^{1}The Laboratory of Physiological Hygiene, University of Minnesota, Minneapolis, Minnesota

^{**}With the assistance of Dr. W. J. Bushard and Dr. H. G. Peterson of the Division of Ophthalmology and Dr. J. C. Franklin and Mr. A. Butler of the Laboratory of Physiological Hygiene.

The purpose of this study was to investigate the relation of the level of illumination intensity to performance and fatigue in visual work. The question of illumination requirements is of considerable importance from the point of view of physiological optics, industrial physiology, and public health. Yet a large part of the research done in the past leaves much to be desired in regard to general approach and specific testing techniques as well as experimental analysis and statistical evaluation. In the present series six illumination levels (2, 5, 15, 50, 100, and 300 footcandles) were studied in repeated experiments in six normal subjects. The work task involved recognition of fine details (letters) and reproduced the essential features of a conveyor inspection operation. In addition to the studying of various criteria of work performance and their change in the course of 2 hours of work, a large battery of visual tests was applied before and after the work so as to characterize the degree of fatigue. Varied illumination affected the performance more than the functional criteria of fatigue. Only one function, the recognition time for stimuli of threshold size, showed a decreasing degree of fatigue up to 300 ft.-c, all other variables, including performance, which changed with a changing level of illumination showed an optimum at 100 ft.-c. The demonstration of an optimum level of illumination makes highly questionable the current practice of recommending “minimum” levels of illumination for industrial jobs, since deterioration of performance and increase in fatigue may result when the optimal level is exceeded. The optimum of 100 ft.-c for the present strenuous visual task is at or below the minimum which should be recommended on the basis of the code of the Illuminating Engineering Society.

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Units of functions and the “desirable” values of the scores.

Retinal functions

Units

“Desirable” values

Recognition time

Seconds

Lower

Flicker fusion frequency

Flashes per second

Higher

Brightness discrimination, green

Arbitrary units

Lower

Brightness discrimination, red

Arbitrary units

Lower

Ophthalmological Tests

Abduction

Diopters

Higher

Adduction

Diopters

Higher

Vertical divergence

Diopters

Higher

Accommodation near point

Centimeters

Lower

Convergence near point

Centimeters

Lower

Performance Criteria

Performance average

No. of correct letters

Higher

Performance drop

No. of correct letters

Lower

Performance range

No. of correct letters

Lower

Blinking Rate

Blinking rate average

Blinks per minute

Lower

Blinking rate difference start-finish

Blinks per minute

Lower

Questionnaire Score

Arbitrary units

Lower

Ophthalmographic Measurements

Movement phase

1/100 sec.

Lower

Angular velocity

Arbitrary units

Higher

No. of movement

Rate per second

Higher

Fixation phase

1/100 sec.

Lower

Extent of movement

Arbitrary units

Lower

Discrepancy

Arbitrary units

Lower

Table II

Mean scores for performance, blinking rate, and questionnaire score at three levels of illumination, and significance of the fatigue changes taking place during 2 hours of visual work.

$\overline{d}$ equals the mean change in 36 pairs of measurements made at the start and at the end of the visual work; negative
$\overline{d}$ values express deterioration. F expresses the statistical significance of the mean changes (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table III

Visual functions which exhibited significant trends with variation of the illumination level.

$\overline{\mathrm{\Delta}}$ equals the mean difference between 36 pairs of changes (d) at the two compared levels of illumination.
Negative
$\overline{\mathrm{\Delta}}$’s indicate greater deterioration under the first of the conditions compared. F expresses the statistical significance of the mean difference (
$\overline{\mathrm{\Delta}}$) between the two compared levels of illumination. F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table IV

Fatigue changes in retinal functions resulting from 2 hours of visual work at three levels of illumination.

$\overline{d}$ equals the mean change in 36 pairs of measurements made before and after visual work. Negative
$\overline{d}$ values express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table V

Change in several ophthalmological routine tests at three levels of illumination.

$\overline{d}$ equals the mean change between 36 pairs of measurements made before and after visual work. Negative
$\overline{d}$’s express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table VI

Fatigue changes in ophthalmographic functions at three levels of illumination.

$\overline{d}$ equals the mean change between 12 pairs of measurements made before and after visual work. Negative
$\overline{d}$’s express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.84. F at 1 percent of significance is 9.65.
For 36 pairs of measurements F at 1 percent of significance is 7.44.

Table VII

Average preference scores for 5 levels of illumination intensity. The smaller the score, the more preferred the condition.

Indicates significance of the mean differences at the 5 percent level.
Indicates significance at 1 percent level. In testing the significance of the differences the combined sample of subject was used (N = 20).

Tables (7)

Table I

Units of functions and the “desirable” values of the scores.

Retinal functions

Units

“Desirable” values

Recognition time

Seconds

Lower

Flicker fusion frequency

Flashes per second

Higher

Brightness discrimination, green

Arbitrary units

Lower

Brightness discrimination, red

Arbitrary units

Lower

Ophthalmological Tests

Abduction

Diopters

Higher

Adduction

Diopters

Higher

Vertical divergence

Diopters

Higher

Accommodation near point

Centimeters

Lower

Convergence near point

Centimeters

Lower

Performance Criteria

Performance average

No. of correct letters

Higher

Performance drop

No. of correct letters

Lower

Performance range

No. of correct letters

Lower

Blinking Rate

Blinking rate average

Blinks per minute

Lower

Blinking rate difference start-finish

Blinks per minute

Lower

Questionnaire Score

Arbitrary units

Lower

Ophthalmographic Measurements

Movement phase

1/100 sec.

Lower

Angular velocity

Arbitrary units

Higher

No. of movement

Rate per second

Higher

Fixation phase

1/100 sec.

Lower

Extent of movement

Arbitrary units

Lower

Discrepancy

Arbitrary units

Lower

Table II

Mean scores for performance, blinking rate, and questionnaire score at three levels of illumination, and significance of the fatigue changes taking place during 2 hours of visual work.

$\overline{d}$ equals the mean change in 36 pairs of measurements made at the start and at the end of the visual work; negative
$\overline{d}$ values express deterioration. F expresses the statistical significance of the mean changes (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table III

Visual functions which exhibited significant trends with variation of the illumination level.

$\overline{\mathrm{\Delta}}$ equals the mean difference between 36 pairs of changes (d) at the two compared levels of illumination.
Negative
$\overline{\mathrm{\Delta}}$’s indicate greater deterioration under the first of the conditions compared. F expresses the statistical significance of the mean difference (
$\overline{\mathrm{\Delta}}$) between the two compared levels of illumination. F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table IV

Fatigue changes in retinal functions resulting from 2 hours of visual work at three levels of illumination.

$\overline{d}$ equals the mean change in 36 pairs of measurements made before and after visual work. Negative
$\overline{d}$ values express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table V

Change in several ophthalmological routine tests at three levels of illumination.

$\overline{d}$ equals the mean change between 36 pairs of measurements made before and after visual work. Negative
$\overline{d}$’s express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.13. F at 1 percent of significance is 7.44.

Table VI

Fatigue changes in ophthalmographic functions at three levels of illumination.

$\overline{d}$ equals the mean change between 12 pairs of measurements made before and after visual work. Negative
$\overline{d}$’s express deterioration. F expresses the statistical significance of the mean differences (
$\overline{d}$). F at 5 percent of significance is 4.84. F at 1 percent of significance is 9.65.
For 36 pairs of measurements F at 1 percent of significance is 7.44.

Table VII

Average preference scores for 5 levels of illumination intensity. The smaller the score, the more preferred the condition.

Indicates significance of the mean differences at the 5 percent level.
Indicates significance at 1 percent level. In testing the significance of the differences the combined sample of subject was used (N = 20).