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46 ft. L. (not shown in the tables), which gives an outside scene to dial luminance ratio of about 12 to 1. Several automobiles had ratios above 30 to 1. According to Duke Elder(4) "... the visual acuity improves slowly as the surrounding illumination is raised to just below the level of that of the test object: when it is raised beyond this point, there is a rapid fall in performance (Lythgoe).” The critical instrument panel details are the darkest areas on the dash in almost every case studied. The following is from the IES Handbook (5): "Current good lighting practice has established that best results are obtained when brightness variation of adjacent areas, particularly within the working field, does not exceed 3 to 1, the work "(speedometer)" being brighter." (Not shown by this study, but very easily observed in certain automobiles, is the camouflaged nature of the speedometer pointer which is more difficult to see in the daytime than are the numbers on the dial). It is clear that brightness ratios are not ideal, in addition they are reversed from what is recommended. The average glare in the field of view of 20,047 ft. L. is explained by chromium horn rings and trim and by chrome plated windshield wiper ams, etc. So long as these are permitted near the driver's line of sight, certain sun positions are going to produce annoying and even incapacitating glare reflections.
The average distance of 72 cm. of the driver's eyes to the dash panel calculates to be 1.4 diopters which is the stimulus to accommodation provided by the numbers on the various dials. For presbyopes with bifocals, the dials are likely to be blurred with either portion of the glasses. For a presbyope with trifocals, this distance is within the range at which he should see clearly. For the younger emmetrope, accommodation time, in addition to adaptation time, is a factor in clearing the speedometer numerals. According to Borish (3) a 1.5D. bluced image will reduce Snellen visual acuity to 20/80 (when a target of 100% contrast is used, the surround luminance is nearly ideal and the patient is permitted time to adapt). The average speedometer number size is equivalent to about 20/300, but at the low levels of dash panel illumination in the daytime, the probability of being out of focus, and with the adverse effects of the ever present chromium glare sources, even these large speedometer numbers are too small for quick viewing. The odometer which has 20/80 sized numbers surely will require a much longer time to read than does the speedometer in most of the cars tested.
The average windshield transmission of 86.4% seems adequate in view of a 70% minimum industry standard. On the other hand, if we add to the unnecessary 15.5% average reflectance from the back of the windshield toward the eyes, the 3.5% and 1.3% average values of light scattered by the dirt on the windshield, we obtain 20.3% useless light scattered toward the driver's eyes. The average scene luminance of 683 ft. L. becomes 590 ft. L. when viewed through the 86.4% transmission windshield. The useless light (20.3% of 683) is 138 ft. L. This means that objects of 100% contrast can never be seen by the driver of the average automobile at more than about 81% contrast during the day
590 + 138
Special conditions such as dark objects on asphalt pavement or automobiles with excessive windshield reflectances, e.g., cars No. 5, 35, 43, 45 and 55 (and no doubt others at certain sun positions) will produce very low contrasts.
While dirt on the windshield is, on the average, unimportant, certain notable exceptions were found (see cars 35, 39 and 56). It is known that tobacco smoke is hygroscopic, and smoke residues on the inside of the windshield will probably cause greater light scattering, at high atmospheric humidities, than noted here,
The figures 2 through 6 show various manufacturers' concepts of a modem space age automobile's control panel! It is apparent that standardization of basic controls, instruments and locations would be helpful and would remove this most critical visual area from the hands of the car stylist and permit life saving basic improvements. The numbers in each photo indicate areas corresponding to the luminances given in the figure legends. Fourteen to 20 measurements were made on each automobile dash panel and the sites chosen were marked on a transparent overlay of a photograph of the dash panel. The camera distance was about 1 meter from the speedometer in each case. The object marked S is a standard reflector used with the Spectra Brightness Spot Meter to determine the illuminance falling on the instrument panel.
The photographs and brightness readings paint a clear picture. Figures 2 and 3 show cars number 5 and 34. While car No. 5 has nearly the lowest (12 ft. L.) instrument panel brightness of all tested, car No. 34 has nearly the highest. The author has driven a vehicle similar
Figure 3, Cor No. 34. S is the standard reflector. The ft. L. luminance
levels are: 1) 125; 2) 135; 3) 230; 4) 1700. Note the need to
to No. 5 on a trip and the frustratingly long time required to adapt to the dark speedometer area was still further extended by the need to search for the meter needle which was even less visible. Car No. 34 is much becter in this respect, since the speedometer dial is ac 125 ft. L., however the extensive dash area immediately below is 1700 ft. L., a lumi gance ratio of over 13 to 1! Since task and surround luminance ratios 360 l or less are considered desirable and since 10 to 1 is considered as the upper limit, it is apparent that serious seeing problems are present in these two cars.
Figure 4, Ca: No. 15. S is the standard reflector. The ft. L. Iuminance
levels are: 1) 100; 2) 90; 3) 410; 4) 1600. At 3 the glass reflection has blocked out the speedometer numbers. The numbers ore of a particularly low contrast on this model.
Figures 4, 5 and 6 show some of the variations in panel design characteristic of the industry. Note the extreme range of glare intensities in figure 5 and the brighter, more uniform appearance of the da sh in figure 6. Even in the figure 6 the surround (3) is brighter than the dial shemselves!
Figure 5, Car No. 41. S is the standard reflector. The ft. L. luminance
levels are: 1) 35; 2) 120; 3) 52; 4) 1000; 5) 26,000; 6) 2,500;
Figure 6, Car No. 40. s is the standard reflector. The ft. L. luminance
levels are: 1) 100; 2) 150; 3) 175; 4) 7,200; 5) 38.
The photographs in figures 7 and 8 offer dramatic evidence of the windshield reflectances shown in Table III in the columns labeled dash reflection. Figure 7 shows a man standing on a tree-covered street. The photograph was taken with black velveteen lying on the dash cop. Figure .8 shows the same scene without the velveteen light absorber. As a pedestrian in broad daylight, he wouldn't suspect that he was invisible to the driver.