit6 des chaines de production).
EINE
METHODE ZUR VORAUSSAGEOKONOMISCHER FOLGEN BE1 ANDERUNGEN AM SICHTBEREICH
Dieser Aufsatz beschreibt ein Verfahren, das ijkonomische Wiungen auf Sichtbarkeit am Arbeitsplatz bei Veriin- derungei der Beleuchtung voraussagt. Die Methode macht Gebrauch von dem "Relative Visual Performance"
-
Modell von Rea und Ouellette (1988). Das Modell sagt visuelle Verarbeitungszeit-
Tvis-
in Abhiingigkeit von Objektgriisse. Kontrast. Helligkeitsanpassung und Alter des Beobachters voraus. Wir haben Tvis fur alphanume- rische Aufgabenproben berechnet, die an verschiedenen gesch~tlichen ~ostsortierun~sstellen gemessen worden sind. Diese Werte wurden mit Tvis -Werten fur die gleichen Aufgabenproben, die unter Bezugsvehdtnissen "on gleichmiissiger Hochleistungsbeleuchtung bemessen d e n , verglichen. Dieser Vergleich gab eine Bewertung uber die Nachteile in visueller Verarbeitungszeit fiir jede Probe bei den Arbeitsverhdtnissen in Vergleich zu den Bezugsverhdtnissen. Die hier aufgefuhrte Methode hat auch einen weitgehenderen heuristischen Wert. da sie spezifische und teschige Hypothesen beziiglich der Folgen fiir Sicht bei Beleuchtungsiinderungen auch in anderen Anwendungsbereichen, wo schnelle und genaue visuelle Verarbeitung kritisch ist (2.b. Strassenbe- leuchtung. Giiteregelung am Fijrderband), angewandtThe idea behind this paper stems from two simple principles. Fmt, the transmission and processing of n e u k information takes time. This is true for all psychological, motor, and sensory processes, vision included. Second, time is money. So, if we could measure or estimate the time required to see an object, and assign a monetary value to this elapsed time, we could predict the economic costs or benefits of increases or decreases in visibility. Lighting designers and illuminating engineers have been unable to apply these obvious principles, however, for at least two reasons. Fmt, most visibility models do not provide estimates of the time required to process visual information. Second, practical mtkurement tools for characterizing the salient features of objects that might affect visual processing time have until recently been unavailable. This paper describes a method for predicting the economic consequences of changes in visibility, based on visual processing time, Tvis, predictions from the RW model1.
2. METHODS AND PROCEDURES
In Canada, six-chmter alphanumeric strings are used to sort mail (e.g. "KIA OR6"). Two samples of this postal code string were measured. One sample was printed on white paper (p = .75), using a laser printer; the other sample was printed on brown manila paper (p = .57), using a dot matrix printer. Both samples were measured under ambient conditions at a manual mail sorting station (view distance about lm; view angle about 30 degrees), and again under a laboratory reference condition of high- intensity uniform lighting (same distance and angle). All measurements were taken using a computer-based video photometer2, consisting of a calibrated solid-state video camera, image capture board, and custom software. The device resolves a visual scene into a quarter-million luminance measurements and stores these in the computer. the software then calculates Tvis for the postal code strings under the two different lighting conditions.
3. RESULTS AND DISCUSSION
Table 1 shows values of Tis (in milliseconds) for two worker ages, two different observed illuminances (263 and 1480 lux). and predicted values for 800 lux. Most samples would take longer to see under field conditions, than under laboratory reference conditions, or had 800 lux been pro- vided in the field. Existing field conditions exact a penalty in visual processing time. This penalty can be used to
Table 1. Th
M
M
E = Observed* 8001w Observed* 8 0 0 1 ~ High Contrast Task (Contrast = .81)
Lab 327 331 331 336 Field 342 334 348 339
Law Contrast Task (Contrast
=
37)Lab 384 390 391 398 Field 432 418 443 427
*
Observed illuminance was 263 and 1480 lux at field and laboratory reference condition, respectively.estimate minimum increased times to sort mail arising from increases in the visual requirements of the task. For example. 48 milliseconds additional time is required to process the average single character in the postal code
string for the lower contrast task, at field conditions relative to the laboratory reference condition (432
-
384). The penalty for a six-character string is thus 48 x 6=
288ms. Since each piece of mail is handled 3.5 times the penalty is 1.008 seconds (.288 x 3.5). If 1,000.000 pieces are processed daily, the overall estimated penalty is 280 hours.
Obviously it may not be feasible to supply laboratory reference conditions at the workstation. Tvis values can
be used to estimate the change in processing time for different site illuminances. For example, increasing the illuminance to 800 lux would save 14 milliseconds (432
-
418). for a 20 year old worker. Applying similar calculations as above suggests a potential saving of 81.6 hours.Similar calculations applied to other samples, measured at different sites, were used to evaluate the economic implications of delamping. The client did not delamp, because the costs of increased visual processing were greater than the savings from delamping. Ideally, other criteria relating to pleasantness and occupant satisfaction would be included in design decisions. Nevertheless, the method described has general applicability wherever it is necessary to tailor illumination for quick visual response, such as might be required by the military, transportation industries, athletics, and visual inspection for quality control.
REFERENCES
[I]
REA
MS & OUELLEITEUI.
Visual performance using reaction times. Ltg. Res. & Tech. 20, pp. 139-153, 1988.[2] REA MS &
JEFFREY
IG: A new luminance and image analysis system for lighting and vision, J. Illum. Eng. Soc. 19, pp. 64-72. 1990.ACKNOWLEDGEMENTS
We would like to thank Mr. Vytas Treciokas for arranging the site visits. It is also a pleasure to acknowledge the assistance of Mr. Ralston Jaekel, in all phases of project planning and execution.
Dale K. Tiller @.Phil.), MJ. Ouellette (B.Sc. Hons.) Institute for Research in Construction
National Research Council of Canada Ottawa, Ontario, KIA OR6
I.C. Pasini (M.Sc., P.Eng.)
Architecture & Engineering Services Public Works Canada