An Assessment of the Blackwell visual task evaluator, Model 3X

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An Assessment of the Blackwell visual task evaluator, Model 3X

Rea, M. S.; Ouellette, M. J.

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1 6 0

National R w c h Conseil nation4

Council Canada

de

rmherches

Canada

AN ASSESSMENT

OF

THE B8LACKWELL

VISUAL TASK EVALUATOR, MODEL

3 X

by hdS Rea and

U

J.

Ouetlette

DBR Paper No.

1160 Division of

Buttding Research

Price

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NATIONAL RESEARCH COUNCIL CANADA D I V I S I O N OF BUILDING RESEARCH

AN ASSESSMENT OF

THE

BLACKWELL VISUAL TASK EVALUATOR, MODEL 3X

b y M.S. R e a a n d M.J. Ouellette

ANALYZED

DBR P a p e r N o . 1160 of the 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 O t t a w a , F e b r u a r y 1984

ABSTRACT

The CIE has recently published a method for assessing the

Visibility Level (VL) of realistic visual stimli (as defined by

Commission Internationale de l'gclairage, 1981

1.

To implement the

system, a visibility meter is required; the Blackwell Visual Task

Evaluator, Model 3X was designed to be used with the CIE method.

At the request of the Chairman of the CIE Technical Committee

3.1

on visual performance, our laboratory undertook an independent

evaluation of the instrument. The findings are reported here.

While there are several idiosyncratic features of the Visual Task

Evaluator, many of these findings are applicable to other

visibility meters.

La CIE a rbcemment publie une methode pour dbterminer le

niveau de visibilit'e des stimli visuels rgels (tels que dgfinis

par la Commission internationale de l'bclairage, en 1981).

Comme cette mdthode requiert l'utilisation d'un appareil de mesure

de la visibilitd, on a consu un appareil special

:

le "Blackwell

Visual Task Evaluator, mod8le

3X".

A

la dernande du pr'esident du

Comite technique 3.1 de la CIE sur la performance visuelle, nos

laboratoires ont proc'ed6

3

une gvaluation de l'instrunent.

On trouvera ici les rgsultats de cette 6valuation. Quoique le

"Visual Task Evaluator" poss3de plusieurs caract'eristiques qui

lui sont propres, bon nombre de ces conclusions s'appliquent

3

TABLE OF CONTENTS

Page

1.

INTRODUCTION

2. GENERAL CHARACTERISTICS

3.

CENTER AND SURROUND FIELDS

3.1

General description

3.2

The brightnesses of the fields

3.3

The operator's tasks

3.4

Field luminances

4.

OPERATION OF THE OUTPUT CONTROL

4.1

Control dial

4.2

Luminance

4.3

Veiling luminance

4.4

Color

5. OPERATION OF THE CONTRAST CONTROL

5.1

Control wheel

5.2

Transmission

5.3

Veiling luminance

6.

OPTICAL FIDELITY

7. DETERMINATION OF ADAPTATION LUMINANCE

8.

BRIGHTNESS MATCHES

8.1

Luminance and brightness

8.2 Task field complexity

9.

MAGNIFICATION

10.

PULSING PROCEDURE

10.1

Appearance

10.2

Temporal characteristics

10.3

Luminance transients

11.

POLARIZATION

12.

USE

AS

A FIELD INSTRUMENT

13.

SUMMARY

AND

CONCLUSIONS

REFERENCES

Table I

Fields seen through VTE

AN ASSESSMENT OF THE BLACKWELL VISUAL TASK EVALUATOR, MODEL 3X*

1. INTRODUCTION

The Model 3X Rlackwell V i s u a l Task E v a l u a t o r (VTE) i s p a t t e r n e d a f t e r s e v e r a l e a r l i e r v e r s i o n s o f t h e VTE (Blackwell, 1959, 1970; Blackwell e t a l , 1964); t h e v a r i o u s v e r s i o n s of t h e VTE were, i n t u r n , p a t t e r n e d a f t e r s e v e r a l e a r l i e r v i s i b i l i t y m e t e r s ( J o n e s , 1920;

B e n n e t t , 1931; Luckiesh and Moss, 1934; C o t t r e l l , 1951). The purpose o f any of t h e s e i n s t r u m e n t s i s t o r e n d e r complex, r e a l i s t i c s t i m l i t o some v i s u a l t h r e s h o l d by a r e d u c t i o n i n o p t i c a l t r a n s m i s s i o n (Dunbar, 1939; Eastman and Guth, 1960). Almost a l l o f t h e l a t e r v i s i b i l i t y m e t e r s

( F i n c h , 1957; Eastman, 1968; S l a t e r , 1975; Levy and Nelder, 1976; O'Donnell e t a l , 1976), i n c l u d i n g a l l of t h e v e r s i o n s o f t h e VTE, a t t e m p t t o reduce v i s u a l s t i m u l i t o a t h r e s h o l d c o n t r a s t . These i n s t r u m e n t s a r e c a p a b l e of a d d i n g l i g h t t o t h e v i s u a l f i e l d a s

t r a n s m i t t e d l i g h t from t h e e x t e r n a l world i s reduced. I n t h i s way t h e a d a p t a t i o n l e v e l of t h e o p e r a t o r i s h e l d r e l a t i v e l y c o n s t a n t w h i l e t h e c o n t r a s t of t h e s t i m u l u s i s modulated.

The CIE method (Commission I n t e r n a t i o n a l e de l l E c l a i r a g e , 1981; I l l u m i n a t i n g E n g i n e e r i n g S o c i e t y of N o r t h America, 1981) i s b a s e d upon t h e d e t e r m i n a t i o n of t h e t h r e s h o l d c o n t r a s t of a t a s k . While i t i s p o s s i b l e t o e v a l u a t e t h e t h r e s h o l d c o n t r a s t of a b s t r a c t t a s k s by o t h e r psychophysical t e c h n i q u e s , i t was proposed i n t h e CIE method t h a t r e a l i s t i c , complex v i s u a l t a s k s s h o u l d b e e v a l u a t e d w i t h c o n t r a s t r e d u c i n g v i s i b i l i t y meters. T h e r e f o r e , v i s i b i l i t y m e t e r s l i k e t h e VTE a r e a n e s s e n t i a l t o o l i n t h e CIE method f o r a s s e s s i n g v i s u a l t a s k s of i n t e r e s t t o l i g h t i n g p r a c t i t i o n e r s . I n f a c t , t h e VTE w a s d e s i g n e d t o implement t h e C I E method.

According t o t h e CIE method (1981), V i s i b i l i t y Level (VL) i s a measure of how e a s y t h e t a s k i s t o s e e , I n t h i s system, t h e VL of a t a s k i s simply t h e r a t i o of i t s p h y s i c a l c o n t r a s t (C) t o i t s t h r e s h o l d c o n t r a s t

(c).

Thus:

VL is e q u a l t o u n i t y a t d e t e c t i o n t h r e s h o l d , and i t i n c r e a s e s as a l i n e a r f u n c t i o n of t a s k c o n t r a s t . For t h i s r e a s o n , VL h a s been c a l l e d t h e ' c o n t r a s t m e t r i c ' of v i s i b i l i t y .

To be i n correspondence w i t h Equation ( I ) , a v i s i b i l i t y meter s h o u l d reduce t h e c o n t r a s t of t h e r e a l t a s k t o t h r e s h o l d w i t h p e r f e c t o p t i c a l f i d e l i t y and w i t h o u t a f f e c t i n g t h e o p e r a t o r ' s a d a p t a t i o n l e v e l . For

*

T h i s p r o j e c t was undertaken a s an assignment from t h e Chairman,

P r o f . D r . H. Bodmann, of t h e Commission I n t e r n a t i o n a l e d e l t E c l a i r a g e , T e c h n i c a l Committee 3.1.

s u c h a n i d e a l v i s i b i l i t y meter, VL i s simply i n v e r s e l y p r o p o r t i o n a l t o i t s ' c o n t r a s t t r a n s m i t t a n c e ' (CT). That is:

This paper p r e s e n t s an assessment of t h e l a t e s t VTE, Model 3X, a s a t o o l f o r a c c u r a t e l y determining VL. Although t h e r e i s no a t t e m p t i n t h i s paper t o e v a l u a t e t h e v a l i d i t y of t h e VL formalism, i t i s c l e a r l y necessary t o a s s e s s c o n t r a s t reducing v i s i b i l i t y m e t e r s l i k e t h e VTE. Without v e r i f i c a t i o n of t h e i r accuracy, i t i s impossible t o e s t i m a t e t h e u n c e r t a i n t y a s s o c i a t e d w i t h t h e determined VL f o r t h e t a s k of i n t e r e s t , even f o r an i d e a l observer. There i s a l s o no a t t e m p t t o compare t h e VTE w i t h o t h e r v i s i b i l i t y meters. However, many of t h e p o i n t s made i n t h i s

r e p o r t a r e a p p l i c a b l e t o o t h e r i n s t r u m e n t s of t h i s type.

2. GENERAL CHARACTERISTICS

The

VTE

is a c t u a l l y two p i e c e s of apparatus: an o p t i c a l u n i t and a c o n t r o l u n i t ( F i g u r e 1). The o p t i c a l u n i t i s b a s i c a l l y a r e f r a c t i n g t e l e s c o p e combined w i t h a system of v a r i a b l e f i l t e r s (Figure 2).

O b j e c t s i n t h e e x t e r n a l world viewed through t h e VTE e y e p i e c e c a n b e made t o appear b r i g h t e r o r dimmer by a d j u s t i n g a ' c o n t r a s t c o n t r o l ' wheel ( F i g u r e 1) t h a t d r i v e s a p a i r of v a r i a b l e d e n s i t y f i l t e r s (X i n Figure 2). Light produced by an i n t e r n a l lamp and t r a n s m i t t e d through a n o t h e r , mechanically l i n k e d p a i r of v a r i a b l e d e n s i t y f i l t e r s

(Y

i n Figure 2) keeps t h e b r i g h t n e s s of t h e viewed f i e l d r e l a t i v e l y c o n s t a n t w h i l e t h e c o n t r a s t of t h e viewed o b j e c t i s reduced.

The c o n t r o l u n i t r e g u l a t e s t h e v a r i o u s components of t h e o p t i c a l u n i t . Meters on t h e c o n t r o l u n i t g i v e t h e o p e r a t i o n a l s t a t u s of t h e o p t i c a l u n t t . I n t h i s r e p o r t t h e c o n t r o l u n i t i s n o t s p e c i f i c a l l y e v a l u a t e d ; i n t h e subsequent d i s c u s s i o n t h e t e r m VTE w i l l b e used t o d e s c r i b e only t h e o p t i c a l u n i t .

3. CENTER AND SURROUND FIELDS

Through t h e eyepiece, t h e VTE o p e r a t o r can s e e two d i s t i n c t luminous regions: a c i r c u l a r c e n t e r f i e l d and a n a n n u l a r surround f i e l d

(Figure 3).

3.1 General d e s c r i p t i o n

The surround f i e l d has an o u t e r diameter of about 16'. The i n n e r diameter, and t h u s t h e d i a m e t e r of t h e c e n t e r f i e l d , can be, nominally,

1.5, 3 o r 6 O , depending upon t h e s i z e of t h e s e l e c t e d f i e l d a p e r t u r e ( F i g u r e 3).

Depending upon t h e p o s i t i o n of m i r r o r s h u t t e r s A , B and C ( F i g u r e 2) t h e c e n t e r f i e l d of t h e VTE w i l l g i v e a view e i t h e r of

[

1

]

a luminous p a t c h o r [ 2 ] some proport i o n of luminance from t h e e x t e r n a l world and of a superimposed luminous v e i l produced by t h e i n t e r n a l lamp. For d e s c r i p t i v e c l a r i t y , t h e s e two views w i l l be c a l l e d t h e s u b s t i t u t i o n and t h e t a s k f i e l d s , r e s p e c t i v e l y .

3.2 The b r i g h t n e s s e s of t h e f i e l d s

The b r i g h t n e s s e s of t h e surround and s u b s t i t u t i o n f i e l d s a r e determined by: [ l ] t h e o u t p u t of t h e i n t e r n a l l i g h t s o u r c e , 2 t h e p o s i t i o n of t h e 'output c o n t r o l ' d i a l on one s i d e of t h e VTE 3

1

t h e p o s i t i o n of t h e 'mode c o n t r o l ' f i l t e r [ 4

1

t h e t r a n s m i s s i o n s and r e f l e c t a n c e s of o p t i o n a l chromatic f i l t e r s i n s e r t e d i n t h e f i l t e r n a c e l l e and [ 5 ] t h e t r a n s m i s s i o n s and r e f l e c t a n c e s of t h e o p t i c a l components i n t h e two paths. The b r i g h t n e s s of t h e t a s k f i e l d depends on [ l ] t h e b r i g h t n e s s ( e s ) of t h e e x t e r n a l world, [ 2 ] t h e o u t p u t of t h e i n t e r n a l s o u r c e , and [ 3 ] t h e f i x e d and t h e v a r i a b l e t r a n s m i s s i o n s and r e f l e c t a n c e s of t h e o p t i c a l components i n t h e p a t h of t h e c e n t e r f i e l d . The b r i g h t n e s s of t h e e x t e r n a l world can be reduced w i t h t h e c o n t r a s t c o n t r o l wheel on one s i d e of t h e VTE. T h i s wheel c o n t r o l s t h e p o s i t i o n of t h e v a r i a b l e d e n s i t y f i l t e r s

(X,

F i g u r e 2) t h a t a t t e n u a t e p a r t of t h e l i g h t coming from t h e e x t e r n a l world. A s t h e b r i g h t n e s s of t h e image of t h e e x t e r n a l world i s reduced, l i g h t from t h e i n t e r n a l s o u r c e i s added t o t h e t a s k f i e l d through t h e mechanically l i n k e d v a r i a b l e d e n s i t y f i l t e r s

(Y).

This keeps t h e b r i g h t n e s s of t h e t a s k f i e l d roughly c o n s t a n t a s t h e c o n t r a s t i s reduced.

3.3 The o p e r a t o r ' s t a s k s

When making v i s i b i l i t y measurements w i t h t h e VTE, t h e o p e r a t o r must match t h e b r i g h t n e s s of t h e a n n u l a r surround f i e l d w i t h t h e b r i g h t n e s s of one o r more luminous r e g i o n s i n t h e t a s k f i e l d . To do t h i s t h e o p e r a t o r must f i r s t s e t t h e c o n t r a s t c o n t r o l wheel t o t h e p o s i t i o n t h a t g i v e s t h e maximum t r a n s m i s s i o n of l i g h t from t h e e x t e r n a l world.

The surround f i e l d b r i g h t n e s s i s t h e n a d j u s t e d t o match t h e b r i g h t n e s s of t h e view of t h e e x t e r n a l world. This can be done by modulating t h e o u t p u t of t h e i n t e r n a l s o u r c e , by u s i n g a f i x e d r a n g e f i l t e r and by a d j u s t i n g t h e p o s i t i o n of t h e output c o n t r o l d i a l . A s noted above, t h e s e manipulations a l s o determine t h e b r i g h t n e s s of t h e s u b s t i t u t i o n f i e l d . The brightness-matched surround and s u b s t i t u t i o n f i e l d s a r e i n t e n d e d t o keep t h e VTE o p e r a t o r ' s a d a p t a t i o n l e v e l c o n s t a n t w h i l e making t h e v i s i b i l i t y measurements.

A subsequent t a s k i s t o reduce t h e c o n t r a s t of t h e image i n t h e t a s k f i e l d t o some v i s u a l t h r e s h o l d

(C),

u s u a l l y t h e boundary between v i s i b l e and i n v i s i b l e . A s n o t e d i n S e c t i o n 2, t h e t r a n s m i s s i o n of t h e VTE i s reduced w h i l e a v e i l i s added t o t h e t a s k f i e l d . I d e a l l y , t h e VTE should e x a c t l y r e p l a c e t h e a t t e n u a t e d luminance from a uniform f i e l d i n t h e e x t e r n a l world w i t h luminance from a uniform v e i l c r e a t e d by t h e i n t e r n a l source; i n t h i s way t h e t o t a l luminance i n t h e t a s k f i e l d would remain c o n s t a n t . Such a system would modulate t h e c o n t r a s t of t h e image of t h e e x t e r n a l o b j e c t w i t h o u t a f f e c t i n g i t s background b r i g h t n e s s . Theref o r e , when t h e o p e r a t o r s e t s

C ,

t h e ' c o n t r a s t t r a n s m i t t a n c e ' (CT) of an i d e a l VTE could be used t o unambiguously determine t h e VL of t h e t a s k of i n t e r e s t .

3.4 F i e l d luminances

A problem w i t h t h e VTE, d i s c u s s e d i n some d e t a i l i n subsequent s e c t i o n s , i s t h a t t h e luminances of t h e v a r i o u s f i e l d s a r e r a r e l y , i f

e v e r , t h e same. Table I shows t h e r e l a t i v e luminances of t h e surround and c e n t e r f i e l d s and d e s c r i b e s t h e c o n d i t i o n s under which t h e y were

obtained. A l l of t h e measurements r e p o r t e d h e r e and t h o s e i n subsequent s e c t i o n s were o b t a i n e d w i t h a focused P r i t c h a r d photometer c a r e f u l l y p o s i t i o n e d a t t h e

VTE

eyepiece.

4. OPERATION OF THE OUTPUT CONTROL

The output c o n t r o l d i a l d r i v e s a p a i r of v a r i a b l e d e n s i t y f i l t e r s ( Z i n F i g u r e 2). These f i l t e r s l a r g e l y determine t h e b r i g h t n e s s of t h e surround and s u b s t i t u t i o n f i e l d s , a s w e l l a s t h e b r i g h t n e s s of t h e luminous v e i l , f o r a g i v e n s e t t i n g of t h e d i a l . 4.1 Control d i a l The o u t p u t c o n t r o l d i a l ( F i g u r e s 1 and 2) i s n o t s u i t a b l e f o r ,

p r e c i s e measurement and r e p r o d u c t i o n of surround and s u b s t i t u t i o n f i e l d luminances. P r e c i s i o n i s l i m i t e d by a l a r g e s e p a r a t i o n (up t o 2 mm) between t h e analogue s c a l e and t h e p o i n t e r . Backlash i n t h e o u t p u t c o n t r o l d i a l is e s p e c i a l l y troublesome; b r i g h t n e s s s e t t i n g s must always be approached from t h e same d i r e c t i o n t o o b t a i n r e p r o d u c i b l e r e a d i n g s . 4.2 Luminance

The output c o n t r o l d i a l d r i v e s v a r i a b l e d e n s i t y f i l t e r s which v a r y t h e luminance of t h e surround annulus and t h e s u b s t i t u t i o n f i e l d

( F i g u r e 2). The r e l a t i v e luminance of t h e f i e l d s f o r s e v e r a l o u t p u t c o n t r o l s e t t i n g s i s shown i n F i g u r e 4. The manufacturer's d a t a a g r e e w e l l w i t h those o b t a i n e d i n our l a b o r a t o r y .

The a b s o l u t e luminances of t h e VTE surround and s u b s t i t u t i o n f i e l d s were about 7% lower t h a n r e p o r t e d by t h e manufacturer.

4.3 V e i l i n g luminance

A s noted i n S e c t i o n 3, t h e VTE procedures r e q u i r e t h e o p e r a t o r t o vary t h e luminance of t h e surround (and t h e r e f o r e t h e s u b s t i t u t i o n ) f i e l d u n t i l t h e surround b r i g h t n e s s matches t h e t a s k b r i g h t n e s s . However, t h i s m a n i p u l a t i o n i s n o t independent of t h e c o n t r a s t

t r a n s m i t t a n c e . An added v e i l i n g l i g h t i s a p p l i e d t o t h e t a s k f i e l d a s t h e b r i g h t n e s s of t h e surround f i e l d i s increased.

A photometer was p o s i t i o n e d a t t h e

VTE

e y e p i e c e t o measure t h e luminance change of t h e t a s k f i e l d a s t h e o u t p u t c o n t r o l was varied. The c o n t r a s t c o n t r o l d i a l was set a t '000' t o provide maximum

t r a n s m i s s i o n through t h e VTE; t h e mode c o n t r o l f i l t e r was s e t on LOW.

F i g u r e 5 shows how t h e luminance of a dark t a s k f i e l d would change a s a f u n c t i o n of t h e o u t p u t c o n t r o l s e t t i n g . As t h e o u t p u t c o n t r o l s e t t i n g becomes lower (and t h e b r i g h t n e s s of t h e surround f i e l d becomes h i g h e r ) a n added luminous v e i l i s a p p l i e d t o t h e c e n t e r f i e l d . Apparently, s t r a y l i g h t , modulated by t h e o u t p u t c o n t r o l d i a l , i n c r e a s e s t h e

luminance of t h e VTE t a s k f i e l d . Also, when t h e o u t p u t of t h e lamp i s i n c r e a s e d o r when t h e mode c o n t r o l f i l t e r ( F i g u r e 2) i s set on HIGH, t h e

luminance in the task field produced by the stray light will be even

greater than that shown in Figure

5.

This stray light in the task field also reduces the contrast

transmittance of the VTE.

A

contrast transmittance value of 0.942 was

reported in the Manual for the instrument at maximum transmission

through the contrast control wedge. The manufacturer has not published

any information as to how this value was obtained or why it is less than

unity. In a personal commnication, however, the manufacturer has

indicated that, indeed, stray light from the internal source was

measured and this produced a contrast transmittance value less than

unity.

It is implicit in the published information and from the

personal communication, that the contrast reduction due to stray light

should be a constant for the instrument.

Figure

6 shows the relationship between the output control and the

percent contrast reduction due to stray light. Estimates of contrast

reduction were derived from the ratio of the ordinate values in Figure 4

with the ordinate values in Figure

5. Contrast reduction is not

constant, nor is it a monotonic function of the output control. If the

output control setting is low, as with brightness matches between the

surround field and bright target backgrounds, the contrast of the task

will be reduced by as much as

5% because of stray light. This contrast

reduction value is close to that which would be expected from the

contrast transmittance value of 0.942 reported by the manufacturer.

Based upon the personal communication with the manufacturer and this

close numerical correspondence, it seems reasonable to infer that the

contrast transmittance value reported in the manual was obtained with an

output control setting of '0' and using photometric conditions similar

to those reported above. However, Figure

6

shows that contrast

reduction is less, and, therefore, contrast transmittance improves, with

higher output control settings. Such conditions would occur when

brightness matches were made between the surround field and dim target

backgrounds. If one were to assume a constant contrast transmittance of

0.942, contrast thresholds would be systematically underestimated for

these darker target backgrounds. In short, if precise estimates of

contrast threshold are desired, the relationship between contrast

reduction and output control setting should be defined (as in Figure 6).

4.4

Color

The color of the surround and substitution fields will obviously

change with the voltage supplied to the incandescent internal lamp.

Because they are not spectrally flat, the mode control filter (HIGH or

LOW) and the output control filters will also affect the color of the

fields. Figure

7 shows the change in CIE chromaticity coordinates of

the surround annulus for HIGH and LOW modes for different output control

settings at a constant lamp voltage. These are not drastic changes but

are noticeable to a color normal observer.

5. OPERATION OF THE CONTRAST CONTROL

The contrast control wheel drives the variable density wedges (X and

Y in Figure 2), and these wedges largely determine the contrast

transmittance (CT) of the VTE.

5.1

Control wheel

The contrast control wheel (Figures 1 and 2) is rotated to place a

contrast-reducing veil on the view of the external world seen through

the VTE. The wheel linkage to the variable density wedges does not

operate smoothly in some places. When setting thresholds, this can be

an annoyance for smooth bracketing of the contrast threshold. Also, and

probably more important, these rough spots can serve as clues to the

subject, like dust on the wedge (Section 12), for reproducing threshold

settings.

5.2

Transmission

Figure 8 shows the relative transmission of the contrast control

wedge as supplied by the manufacture+ and as measured in our

laboratory. These readings agree very well. Contrast transmittance

(CT)

is

closely related to the relative transmission (RT) in Figure 8.

In a perfect visibility meter that exactly substituted luminance from an

internal source for luminance attenuated in the external world, CT

=

RT.

An absolute transmission of 0.104 was obtained when the contrast

control digital volt meter

(DVM)

reading was '000'.

This agrees well

with a transmission of 0.101 supplied by the manufacturer.

5.3

Veiling luminance

A fundamental requirement for the VTE operation is that the

luminance of the task field is constant for any setting of the contrast

control wheel. Thus, the added veiling luminance should exactly

compensate for the subtracted task luminance as the contrast control

wheel is rotated (and vice versa).

No data have been supplied by the

manufacturer for this particular instrument that would validate that

requirement. In an earlier publication by Blackwell (1970), however,

such data were presented for a Model 3 VTE. In Figure 22 of that paper,

log 'relative luminance' of the task field was plotted as a function of

the contrast control setting. From a casual examination of the highly

compressed ordinate in that figure and from the description in the text,

one would conclude that, indeed, the luminance of the task plus the

luminance from the veil is constant for any setting of the contrast

control wedge. In fact, this conclusion would be incorrect.

Measurements were made of the total luminance produced in the

center field. First, the internal lamp was turned off and the contrast

*

The manufacturer actually presents 'contrast transmittance' values for

different contrast control readings.

c o n t r o l set a t '000' t o provide maximum t r a n s m i s s i o n t o t h e e x t e r n a l

world. The luminance of a uniform w h i t e c a r d was t h e n measured through

t h e c e n t e r f i e l d . Next, a s e t t i n g of '1600' on t h e c o n t r a s t c o n t r o l w a s

e s t a b l i s h e d , and t h e o u t p u t c o n t r o l d i a l a d j u s t e d u n t i l t h e luminance of t h e c e n t e r f i e l d (now determined p r i m a r i l y by t h e v e i l i n g f i e l d ) was

e q u a l t o t h e c e n t e r f i e l d luminance recorded p r e v i o u s l y . Luminances f o r

c o n t r a s t c o n t r o l r e a d i n g s between '1600' and '000' were t h e n measured.

T h i s procedure was r e p e a t e d s e v e r a l times. The ' h i g h extreme' and 'low

extreme' measurements a r e p r e s e n t e d i n F i g u r e 9.

This f i g u r e r e v e a l s two d i s t i n c t problems w i t h t h e VTE. F i r s t , t h e f a c t t h a t t h e r e a r e two c u r v e s r a t h e r t h a n one demonstrates s l i p p a g e i n

t h e counterbalanced, c o n t r a s t c o n t r o l wedges

(X

and

Y

i n F i g u r e 2).

Even though t h e e x a c t p o s i t i o n of one wedge i s known from t h e p o t e n t i o -

meter a t t a c h e d t o i t and t h e a s s o c i a t e d d i g i t a l v o l t meter, t h e o t h e r , mechanically l i n k e d , wedge can s l i p , producing v a r i o u s luminances f o r a p a r t i c u l a r c o n t r a s t c o n t r o l s e t t i n g . Second, t h e two curves have a c h a r a c t e r i s t i c d i p n e a r t h e '300' s e t t i n g . T h i s d i p corresponds t o a luminance r e d u c t i o n of about 20%. Thus, t h e t a s k and v e i l i n g f i e l d s a r e n o t a c c u r a t e l y s u b s t i t u t e d . T h i s 20% r e d u c t i o n i s q u i t e i m p o r t a n t when a s s e s s i n g VL f o r dark t a r g e t s on b r i g h t backgrounds, b u t , p a r e n t h e t i - c a l l y , such a r e d u c t i o n a p p e a r s i n s i g n i f i c a n t when p l o t t e d on a f o u r decade l o g a r i t h m i c o r d i n a t e , such a s was p r e s e n t e d i n Figure 22 of t h e Blackwell (1970) paper.

It should be noted t h a t t h e r e l a t i v e luminance a s s o c i a t e d w i t h t h e

'1600' s e t t i n g on t h e ' h i g h extreme' curve i s n o t e q u a l t o 1.0 a s might

be expected from t h e measurement procedures d e s c r i b e d above i n t h i s s e c t i o n . I n S e c t i o n 4.3, i t was p o i n t e d o u t t h a t s c a t t e r e d l i g h t from t h e i n t e r n a l s o u r c e produced an added v e i l on t h e t a s k f i e l d . The magnitude of t h e v e i l i n g l i g h t depended upon t h e o u t p u t c o n t r o l s e t t i n g

(Figure 9 ) . For t h e o u t p u t c o n t r o l s e t t i n g needed t o match t h e

luminance of t h e w h i t e c a r d s e e n i n t h e t a s k f i e l d , a v e i l of s c a t t e r e d l i g h t i n c r e a s e d t h e c e n t e r f i e l d luminance by t h r e e p e r c e n t when t h e

c o n t r a s t c o n t r o l s e t t i n g provided maximm t r a n s m i s s i o n ( a t '000' ).

6. OPTICAL FIDELITY

O p t i c a l systems a r e n o t p e r f e c t . Refracted r a y s do n o t always come

t o f o c u s i n t h e same p l a n e because of s c a t t e r , a b e r r a t i o n s and d i f f r a c t i o n . N a t u r a l l y VTE o p t i c s s u f f e r from t h e s e l i m i t a t i o n s a s well. F u n c t i o n a l l y t h e s e o p t i c a l i n f i d e l i t i e s reduce t h e t r a n s m i t t e d c o n t r a s t of v i s u a l t a r g e t s . Smaller t a r g e t s o r t h o s e w i t h f i n e d e t a i l ( i e . s t i m u l i having h i g h e r s p a t i a l f r e q u e n c i e s ) a r e more a f f e c t e d by t h e s e o p t i c a l i n f i d e l i t e s t h a n a r e l a r g e r stimuli ( i . e . , t h o s e w i t h lower s p a t i a l f r e q u e n c i e s ) . Thus, t h e c o n t r a s t t r a n s m i t t a n c e of t h e VTE w i l l depend upon t h e s i z e , o r t h e s p a t i a l frequency c o n t e n t , of t h e t a s k being viewed through t h e instrument.

The c o n t r a s t of a luminous border can be measured through and w i t h o u t t h e VTE, and t h e r a t i o of t h e s e two c o n t r a s t s c a n b e considered

a s t h e c o n t r a s t t r a n s m i t t a n c e of t h e VTE f o r t h a t stimulus. C o n t r a s t t r a n s m i t t a n c e v a l u e s of t h i s t y p e were o b t a i n e d f o r two s t i m l i of

0.5O, on a white background. The o t h e r comprised two l a r g e p a t c h e s of paper, one b l a c k and one white; each p a t c h completely f i l l e d t h e VTE f i e l d of view (6O). The VTE c o n t r a s t c o n t r o l d i a l was set f o r maximum t r a n s m i s s i o n and t h e i n t e r n a l l i g h t s o u r c e was t u r n e d o f f . The c o n t r a s t s of both s t i m u l i measured under t h e s e c o n d i t i o n s w i t h and w i t h o u t t h e VTE a r e shown i n Table 11. One can s e e from t h e s e d a t a t h a t t h e c o n t r a s t of t h e l e t t e r ( w i t h t h e h i g h e r s p a t i a l f r e q u e n c i e s ) a g a i n s t i t s background i s reduced by about 4%. The c o n t r a s t of t h e o t h e r s t i m u l u s , w i t h l o w s p a t i a l f r e q u e n c i e s , was n o t reduced a t a l l , i n d i c a t i n g t h a t t h e VTE s e l e c t i v e l y reduces t h e c o n t r a s t of t a s k s having h i g h e r s p a t i a l f r e q u e n c i e s . Again, t h i s r e s u l t i s t y p i c a l of o p t i c a l systems (Langf ord, 1977).

These r e s u l t s , and those o b t a i n e d by o t h e r s from a v a r i e t y of

o p t i c a l systems, i n d i c a t e t h a t t h e a b s o l u t e t h r e s h o l d s f o r s m a l l t a r g e t s o b t a i n e d through t h e VTE, o r any o t h e r o p t i c a l v i s i b i l i t y meter, w i l l be h i g h e r t h a n t h o s e t h a t would be o b t a i n e d w i t h o u t i n t e r v e n i n g o p t i c s

( i . e . , f o r f r e e viewing). More i m p o r t a n t l y , however, t h e s e t h r e s h o l d s w i l l be d i f f e r e n t i a l l y h i g h e r depending upon t h e s p a t i a l frequency

c o n t e n t of t h e s t i m u l u s being evaluated. Thus, a simple c o r r e c t i o n f a c t o r l i k e t h a t i m p l i e d by t h e c o n t r a s t t r a n s m i t t a n c e v a l u e i s n o t v a l i d . This d i f f e r e n t i a l a t t e n u a t i o n of s p a t i a l f r e q u e n c i e s i m p l i e s t h a t t h e C I E method (1981) does n o t adequately c h a r a c t e r i z e VL because t h r e s h o l d c o n t r a s t

(C)

i s n o t p r o p e r l y d e f i n e d f o r s p a t i a l l y d i f f e r e n t s t i m u l i . More s p e c i f i c a l l y , two t a s k s e s t i m a t e d by t h e VTE t o have t h e same v a l u e of

C,

b u t w i t h d i f f e r e n t s p a t i a l f r e q u e n c i e s , w i l l n o t

n e c e s s a r i l y be e q u a l i n

VL

i n f r e e viewing c o n d i t i o n s , due t o t h i s d i f f e r e n t i a l a t t e n u a t i o n of s p a t i a l f r e q u e n c i e s .

7. DETERMINATION OF ADAPTATION LUMINANCE

According t o CIE 1912.1, one must be c a r e f u l t o c o r r e c t l y r e l a t e c o n t r a s t t h r e s h o l d

( e )

v a l u e s t o a d a p t a t i o n luminance. I n t h i s r e g a r d t h e r e i s an e r r o r i n t h e VTE I n s t r u c t i o n Manual (June, 1978). I n S e c t i o n I V of t h e Manual, e n t i t l e d "Determination of t a s k background luminance", t h e manufacturer d e s c r i b e s a procedure f o r e s t i m a t i n g t h e t a s k ( a d a p t a t i o n ) luminance. The method f o r o b t a i n i n g t h e t a s k luminance i s s i m i l a r t o t h a t used w i t h a Macbeth illuminometer (Wyszecki and S t i l e s , 1982). A b r i g h t n e s s match i s made between a surround f i e l d of v a r i a b l e , but known, luminance and t h e t a s k f i e l d * . From t h i s match t h e luminance of t h e t a s k f i e l d i s determined. But t h e formula f o r t h i s method u s i n g t h e VTE i s i n c o r r e c t i n t h e Manual.

The manufacturer w r i t e s :

*

D i f f i c u l t i e s w i t h t h e b r i g h t n e s s matching procedure a r e d e s c r i b e d i n S e c t i o n 8 below.

where:

TL

is t h e t a s k luminance.

LK i s t h e maximum luminance of t h e surround f i e l d (i.e., maximum t r a n s m i s s i o n of t h e o u t p u t c o n t r o l w e d g e s ) f o r a g i v e n mode c o n t r o l f i l t e r . This i s known by p r i o r c a l i b r a t i o n .

T i s t h e t r a n s m i s s i o n of t h e o u t p u t c o n t r o l wedge.

Tc i s t h e t r a n s m i s s i o n of t h e v a r i o u s cubes i n t h e o p t i c a l p a t h of t h e VTE.

To be c l e a r ,

TL

must f i r s t be b e t t e r defined. I f t a s k luminance

i s t o be t a k e n a s t h e luminance of t h e a c t u a l t a s k w i t h no i n t e r v e n i n g o p t i c s , t h e n t h e formula should be w r i t t e n as:

where:

TVTE i s t h e a b s o l u t e t r a n s m i s s i o n of t h e VTE (0.104, S e c t i o n 5.2).

A l l o t h e r terms a r e d e f i n e d a s i n Equation (3).

I f t a s k luminance i s t o be t a k e n a s t h e luminance of t h e surround f i e l d , and hence t h e brightness-matched t a s k f i e l d , t h e n t h e f o r u u l a must be w r i t t e n as:

where a l l terms a r e d e f i n e d a s i n Equation ( 3 ) .

From t h e t e x t i n S e c t i o n I V of t h e VTE I n s t r u c t i o n Manual, i t

a p p e a r s t h a t t h e manufacturer i n t e n d e d t o d e f i n e t a s k luminance (TL) a s i n Equation (4). Therefore, one must be c a r e f u l t o r e l a t e t h e c o r r e c t a d a p t a t i o n luminance t o c o n t r a s t t h r e s h o l d

(c).

The a d a p t a t i o n

luminance f o r

C

i s n o t t h e luminance of t h e a c t u a l t a s k ( d e f i n e d

-

i n

Eq. 4) nor i s i t a s d e f i n e d i n Equation (3). Rather, i t i s t h e

luminance d e f i n e d i n Equation (5), t h e t a s k f i e l d luminance p e r c e i v e d a t t h e eyepiece of t h e VTE. To r e l a t e

?!

t o t h e a c t u a l t a s k luminance

d e f i n e d by Equation ( 4 ) , one h a s two a l t e r n a t i v e s . One can e i t h e r i n c r e a s e t h e a c t u a l t a s k luminance by a f a c t o r of 9.6 (1/0.104, S e c t i o n 5.2) and o b t a i n a new

c,

o r one can assume a r e l a t i o n s h i p between

C

and a d a p t a t i o n l e v e l ( l i k e t h e V i s i b i l i t y r e f e r e n c e f u n c t i o n i n

CIE

19/2.1) and t h e n mathematically d e r i v e a t t h e a c t u a l t a s k luminance.

8. BRIGHTNESS MATCHES

Brightness matching of t h e surround and c e n t e r f i e l d s i s q u i t e e a s y when t h e s e juxtaposed f i e l d s a r e uniform and when t h e y have t h e same

s p e c t r a l composition. I n most s i t u a t i o n s where t h e

VTE

would be used, however, n e i t h e r of t h e s e c o n d i t i o n s e x i s t .

8.1 Luminance and b r i g h t n e s s

One of t h e o p e r a t o r ' s f i r s t t a s k s when o p e r a t i n g t h e VTE

( S e c t i o n 3.3) i s t o make a b r i g h t n e s s match between t h e t a s k and

surround f i e l d s . B r i g h t n e s s matches a r e n o t n e c e s s a r i l y luminance

matches, however. The l a r g e r t h e c o l o r d i f f e r e n c e between two e q u a l l y

b r i g h t f i e l d s , t h e l a r g e r t h e luminance d i f f e r e n c e (Alman, 1977).

The c o l o r of t h e surround f i e l d , produced by t h e i n t e r n a l s o u r c e ,

i s

n o t u s u a l l y metameric w i t h t h e c o l o r ( s ) of t h e view of t h e e x t e r n a l world a s

s e e n i n t h e t a s k f i e l d . Consequently, t h e t a s k and surround f i e l d s may

n o t have e q u a l luminances, even though t h e i r b r i g h t n e s s e s a r e matched, because of t h e c o l o r d i f f e r e n c e s i n t h e two f i e l d s .

The C I E 1912.1 formalisms d e f i n e a d a p t a t i o n l e v e l i n terms of

luminance. These formalisms r e q u i r e v i s i b i l i t y measurements of

r e a l i s t i c s t i m u l i a t a s p e c i f i e d a d a p t a t i o n luminance, but t h e VTE methodology f o r a s s e s s i n g a d a p t a t i o n l e v e l i s based upon b r i g h t n e s s .

Therefore, t h e r e i s an i n c o n s i s t e n c y between t h e CIE formalisms and t h e

VTE methodology which was developed t o implement t h e CIE system. To circumvent t h e problem of heterochromatic b r i g h t n e s s matches w i t h t h e VTE r e s u l t i n g i n d i f f e r e n t luminances f o r t h e t a s k and surround

f i e l d s , t h e manufacturer makes t h e following recommendation (p. 8 , VTE Manual) :

"When t h e r e is a l a r g e and troublesome c o l o r d i f f e r e n c e between

t h e luminance of t h e t a s k background and t h e luminances produced w i t h i n t h e VTE, c o n s i d e r a t i o n should be given t o t h e u s e of chromatic f i l t e r s t o reduce o r e l i m i n a t e t h e observed c o l o r

d i f f e r e n c e s . Chromatic f i l t e r s may be used t o modify t h e 2848OK

c o l o r temperature of a l l beams produced w i t h i n t h e VTE by b e i n g i n s e r t e d i n t o t h e f i l t e r n a c e l l e

.

.

.

."

These "chromatic" f i l t e r s a r e b e t t e r d e s c r i b e d a s "conversion" o r

" l i g h t balancing" f i l t e r s (Eastman Kodak, 1973). Kodak p r o v i d e s

a

d e s c r i p t i o n of t h e s e f i l t e r s , and t h i s should be used t o supplement t h e VTE o p e r a t i o n because no d e t a i l s a r e g i v e n i n t h e VTE Manual o r i n r e l a t e d published a r t i c l e s a s t o how colored f i l t e r s could be used i n

t h e VTE t o make t a s k and surround f i e l d s t h e same c o l o r . Two p o i n t s

should be made about t h e s e f i l t e r s , however. F i r s t , i t i s u n l i k e l y t h a t

a n e x a c t c o l o r match w i l l be o b t a i n e d between t a s k and surround f i e l d s because only a l i m i t e d number of f i l t e r s a r e a v a i l a b l e t o a d j u s t t h e

c o l o r temperature. Thus, only a crude c o l o r match can b e expected i n

many circumstances. Secondly, d i s c o n t i n u o u s s o u r c e s such a s f l u o r e s c e n t

lamps may have c o r r e l a t e d c o l o r t e m p e r a t u r e s t h a t w i l l n o t b e e x a c t l y c o r r e c t e d by t h e recommended conversion o r l i g h t balancing f i l t e r s .

There a r e i n c o n s i s t e n c i e s between t h e manufacturer's c o l o r matching recommendation and h i s luminance c a l i b r a t i o n recommendation ( S e c t i o n 7). A s t h e c o l o r c o r r e c t i n g o r l i g h t balancing f i l t e r s a r e i n s e r t e d i n t o t h e

the output control wedge must be manipulated to increase the luminance

of the surround. This luminance increase changes the color of the

surround field slightly (Figure 7), and new color filters may be

required to maintain a color match. This may be a minor problem because

the color shifts are small when the output control wedge is varied.

There is a more serious difficulty with this recommendation.

Inserting the color filters upsets the calibration of the surround

luminance provided by the manufacturer (Section 7).

According to the

VTE Manual, the surround luminance should be used to determine the task

luminance, and thus define adaptation level, but the actual adaptation

luminance will be unknown without additional photometric measurements.

It is unlikely, despite the instructions in the VTE Manual, that

one can accurately predict task or surround field luminances from the

manufacturer's calibration and simple transmission measurements of the

color filters because of the multiple reflections between color filters

in the nacelle. One should make surround luminance measurements at the

eyepiece for every stimulus condition. These measurements are both

time-consuming and troublesome.

These measurements can be avoided if one knows the absolute

transmission of the VTE and the actual luminance of the task. Simply

multiplying the luminance of the external field by the transmission of

the VTE determines the task field luminance at the eyepiece.

where:

TL is task luminance at the eyepiece (similar to that described

in Equation

5).

FL

is the luminance of the external field.

All other terms are as defined in Equation

(4).

This approach avoids the problem of continually having to do

photometric measurements at the eyepiece. With this approach, however,

an independent luminance determination of the external field stimulus

must always be made, and, therefore, an additional instrument

(i.e., a luminance meter) must be employed.

It should also be noted that Equation (6) is, strictly speaking,

a different expression for task luminance than Equation

(5) (Section

7).

But when both the brightness and the color of the task and surround

fields are the same, their luminances are the same.

The previous discussion in this section has dealt with the

relationship between luminance and brightness for the task and the

surround fields.

A more important problem is associated with the

relationship between the luminance and brightness of the tasks and the

substitution fields. Assuming that the surround and substitution fields

have t h e same luminous p r o p e r t i e s * , t h e n t h e t a s k and s u b s t i t u t i o n f i e l d s w i l l h a v e d i f f e r e n t luminances when t h e t a s k and s u r r o u n d f i e l d s a r e brightness-matched b u t a r e of d i f f e r e n t c o l o r s .

The t r a n s i e n t c h a n n e l i n t h e human v i s u a l s y s t e m h a s a s p e c t r a l s e n s i t i v i t y s i m i l a r o r i d e n t i c a l t o V A (luminance). T h i s c h a n n e l

responds b e s t t o sudden changes i n luminance; a b u r s t of a c t i v i t y o c c u r s ,

a t t h e o n s e t and o f f s e t of t h e change (Legge, 1978; I n g l i n g , 1978). When t h e s u b s t i t u t i o n and t a s k f i e l d s a r e of d i f f e r e n t luminances, and a r e a l t e r n a t e d t e m p o r a r i l y i n t h e

Vm,

t h e n t r a n s i e n t s i g n a l s w i l l be produced i n t h e v i s u a l system. A s n o t e d i n S e c t i o n 10.3, t h e s e

t r a n s i e n t s i g n a l s e l e v a t e c o n t r a s t t h r e s h o l d s (Boynton and M i l l e r , 1963) and t h u s i n t r o d u c e a b r i g h t n e s s matching a r t i f a c t i n t o t h e VTE t h r e s h o l d d a t a . T h e r e f o r e , i t i s i m p o r t a n t t o e n s u r e t h a t t h e c o l o r , a s w e l l a s t h e b r i g h t n e s s of t h e t a s k and t h e s u b s t i t u t i o n f i e l d s a r e t h e same.

A s d i s c u s s e d above, t h i s e n s u r e s t h a t t h e luminances a r e t h e same. 8.2 Task f i e l d c o m p l e x i t y

It i s d i f f i c u l t f o r t h e s u b j e c t t o make b r i g h t n e s s matches between a complex t a s k ( e . g . , a p e r s o n ' s f a c e ) and t h e surround. The v a r i o u s luminances and c o l o r s .make i t h a r d f o r t h e s u b j e c t t o know what p a r t o r p a r t s of t h e t a s k h e s h o u l d u s e when making t h e b r i g h t n e s s matches.

It i s a l s o d i f f i c u l t f o r t h e e x p e r i m e n t e r t o know what luminances i n t h e complex f i e l d a r e r e l e v a n t t o measure and w e i g h t t o d e s c r i b e t a s k f i e l d luminance ( S e c t i o n 8.1).

I n e a r l i e r v e r s i o n s of t h e VTE, Rlackwell recommended t h a t b r i g h t n e s s matches b e made between t h e s u r r o u n d and a d e f o c u s e d t a s k f i e l d ( R l a c k w e l l , 1970, p. 268). The assumption i m p l i c i t w i t h t h i s method was t h a t a d a p t a t i o n luminance was based upon t h e a v e r a g e

Luminance of t h e t a s k f i e l d . L a t e r i n t h a t paper (p. 2 8 0 ) , t h e a u t h o r recommended magnifying t h e h i g h e s t c o n t r a s t b o r d e r and s e t t i n g t h e ~ u r r o u n d b r i g h t n e s s e q u a l t o t h e h i g h e r luminance. T h i s assumes t h a t a d a p t a t i o n l e v e l i s based upon t h e h i g h e s t luminance. N e i t h e r p r o c e d u r e can be recommended w i t h c o n f i d e n c e on t h e o r e t i c a l grounds.

M u l t i c o l o r e d t a s k s p r e s e n t a problem. F i r s t , i t i s d i f f i c u l t t o know what c o l o r t h e s u r r o u n d f i e l d s h o u l d be f o r a b r i g h t n e s s match. Second, i t i s p o s s i b l e t o have a v i s i b l e boundary w i t h e q u a l luminance on b o t h s i d e s b u t d i f f e r e n t c o l o r s . Thus, VL c a n b e g r e a t e r t h a n u n i t y when t h e luminance c o n t r a s t of t h e f i e l d , a s d e f i n e d i n t h e CIE

formalism, i s below t h r e s h o l d . T h i s f a i l u r e of t h e CIE formalism t o d e f i n e c o l o r c o n t r a s t a t t h r e s h o l d a c t u a l l y makes i t i m p o s s i b l e t o a c c u r a t e l y d e s c r i b e VL f o r any m u l t i c o l o r e d s t i m u l u s .

9. MAGNIFICATION

The s i z e of t h e t a s k f i e l d image produced by t h e VTE o p t i c s i s v a r i a b l e , b u t r a r e l y e q u a l t o t h a t of t h e p h y s i c a l t a s k u n d e r

free-viewing c o n d i t i o n s . T h e r e f o r e , t h e VTE must be f u r t h e r away o r c l o s e r t o t h e t a s k t h a n a free-viewing o b s e r v e r would be f o r a matched v i s u a l angle. This d i f f e r e n c e between t h e l o c a t i o n of t h e VTE a t t h e p r o p e r m a g n i f i c a t i o n and t h e l o c a t i o n of a free-viewing o b s e r v e r may confound t h e v i s i b i l i t y measurements due t o t h e d i f f e r e n t 'body shadow' p o s i t i o n s ( i . e . , l i g h t i n g geometries). F u r t h e r , because t h e a p p a r e n t s i z e of t h e s t i m u l u s i s important t o v i s i b i l i t y ( l a r g e r o b j e c t s a r e g e n e r a l l y more v i s i b l e t h a n smaller o n e s ) , t h e m a g n i f i c a t i o n of t h e VTE must be c o r r e c t l y determined and t h e p o s i t i o n of t h e instrument a d j u s t e d t o match t h e v i s u a l a n g l e of t h e o b j e c t s e e n through t h e VTE and s e e n by free-viewing. (There i s no d i s c u s s i o n of t h e s e p o i n t s i n t h e r e l e v a n t p u b l i s h e d documents.) Therefore, w i t h o u t a n awareness of t h e s e f a c t o r s and t h e proper a t t e n t i o n t o p o s i t i o n t h e VTE, one may f i n d t h a t t h e e s t i m a t e d v a l u e s of VL a r e n o t c o r r e c t l y r e l a t e d t o t h e free-viewing s i t u a t i o n .

10. PULSING PROCEDURE

The VTE i n c l u d e s a p u l s i n g mechanism, purported t o s i m u l a t e f i x a t i o n pauses (Blackwell, 1970).

10.1 Appearance

The VTE may be o p e r a t e d i n a s t a t i c mode (no p u l s i n g ) , a ' f i x e d p u l s e r a t e ' mode o f , nominally, 200 m s pauses, o r i n a ' v a r i a b l e p u l s e r a t e ' mode. N e i t h e r p u l s i n g mode s a t i s f a c t o r i l y mimics b r i e f views f o l l o w i n g s a c c a d i c e y e movements. The m i r r o r s h u t t e r system occludes t h e t a s k from t o p t o bottom and t h e n r e v e a l s i t from bottom t o top. Hence, t h e lower p o r t i o n s of t h e t a s k a r e p r e s e n t e d f o r a p e r c e p t i b l y

longer time t h a n t h e upper p o r t i o n s . Therefore, c r i t i c a l a r e a s of t h e t a s k i n t h e lower p o r t i o n of t h e t a s k f i e l d w i l l have lower t h r e s h o l d s than c r i t i c a l a r e a s a t t h e top of t h e t a s k f i e l d . F i x a t i o n a l pauses a r e not c h a r a c t e r i z e d by t h i s slow up-and-down o c c l u s i o n procedure, b u t probably by an o n s e t and an o f f s e t of t h e whole v i s u a l f i e l d s i m i l a r t o t h a t f o l l o w i n g b l i n k s (Riggs e t a l , 1981).

10.2 Temporal c h a r a c t e r i s t i c s

These important i s s u e s of appearance a s i d e , i t i s d e b a t a b l e whether one should have 200 ms p r e s e n t a t i o n s . T h i s i s a somewhat

a r b i t r a r y p r e s e n t a t i o n time and t a r g e t s p r e s e n t e d f o r s l i g h t l y d i f f e r e n t p e r i o d s w i l l be c h a r a c t e r i z e d by d i f f e r e n t c o n t r a s t t h r e s h o l d s

( K i t t e r l e and Corwin, 1979).

F i g u r e 10 shows t h e luminance modulation of a uniform luminous t a s k f i e l d produced a t t h e e y e p i e c e by t h e VTE i n t h e f i x e d p u l s e r a t e mode. The 'on' and ' o f f ' h a l f c y c l e s a r e n o t e x a c t l y t h e same d u r a t i o n , and n e i t h e r a r e e q u a l t o 200 ms. The c o n t r a s t t h r e s h o l d v a l u e s o b t a i n e d w i t h t h i s ramping procedure w i l l be d i f f e r e n t from t h o s e t h a t would be

o b t a i n e d w i t h a f l a s h f o r i n s t a n c e , because of t e m p o r a l - s p a t i a l i n t e r a c t i o n s i n t h e v i s u a l system (Kelly, 1977).

when t h e VTE i s i n t h e v a r i a b l e p u l s e r a t e mode, t y p i c a l 'on' h a l f - c y c l e s a r e a s shown i n F i g u r e 11. The v a r i a b l e ' o f f ' h a l f - c y c l e s

may range from 0.7 s t o 3.0 s, depending upon t h e d e l a y knob s e t t i n g (minimum and maximum d e l a y , r e s p e c t i v e l y ) . For a g i v e n s e t t i n g of t h e d e l a y knob, however, t h e ' o f f ' h a l f - c y c l e s were n o t always t h e same d u r a t i o n . The l a r g e s t v a r i a t i o n i n t h e ' o f f ' h a l f - c y c l e f o r a p a r t i c u l a r d e l a y knob s e t t i n g was 33%. The consequences of t h i s v a r i a t i o n f o r t h r e s h o l d s were n o t e v a l u a t e d i n t h i s s t u d y .

10.3 Luminance t r a n s i e n t s

The luminance of t h e s u b s t i t u t i o n f i e l d i s h i g h e r t h a n t h a t of t h e surround f i e l d by a f a c t o r of 1.3 (Table I ) . The surround f i e l d i s supposed t o be brightness-matched t o t h e t a s k f i e l d . I g n o r i n g t h e problem d i s c u s s e d i n S e c t i o n 8 on b r i g h t n e s s matches, a luminance t r a n s i e n t w i l l be produced when t h e t a s k and s u b e t i t u t i o n f i e l d s a r e a l t e r n a t e d . T h i s can produce a n a r t i f a c t u a l l y h i g h e r c o n t r a s t t h r e s h o l d t h a n would be expected i f t h e luminances of t h e t a s k and s u b s t i t u t i o n f i e l d s were e q u a l (Boynton and M i l l e r , 1963).

11. POLARIZATION

The VTE o p t i c a l system can, through r e f r a c t i o n and r e f l e c t i o n , change t h e p o l a r i z a t i o n of t h e l i g h t r e a c h i n g t h e o p e r a t o r ' s eye. When focused on an u n p o l a r i z e d f i e l d , t h e VTE produced a h o r i z o n t a l p o l a r i z a t i o n of 1.8% a t t h e e y e p i e c e (Rea, 1981). Plane p o l a r i z e d l i g h t from an e x t e r n a l s o u r c e was reduced by a s i m i l a r amount a f t e r t r a n s m i s s i o n through t h e VTE. T h i s change i s unimportant under most circumstances, but i t should be t a k e n i n t o account i f t h e p o l a r i z a t i o n p r o p e r t i e s of t a r g e t s a r e of i n t e r e s t .

12. USE AS A FIELD INSTRUMENT

The VTE i s f a i r l y heavy, and t h e handles a r e n o t w e l l s i t u a t e d f o r l i f t i n g . It i s d i f f i c u l t t o t r a n s p o r t and cumbersome t o s e t on a

t r i p o d . The VTE i s a l s o a f r a g i l e instrument. The o p t i c s and i n t e r n a l p a r t s r e q u i r e f r e q u e n t realignment and adjustment. F u r t h e r , t h e

instrument i s s u s e p t i b l e t o d u s t and moisture; consequently i t r e q u i r e s r e g u l a r c l e a n i n g and a t t e n t i o n . However, t h e i n s t r u m e n t i s n o t designed f o r convenient s e r v i c i n g ; t h e c h a s s i s cover cannot be removed as a

s i n g l e u n i t . I n s t e a d , t h e s i d e p a n e l s must be i n d i v i d u a l l y dismantled by t h e removal of many screws. F u r t h e r , w i r e s l e a d i n g t o v a r i o u s e l e c t r i c a l components are a t t a c h e d t o t h e s i d e p a n e l s , s o t h e y must b e unsoldered when s e r v i c i n g t h e instrument.

P a r e n t h e t i c a l l y , t h e unavoidable d u s t p a r t i c l e s t h a t g e t on t h e c o n t r a s t c o n t r o l wedges

(X

i n F i g u r e 2) can be e s p e c i a l l y troublesome i n t h i s instrument because s u b j e c t s can o c c a s i o n a l l y u s e them t o reproduce t h r e s h o l d s e t t i n g s . S u b j e c t s merely match t h e l o c a t i o n of d u s t

p a r t i c l e s a c r o s s t r i a l s r a t h e r t h a n s u c c e s s i v e l y e s t i m a t e t h e v i s i b l i t y of t h e t a s k . Even t h e most s i n c e r e s u b j e c t s have d i f f i c u l t y i g n o r i n g them once they have been noticed.

13. SUMMARY AND CONCLUSIONS

The d i f f i c u l t i e s o u t l i n e d above b r i n g i n t o q u e s t i o n t h e a b i l i t y of t h e Blackwell V i s u a l Task E v a l u a t o r , Model 3 X t o p r o v i d e a c c u r a t e d a t a f o r t h e computation of V i s i b i l i t y Level (VL). Some of t h e d i f f i c u l t i e s w i t h t h e c u r r e n t model of t h e VTE can b e e a s i l y overcome o r reduced

(e.g., i n s e r t i o n of a n e u t r a l d e n s i t y f i l t e r i n t o t h e o p t i c a l p a t h of t h e s u b s t i t u t i o n f i e l d ) . Other d i f f i c u l t i e s could n o t be e a s i l y overcome but would r e q u i r e a more s e r i o u s r e d e s i g n of t h e VTE

(e.g., making i t more impervious t o d u s t , redeveloping t h e p u l s i n g a c t i o n , e l i m i n a t i n g s l i p p a g e a t t h e c o n t r a s t c o n t r o l wedges, reducing t h e weight).

There a r e o t h e r , more fundamental, d i f f i c u l t i e s w i t h t h e v i s i b i l i t y meter concept t h a t cannot be e a s i l y o r cheaply overcome. Foremost of t h e s e i s t h a t o p t i c a l systems a r e imperfect; some s p a t i a l f r e q e n c i e s a r e p r e f e r e n t i a l l y reduced i n modulation amplitude

(e.g., c o n t r a s t t r a n s m i t t a n c e ) . Thus, v i s i b i l i t y e s t i m a t e s of c e r t a i n s t i w l i w i l l be b i a s e d r e l a t i v e t o f r e e viewing. Without implementation of very expensive l e n s systems, c a r e f u l alignment and r i g i d mountings, i t i s u n l i k e l y t h a t a completely s a t i s f a c t o r y p o r t a b l e v i s i b i l i t y meter could be c o n s t r u c t e d .

The b r i g h t n e s s matching method ( s e t t i n g surround, s u b s t i t u t i o n and v e i l i n g luminances) a l s o p r e s e n t s s e r i o u s fundamental problems. Complex and colored f i e l d s c r e a t e t h e o r e t i c a l u n c e r t a i n t i e s a s t o t h e a d a p t a t i o n l e v e l of t h e o b s e r v e r s . F u r t h e r , t h e e q u a t i o n s d e s c r i b i n g VL a r e based upon luminance whereas t h e VTE methodology i s based upon b r i g h t n e s s . T h i s could be a s e r i o u s i n c o n s i s t e n c y when t h e VL of c o l o r e d t a r g e t s

(e.g., automobile t a i l l i g h t s o r e g r e s s s i g n a g e ) i s important. I n s h o r t , t h e s e fundamental l i m i t a t i o n s of a l l v i s i b i l i t y meters, i n c l u d i n g t h e VTE, may i n t r o d u c e a r t i f a c t s i n t o t h e t h r e s h o l d estimates t h a t would n o t be p r e s e n t i n r e a l viewing s i t u a t i o n s .

I n sum, both p r a c t i c a l and t h e o r e t i c a l problems w i t h t h e

VTE

w i l l

l e a d t o i n a c c u r a c i e s i n e s t i m a t i n g V i s i b i l i t y Level. The magnitude of t h e s e e r r o r s w i l l depend upon t h e s p a t i a l , temporal and chromatic

c h a r a c t e r i s t i c s of t h e s t i m u l u s a s w e l l a s such p r o p e r t i e s of t h e VTE a s s c a t t e r e d l i g h t , o p t i c a l i n f i d e l i t y , and o p t i c a l - m e c h a n i c a l

n o n l i n e a r i t i e s . W e a r e n o t aware of any r e c e n t review of t h e p r a c t i c a l and t h e o r e t i c a l a s p e c t s of a s s e s s i n g V i s i b i l i t y Level. It seems t h a t some more s e r i o u s ' w o r k i n t h i s a r e a must t a k e p l a c e b e f o r e t h e VTE, o r o t h e r v i s i b i l i t y meters, can be c o n f i d e n t l y used w i t h t h e model

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