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The Luckiesh-Moss Test-Chart Illuminator*
NOTES, CASES, INSTRUMENTS T H E LUCKIESH-MOSS TESTCHART ILLUMINATOR* MATTHEW LUCKIESH, D.SC. AND FRANK
K. Moss Cleveland, Ohio
sides. I n fact, the contrast between the black letters and the white background is strikingly different for these two arrange ments of light sources. A further advan tage of our arrangement is that shallow
-The test-chart illuminator shown in the accompanying illustration was developed at the request of and in accordance with the specifications of the Advisory Com mittee of Ophthalmologists of the EyeHealth Committee of the American Stu dent Health Association acting in coopera tion with the National Society for the Prevention of Blindness. It is designed to illuminate the standard (1932) test-chart of the American Medical Association to a uniform brightness of 10 foot-lamberts with a manufacturing tolerance of plus or minus 2 foot-lamberts. Actually, an un certainty in brightness of this magnitude may introduce a possible uncertainty of only 3 percent in visual acuity. 1 Since the test-chart itself is graded in steps of not less than 20 percent in visual acuity, the range in brightness tolerance of the il luminator is clinically insignificant. T h e chart is illuminated by two 40-watt inside-frost Lumiline lamps placed hori zontally above and below the chart. T h e light from these lamps is directed upon the chart by carefully designed and pol ished parabolic chromium-plated reflectors placed in shallow housings. Since the printed characters of the chart reflect light both specularly and diffusely, maximal contrast 2 between the black letters and white background is obtained by directing the light upon the chart at extremely acute angles. F o r this reason the light sources are placed horizontally above and below the chart rather than vertically along the * From the Lighting Research Laboratory, General Electric Company.
Fig. 1 (Luckiesh and Moss). A new illumi nator for the 1932 A.M.A. chart designed to produce a uniform brightness over the entire chart.
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NOTES, CASES, INSTRUMENTS
lamp housings are required and hence shadows resulting from light sources lo cated elsewhere in the examination room are minimized. Notwithstanding the in herent difficulties of lighting from such acute angles it is practically possible to obtain such a degree of uniformity in brightness that the maximum variation from the average brightness of various areas of the chart does not exceed 5 per cent in either the horizontal or vertical directions. Obviously, the uniformity of the illumination on the chart, as well as the level of illumination, will be influenced by the general lighting in the examination room. However, the visual-acuity ratings obtained in practice should not be sig nificantly altered by the ambient illumina tion, if the latter amounts to only a few foot-candles. All exposed surfaces of the illuminator are coated with a dull crystallized gray finish having a reflection factor of about
40 percent. T h e latter represents a median value between the "white" chart and the average wall surface against which the chart will be viewed. T h u s the distraction of high brightness-contrasts between the illuminator and its surroundings, as well as between the illuminator and the chart, are minimized. A bright fixation light for use in con junction with the Maddox rod is provided by viewing an inside-frosted lamp through a circular aperture having a diameter of one centimeter. This light source is controlled by a separate switch and provides a brightness of 25 candles per square inch. This attachment, located on the bottom lamp housing, is hinged so that it disappears into the lower hous ing when not in use. This equipment may be obtained from T h e Welch Allyn Company of Auburn, New York. Nela Park.
REFERENCES
'Cobb, P. W., and Moss, F. K. Institute, 1928, v. 205, p. 831. 2 Luckiesh, M., and Moss, F. K. pany, 1937.
The four variables of the visual threshold. Jour. Franklin The science of seeing. New York, D. Van Nostrand Com-
K. Moss Cleveland, Ohio
sides. I n fact, the contrast between the black letters and the white background is strikingly different for these two arrange ments of light sources. A further advan tage of our arrangement is that shallow
-The test-chart illuminator shown in the accompanying illustration was developed at the request of and in accordance with the specifications of the Advisory Com mittee of Ophthalmologists of the EyeHealth Committee of the American Stu dent Health Association acting in coopera tion with the National Society for the Prevention of Blindness. It is designed to illuminate the standard (1932) test-chart of the American Medical Association to a uniform brightness of 10 foot-lamberts with a manufacturing tolerance of plus or minus 2 foot-lamberts. Actually, an un certainty in brightness of this magnitude may introduce a possible uncertainty of only 3 percent in visual acuity. 1 Since the test-chart itself is graded in steps of not less than 20 percent in visual acuity, the range in brightness tolerance of the il luminator is clinically insignificant. T h e chart is illuminated by two 40-watt inside-frost Lumiline lamps placed hori zontally above and below the chart. T h e light from these lamps is directed upon the chart by carefully designed and pol ished parabolic chromium-plated reflectors placed in shallow housings. Since the printed characters of the chart reflect light both specularly and diffusely, maximal contrast 2 between the black letters and white background is obtained by directing the light upon the chart at extremely acute angles. F o r this reason the light sources are placed horizontally above and below the chart rather than vertically along the * From the Lighting Research Laboratory, General Electric Company.
Fig. 1 (Luckiesh and Moss). A new illumi nator for the 1932 A.M.A. chart designed to produce a uniform brightness over the entire chart.
1061
1062
NOTES, CASES, INSTRUMENTS
lamp housings are required and hence shadows resulting from light sources lo cated elsewhere in the examination room are minimized. Notwithstanding the in herent difficulties of lighting from such acute angles it is practically possible to obtain such a degree of uniformity in brightness that the maximum variation from the average brightness of various areas of the chart does not exceed 5 per cent in either the horizontal or vertical directions. Obviously, the uniformity of the illumination on the chart, as well as the level of illumination, will be influenced by the general lighting in the examination room. However, the visual-acuity ratings obtained in practice should not be sig nificantly altered by the ambient illumina tion, if the latter amounts to only a few foot-candles. All exposed surfaces of the illuminator are coated with a dull crystallized gray finish having a reflection factor of about
40 percent. T h e latter represents a median value between the "white" chart and the average wall surface against which the chart will be viewed. T h u s the distraction of high brightness-contrasts between the illuminator and its surroundings, as well as between the illuminator and the chart, are minimized. A bright fixation light for use in con junction with the Maddox rod is provided by viewing an inside-frosted lamp through a circular aperture having a diameter of one centimeter. This light source is controlled by a separate switch and provides a brightness of 25 candles per square inch. This attachment, located on the bottom lamp housing, is hinged so that it disappears into the lower hous ing when not in use. This equipment may be obtained from T h e Welch Allyn Company of Auburn, New York. Nela Park.
REFERENCES
'Cobb, P. W., and Moss, F. K. Institute, 1928, v. 205, p. 831. 2 Luckiesh, M., and Moss, F. K. pany, 1937.
The four variables of the visual threshold. Jour. Franklin The science of seeing. New York, D. Van Nostrand Com-