Table I lists the automobiles studied with the position of the sun being inferred from the time of day and the direction of the automobile. For reference, at 12:30 the sun was straight south and was 700 above the horizon.

Table II shows the results of measurements intended to establish an average scene reflectance value. Various earth surfaces were measured using the G.E. meter and neutral density wratten filters. The meter was held horizontally at waist height and aimed at the horizon over the road or field. A 10-inch black shield was placed above the meter to cut out the sky light. Readings obtained were multiplied by two as a rough correction for the loss of 1/2 of the meter's normal receptive field caused by the black shield held over the meter. To obtain the ambient (sun plus sky) luminance the meter was aimed at the zenith and the operator carefully avoided obstructing any of the sky light. The average figure of 11.7% reflectance obtained is, therefore, particularly significant for viewing horizontally along a highway and is the integration of high lights and shadows as encountered during the measurements. The reflectance of 11.7% will be used with the measured ambient illuminance to obtain a value for comparing with the stray light from the windshield, which results from dirt on the outside and inside of the windshield and from light reflected from the windshield surfaces. see Figure 1 on page 10.

In Table III, the car numbered in the first column, is completely described in Table I. The sky illuminance in column 2 is the amount

Figure 2, Car No. 5. S is the standard reflector. The ft. L. luminance levels at the numbers shown are: 1) 12; 2) 17; 3) 11; 4) 22; 5) 22; 6) 40; 7) 950; 8)7500; 9) 2700. Note reflections in windshield. The "venetian blind" at the right is a grill on the top of the dash reflected in the windshield. Note glare from turn signal lever.

of light in foot-candles on a horizontal surface. The average scene value in column 3 is obtained by multiplying the sky luminance value by 11.7% obtained from Table II. The illumination on the dash, column 4, was obtained with the Spectra Brightness Spot Meter by measuring the light reflected from a small white reference plaque attached in the center of the speedometer on each instrument panel, and a suitable correction was made for the coefficient of reflection of the plaque. Columns 5, 6 and 7 show the Maximum, Minimum and Average Luminances of the Instruments and their immediate surroundings. These values are direct measures with the Spectra Brightness Spot Meter. Column 8, the Maximum Glare in the Field of View, was obtained with the Spectra Brightness Spot Meter aimed toward the glare source as the driver would see it. Column 9 gives the Eye to Panel Distance in centimeters with the seat position as found when the car was being measured. Columns 10 and. 11 give the angular size of the vertical dimension of the speedometer and odometer numbers.

Column 12 gives the transmission of the clean windshield as measured by the Spectra Brightness Spot Meter. A white diffusing reference plaque was placed vertically on the car hood. The windshield was cleaned both on the inside and the outside, and black velveteen cloth was placed over the dash panel to eliminate windshield back surface reflections. The light reading through the cleaned windshield was then divided by the light reading taken outside the windshield, of the same plaque in the same position, to obtain the per cent of transmission.

Column 13 gives the values of light reflected toward the driver by the windshield from the top of the dash panel. This is a derived value and is the difference in windshield "transmission" with and without black velveteen over the top of the dash. Column 15 gives the values of light scattered from dirt on the outside of the windshield. These were obtained as the difference between the windshield "transmission" before and after cleaning on the outside. Column 17 gives the values of light scattered by dirt on the inside of the windshield. These values are the difference between the windshield "transmission" before and after cleaning on the inside. Columns 14, 16 and 18 are per cent values derived by comparing columns 13, 15 and 17 with the average scene luminances in column 3.

At the bottom of Table III is the average of the data of all automobiles tested. The average dash luminance of 72 ft. L. compared to the average scene luminance of 683 ft. L. viewed through the average windshield transmitting 86.4%. The luminance difference is about 8 to 1 which is comparable to the special aircraft dash luminance problem (5 to 1) studied by the author(1) using a reaction time measure of ability to recognize a test letter on the dash panel. Such brightness differences prohibit "instantaneous glance" perception and may require fixation for a second or more to see well enough to identify the task. Subsequent return of the attention to the road must result in temporary dazzling due to the heightened sensitivity of the eye needed to see the dash panel. The seriousness of the problem is substantiated by the fact that on the average dash the average speedometer dial luminance is

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 arms, 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. blurred 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 time.

590 590 + 138

= 0.81.

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 modern 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, Car 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 search in the dark" below area 4) for control levers and knobs.

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 better in this respect, since the speedometer dial is at 125 ft. L., however the extensive dash area immediately below is 1700 ft. L., a lumi nance ratio of over 13 to 1! Since task and surround luminance ratios 3 to 1 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, Car No. 15. S is the standard reflector. The ft. L. luminance levels are: 1) 100; 2) 90; 3) 410; 4) 1600. At 3 the glass reflection has blocked out the speedometer numbers. The numbers are 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 dash in figure 6. Even in the figure 6 the surround (3) is brighter than the dial themselves!

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