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[Presented before the Night Visibility Committee of the Highway Research Board, January 1963, Washington, D.C.]
THE RELATIONSHIP BETWEEN NIGHT DRIVING ABILITY AND THE AMOUNT OF LIGHT NEEDED FOR A SPECIFIC PERFORMANCE ON A LOW CONTRAST TARGET
(By Merrill J. Allen, O.D., Ph. D., and William M. Lyle, O.D., M. Sc., American Optometric Foundation Motorists Vision Research Grant, Division of Optometry, Indiana University, Bloomington, Ind.)
That many people have difficulty seeing at night often has been noted and it is gratifying to know that many drivers will not drive at night because they believe they do not see as well as they should.
It has recently been shown (1, 2, 3) that the transmission of the eye is progressively reduced with age, see figure 1(1). This coupled with the reduction in the average pupil size with age can produce a marked reduction in retinal illumination in the older driver. It may be assumed that a specific level of retinal illumination must be maintained at all ages for some standard level of highway night visual performance. Inasmuch as individual differences preclude a prediction of visual performance, it is desirable to measure any loss by some easily administered test. The results of such a test, if expressed in the amount of light needed for a specific visual task can be meaningful to lighting engineers, legislators, automobile licensing agencies, insurance agencies, ophthalmic practitioners, etc. Such a visual performance test would automatically include the effects of scatter and absorption in the ocular media and retinal layers, and the effects of optical irregularities and errors of refraction. Other factors such as the level of adaptation, etc., would also be included.
The test reported here consists of four lines of letters. The top two lines of letters subtend an angle of 10 min. at 3 meters (equivalent to 3/6 Snellen notation). The bottom two lines of letters subtend a visual angle of 5 min. at 3 meters. The second line of large letters has a contrast of 10 percent. The last line of small letters has a contrast of 20 percent.
An 11-inch square photographic film is transilluminated by two 60-watt tungsten lamps in a light box. An opal plastic sheet diffuses the light from the bulbs before it reaches the film. To minimize the effect of room illumination, a 20 percent transmission gray filter covers the photographic target. The luminance of the letter background with filter in place is variable from zero to 100-foot lamberts. A photographic light meter was inserted into a hole in the side of the box to measure the light level. By measuring the light level directly, the electrical circuitry needed is greatly simplified.
An effort was made to evaluate the influence of several variables using this test as a measure of visual performance. Since we already know many of the visual factors and their interrelationships (1, 2, 3, 4, 5, 7), the purpose of the data obtained here is to determine the ability of this instrument to make such measure
The way in which refractive errors affect night visual performance is seen in figures 2-5. It is apparent that optimum performance with the least light is obtained within only a short range of dioptric powers. The curves are labeled to indicate 5 min. test letters (20/20); 10 min. test letters (20/40); 100 percent contrast (high); 20 percent contrast (20/20 low); and 10 percent contrast (20/40 low). For each trial lens the illumination was increased until the particular line of letters could be read. Luminance levels in foot lamberts are obtained by multiplying the ordinate scale values by 0.22.
In figure 4, the wide range of acceptable lens powers as well as the erratic performance of this subject result from his large monocular amplitude of accommodation. When he was required to maintain binocular vision (fig. 5) as he would while driving, his performance was generally better, but over a much smaller lens power range. This is explained by the normal interrelationship between accommodation and convergence.
Figures 6 and 7 show the effects of pupil size upon the light needed for the various visual targets. The pupil diameters on the X axis are arranged according to their squares (area). The need for more light with pupils below 3 millimeters is most evident. Patients being treated for glaucoma with miotics will be handicapped as indicated, due to the resulting pupillary constriction. On the other hand the failure to accept less illuminance with pupil sizes larger than 3 millimeters is undoubtedly due to the aberrations introduced (akin to fig. 2–5) and to the presence of a Stiles-Crawford effect in photopic vision which reduces the efficiency of the rays from the margin of the pupil.
In order to determine the characteristics of the population as measured on this apparatus, 12 instruments have been built and are being used routinely in optometric offices on adults of driving age. So far the few results obtained show an age dependency as one would predict, and even show some correlations with some of the questions being asked of these people. Figure 8 is the questionnaire used. Each answer choice has a number value which is used in totaling the score. Since a great deal of data is expected in the next few months, further comments are out of place at the present time.
SUMMARY AND CONCLUSIONS
A test is described and results presented to show that this instrument is capable of measuring at least two of the several factors that can increase the need for greater task illumination. The 10 percent contrast test letters perhaps best simulate the contrast of objects often encountered at night. The 20/40 level is representative of vision requirements of most drivers' licensing agencies. The brightness levels at which these letters can be recognized with an optimum visual apparatus are not far removed from those being contemplated and actually used in highway lighting systems (6). Even a moderate visual impairment will likely necessitate an increase in the illumination required to see a low contrast object on the highways at night to amounts above those currently available.
From the work of other investigators and from the data presented here, one may conclude that a less than optimum visual apparatus can perform satisfactorily with sufficient light. The conditions for drivers visual acuity testing provide high illumination and high contrast and cannot be expected to indicate poor nighttime visual performance. Indeed if a person barely passes the regular 20/40 test, he must surely have a very poor visual performance at night, whatever may be his visual disability.
1. Boettner, A. E., and J. R. Wolter, M.D., Transmission of the Ocular Media. Technical Documentary Report No. MRL-TDR-62-34, Wright-Patterson Air Force Base, Ohio, May 1962. Also in Investigative Ophthalmology, vol. 1, No. 6, December 1962, pp. 776-783.
2. Said, F. S., and R. A. Weale, The Variation With Age of the Spectral Transmissivity of the Living Human Crystalline Lens, Gerontologia, vol. 3, No. 4, 1959, pp. 213–231.
3. Weale, R. A., Retinal Illumination and Age, Transactions Illuminating Engineering Society (London), vol. 26, No. 2, 1961, pp. 95-100.
4. Fortuin, G. J., Visual Power and Visibility, Doctoral Dissertation, Rijksuniversiteit Te Groningen, 1951.
5. Richards, Oscar W., Seeing for Night Driving, Journal of the American Optometric Association, vol. 32, No. 3, October 1960, pp. 211-214.
6. Rex, Charles H., Effectiveness Ratings for Roadway Lighting, Preprint No. 36, General Electric Co., Hendersonville, N.C., 1962, p. 9.
7. Richards, Oscar W., Night Driving Seeing Problems, American Journal of Optometry and Archives of the American Academy of Optometry, November 1958.