This study, investigating the relation between the eye fatigue and the levels of illumination, is one of the researches to obtain the fundamental materials for the arrangement of the recommendation of illumination in Japan.
In this experiment, the degree of the visual fatigue was estimated with the fluctuation of the accommodation time of the eye which indicates the fatigue of the ciliary muscle in the eye, during the visual task performance. Fig. 2 and Fig. 9 show an apparatus for the measurements of the accommodation time which we designed in purpose of estimating the visual fatigue. The accommodation time was measured befor and after the visual task performance under various levels of illumination, in the range of 50lx. to 2000lx., and the fluctuation rate of the accommodation time was calculated with following formula;fluctuation rate=accommodation time after the visual task/accommodation time befor the visual task
The visual task consisted in the recognition of small letters of 1.5mm. diameter or the fine line in the small circle of 1. 5 mm. diameter, printed on white papers, as shown in fig. 5. The visual task chart (A) in fig. 5 is printed with small letters in random order, and the chart (B) is printed with small circles enclosing a fine line in each circles with 8 kinds of directions, both contrasts are about 96%, the chart (C) is printed with grey ink same as chart (B) which contrast is about 67%.
Using those three kinds of visual task charts the subjects were instructed to check the specific letter or the specific circle. Fig. 8 shows an aspect of the visual task performance.
A space of the visual task performance was illuminated uniformly by 12 fluorescent lamps of 40 W. white in a lamphause coverd with diffusing plate and installed with intensity regulator, as shown in fig. 6 and fig. 7.
Six levels of illumination were adopted for the performance of visual tasks in series of 50, 100, 200, 500, 1000, and 2000lx., on the surface of the chart. The surface of the table and its soroundings were covered with grey clith of 30% reflectance, consequently the visual task chart extends in the grey visual field to 60 degrees in chart (A), to 30 degrees in chart (B) and (C), as shown in Fig. 4.
EXPERIMENT 1: Using the visual task chart (A) the prolongation of the accommodation time was measured with 3 subjects, fig. 10 shows the result of this study, indicating that the prolongation of the accommodation time after the visual task was most remarkable at 50lx. illumination level, and the smallest prolongation was found at 1000lx., without so much difference with 500 and 2000lx.
This results suggested that the favorable level of illumination for this kind of visual task exists above 500lx., never under 200lx.
EXPERIMENT 2: The results of the experiment with the chart (B) were shown in fig. 11, indicating that the prolongation of the accommodation time after the visual task was smallest at 1000 and 2000lx. Comparing this results with that of the exp. 1, no many differences were found, but the tendency that the prolongation at 500lx. was larger than that of at 1000 and 2000lx. This tendency was comfermed by exp. 4.
EXPERIMENT 3: Using the visual task chart (C), printed with greyink same as the chart (B), the prolongation of the accommodation time was measured with 3 subjects. The tendency of the results, as shown in fig. 12, was similar to that of exp. 2, but the prolongations were larger than that of exp. 2 in all levels of illumination.
EXPERIMENT 4: Magnifying the white area of the visual task chart (B) to 60 degrees in the visualfield, the prolongation of the accommodation time was measured with one subject, in the purpose of the determination of the influence of the white area in the background.

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Many studies on the relation between the eye fatigue and levels of illumination have carried out in Japan since 1938, as Fig. 2 showing some results of them. The results of those studies indicate different values of illum. levels as the most favarable level of illumination for the visual tasks.
On the other hand many studies on the relation between the visual acuity and levels of illumination have carried out. Most of all results of those studies on the visual acuity show that the visual acuity becomes better with the increasing of levels of illumination, and as we know, those results have adopted in the modern illum. eng. technique.
Comparing the study on the eye fatigue with that of the visual acuity, we notice that the study on the eye fatigue indicates often the existence of the most favarable level of illumination, as shown schematically curve B in Fig. 1, and incompatible with this fact, almost of all results of studies on the visual acuity never indicate the existence of the best level of illnmination, as curve A in Fig. 1. indicates.
This paradox occurs, I believe, owing to the quality of the illumination, because it would be much easier to make the quality of illumination better at the target for visual acuity measurements than the space of visual tasks, the quality means the glare, the contrast distribution in the visualfield and the color temperature of illuminants. I assumed on this paradox that the curves of full lines in Fig. 1 would removed toward the dotted lines by the improvement of the quality of the illumination.
This study by Dr. Matsui interested me very much in the meaning of the above mentioned matter, and results of this study makes certain of my hypothesis on that paradox.

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