Color and contrast sensitivity after glare from high ...

Color and contrast sensitivity after glare from high-brightness LEDs H.-D. Reidenbach*a a Cologne University of Applied Sciences Research Laboratory on Medical Technology and Non-Ionizing Radiation Betzdorfer Str 2, 50679 Cologne (Koeln), Germany ABSTRACT The color contrast capability was investigated for 3 volunteers with 7 specially developed test charts in red, green, blue, cyan, magenta, yellow and black as a reference, namely without and after glare from 4 colored high-brightness LEDs. Each subject completed 56 tests in order to check especially the ability to discriminate low contrast. It was found that a contrast decrease of one level is equivalent to an increase of about 4 s in the required identification time and in addition a delay time between about 14 s and 16 s has been measured at the beginning of the respective test as a result of the dazzling glare from an LED. In addition trials have been performed with 4 different pseudoisochromatic color plates designed by Ishihara for color vision. These plates have been used to determine temporary color deficiencies after an exposure from a high-brightness LED. For this purpose 40 volunteers have been included in a laboratory test. Color vision was impaired for periods between 27 s and 186 s depending on the applied color plate and respective LED color. Keywords: Color contrast, Ishihara test, identification time, glare, high-brightness LEDs

1. INTRODUCTION It is well known that a dazzling light influences the capability of human vision temporarily. This impairment concerns mainly the visual acuity and color vision. Whereas visual acuity is a measure of the ability to distinguish fine details color vision is the capacity to distinguish objects based on the wavelengths of the light they reflect or emit. Both abilities may be disturbed by blinding glare for relatively long durations, but quantitative data are not easily available, especially not for modern light sources like high-brightness LEDs (HB-LEDs). In general glare is divided into physiological and psychological glare, i.e. disability and discomfort glare, and there exists several CIE publications on this topic1, 2. Whereas disability glare is defined in the International Lighting Vocabulary as glare that impairs the vision of objects without necessarily causing discomfort, discomfort glare is described as glare that causes discomfort without necessarily impairing the vision of objects3. Visual acuity has its meaning as a standard test done by eye care professionals. The perturbation time is normally about 60 s or shorter as far as the impact of glare is regarded. This has been shown for high-brightness LEDs recently4, 5. Since in addition to visual acuity color contrast vision is an important physiological property of the human eye and its reliability is essential especially in occupational health and safety it was the goal to improve the current knowledge especially on color and contrast sensitivity.

2. METHODS Since it has been shown in preliminary investigations that the color perception is disturbed by bright light and that the duration of the impaired color vision depends on the exposure duration and on the dazzling light source themselves, a test procedure has been developed in order to determine the influence of temporary blinding on color vision quantitatively. More precisely the contrast vision has been carried out via presentation of colored optotypes and in addition color vision was investigated separately, namely the duration of disturbance. In a third part the subjective perception of colored test patterns was investigated after irradiation with HB-LEDs of various colors. All 3 test methods have been realized as print and monitor presentation. *[email protected]; phone +49 221 8275-2003; fax +49 221 885256

Ophthalmic Technologies XVIII, edited by Fabrice Manns, Per G. Söderberg, Arthur Ho, Bruce E. Stuck, Michael Belkin, Proc. of SPIE Vol. 6844, 68441R, (2008) · 1605-7422/08/$18 · doi: 10.1117/12.762896

Proc. of SPIE Vol. 6844 68441R-1 2008 SPIE Digital Library -- Subscriber Archive Copy

The use of Landolt C-rings is a well established technique to investigate visual acuity, but it has not been used, to our knowledge, to investigate the color contrast impairment after a blinding glare from HB-LEDs. In order to determine the quantitative consequences of temporary blinding on color and contrast visibility a procedure has been developed where the test charts were based on the so-called Pelli-Robson contrast sensitivity chart but instead of Sloan letters (e.g. O, S, N, Z, C, H, V,. D, K, R) colored Landolt C-rings were used as optotypes. The respective Landolt C-rings were arranged in groups of three each with the same primary color and contrast value and successive groups decreased in contrast by a factor of 1/√2 between 11 % and 1 %, i.e. 11.0 %, 7.9 %, 5.6 %, 4.0 %, 2.8 %, 2.0 %, 1.4 % and 1,0 %. There were 4 rows with 6 rings and randomized orientations of the gap in the ring. The applied 8 low contrast levels have been equated with the simpler numbers 1 to 8, i.e. 1 corresponds to 11 %, 2 corresponds to 7.9 % etc. The Pelli-Robson contrast sensitivity test offers a 90 % reproducibility and a constant stop criterion. On the other hand the disadvantage is that the subjects are able to memorize the letter series relatively easy, especially since many tests should be repeated. The orientation of the respective gap in a Landolt ring is much less memorable. Closer details on the application of Landolt ring optotypes and a method for measuring distance visual acuity under daylight conditions for the purposes of certification or licensing are described in ISO 85966 and it was decided to use modified test charts in order to determine color contrast behavior influenced by an afterimage. Since there were no appropriate color contrast charts commercially available, the respective test charts have been produced on the basis of our own conception and specifications. Photoshop was used in TIFF-format. The typographic realization demanded some skill therefore and various prints were necessary in order to achieve the required Weber contrast values and grading. The control has been done with a luminance meter and in addition level 1 of all 7 colors, i.e. red, green, blue, cyan, magenta, yellow, and black as a reference, was spectroscopically measured and the results illustrated in a CIELab color diagram. CIELab is a color space which has been established in 1976 by CIE. The low contrast capability was investigated in a time-consuming trial with 3 volunteers with charts in all 7 colors. Black C-rings have been used as a reference in all investigations. All measurements were performed under accurately defined and constant D65 illumination, which correspond to daylight under special color temperature conditions, namely without and after glare from 4 colored LEDs, i.e. each subject completed 56 tests. The subjective color perception was determined in another test series with 6 colored charts in the primary colors red, green, blue, cyan, magenta and yellow. These charts had the dimensions 35 mm x 35 mm, printed on white cards and were therefore covered totally from the afterimage produced by the respective HB-LED. The various colors were presented to the subjects during cycles of about 15 s until they finally gave the correct answer of the respective color. In addition tests have been performed with 4 different pseudoisochromatic color plates according to the Ishihara color test in order to determine the temporary color deficiency after exposure from a colored high-brightness LED. In this case 40 volunteers have been included in a laboratory test. Fig. 1 (see color plate on last page)shows the applied 4 test charts. It has been taken into account that tests for the most widespread color deficiencies are represented in at least one chart. Since the recognition value is high for Ishihara charts they could be applied only once per subject. As light sources 4 bright HB-LEDs have been used, namely at 455 nm (royal blue), 520 nm (green), 593 nm (amber) and 638 nm (red). In order to get a sufficiently large blinding light source the respective LED was positioned in the focal point of a collimator and a collimated ray beam was achieved. The collimator represented the virtual source simultaneously. All exposures have been performed with only one single duration of 5 s, particularly to restrict the number of tests per subject reasonably acceptable. The selected optical power behind the 7-mm aperture was limited to 4 mW, since not all color HB-LEDs were able to deliver higher outputs. According to the international standard IEC 60825-1: 2001-08 the maximum permissible power is 30.9 mW, if the LED is treated as an extended source under laser conditions, which is now no longer valid as far as standards are regarded, since LEDs are included from now on in the new standard IEC 62471 and treated as lamps in most cases8. The subjects belonged to lab employees and students and to the age group between 20 and 40 years, where usually normal contrast sensitivity exists. All subjects became tested concerning their color vision and ametropes carried their eyeglasses or contact lenses during the tests. They all got the essential information concerning the test procedure and the whole purpose and participated in the investigations after written consent only. The subject’s eye was at a distance of 10 cm from the aperture during the irradiation. Upon completion he/she took a seat in front of the test monitor or opposite to one of the persons “providing” color charts or he/she stood in front of the Landolt C-ring contrast charts, which were positioned at a distance of 3 m at the lab wall. The size of these test charts

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was 594 mm × 420 mm (A2 wide format), which corresponds to the expected perceived afterimage size according to Emmert’s law. The time duration of less than 3 s to move from the irradiation apparatus to the respective test site was insignificant since it was less than the time needed to identify an object after glare. The mean luminance of the charts was 85 cd/m2 and the illuminance about 310 lx. Besides the printed charts the Landolt C-rings became realized on a CRT-monitor too. In this case instead of ring diameter of 52 mm 12 mm have been used, which corresponds to an angle of vision (angular subtense) of 1° in a distance of 70 cm. The observation took place at 310 lx and a mean luminance of 112 cd/m2. In order not to falsify the results by still existing afterimages, i.e. insufficient readaptation, a pause of at least 30 minutes has been kept between two successive irradiations. Therefore time-consuming investigations could not be avoided.

3. RESULTS 3.1 Identification times of various contrast levels Due to the time-consuming tests only 3 subjects were able to perform all tests. In total each subject had to take part in 56 tests, since there existed 6 colored charts and a black one for reference purposes and 4 HB-LEDs with different colors have been used as blinding light sources. In addition each subject had to perform a test cycle before he/she became dazzled. Fig. 2 shows the results obtained for the normal and disturbed identification times, i.e. the time duration which is needed under normal vision conditions and after a blinding irradiation from a HB-LED.

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As can be clearly seen from fig. 2 there is a remarkable delay of about 18 s for the start of identification of contrast level 1 (11 % contrast) due to blinding compared with only 3 s under normal vision conditions. Up to level 4 the difference between both conditions is about 2 s only, but increases at lower contrast steps (higher contrast levels). From the comparison of the influence on the colored and black Landolt C-rings it can be seen that perceptible deviations arise at contrast level 4, i.e. at less than 4 % contrast value. Finally glare results in a mean increase up to 16 s in identification time, which is 6 s more compared to black rings and about 10 s more compared to non dazzling conditions. For contrast level 8, which is equivalent to only 1 % contrast value, no correct readings were given under all tested conditions. Yellow has been excluded in fig. 2 since for this color the identification times were 35 s for contrast level 1, 12 s for level 2, 15 s for level 3, 16 s for level 4 and 27 s for level 5, and therefore the mean value would have been affected to strong by this single color. Contrast level 6 could not be identified at all, if the Landolt C-rings were presented in yellow. Yellow has been the color which showed the largest individual differences for all 3 subjects. A closer look on the results for the other colors has shown that a reduction of contrast becomes effective at different levels for the various colors. Especially red, cyan and magenta showed an increase already at level 5 of about 10 s under afterimage conditions compared to the normal vision situation, whereas blue and green did show an increase of 5 to 7 s not earlier than at level 7, which is nearly the same for the conventional black Landolt C-rings. The order of the influence of a bright light source, like a HB-LED, increases from black to blue, green, magenta, cyan, red to yellow, with partly small differences only. The same sequence has been found for the colored rings under normal, undisturbed conditions, i.e. blinding by colored LEDs intensifies the effect. In addition there was only a minor difference as far as the test with a wall-chart and a monitor illustration are regarded. If instead of the colors of the Landolt C-rings the colors of the stimulating LEDs are regarded the respective results are shown in fig. 3 (see color plate on last page). It might be elicited from fig. 3 that contrast vision is most disturbed after an irradiation from a green LED, but the difference is relatively small, i.e. between 1 s at level 3 and up to about 3 s at higher contrast levels. A fundamental relation between color of the C-ring and the LED could not be derived from the existing data, although the higher spectral visibility of the green LED seems to indicate a correlation between visibility, brightness and disturbance of color contrast vision. As a total result of this part of the investigations it might be stated that there exists an increase of identification time of about “4 s/1 contrast step” below a contrast value of about 4 % for color contrast, compared to about 3.6 s at the same contrast decrease for black on white illustrations. But in addition there is a difference concerning the absolute values of the identification times if black and colored illustrations are regarded. The isolated effect of blinding, especially for lower color contrast ratios, results in an increase of about 3 s per decrease of contrast by a factor of 1/√2, i.e. one step in contrast. 3.2 Color vision disturbance measured with pseudoisochromatic charts Pseudoisochromatic charts according to fig. 1 have been used in order to determine the duration of impairment of color vision after glare with HB-LEDs of various colors. Due to the fact that each chart could be shown only once to each subject it was not possible that each subject evaluated all charts for all 4 colors of the applied HB-LEDs. A total of 40 subjects have been tested and mean values are provided for 5 subjects per chart and LED-color. The results indicating the time needed in order to identify the correct symbols on the respective Ishihara chart are given as histograms for each color. Fig. 4 shows an example of these investigations, namely the case where chart 4 in fig. 1 has been used as test chart and all 4 colored LEDs have been applied as stimulating light source.


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As might be clearly seen in this case where blue dominates in the somewhat hidden number 49 of chart 4 in fig. 1 it is not really surprising that the blue LED produced the longest time duration in order to identify just this test chart, whereas all other 3 colors resulted in about the same identification times. A closer look on all results of this part of the investigations on color vision impairment shows that about the same results have been achieved whether the pseudoisochromatic charts have been presented in print or monitor version. The range of identification times was between 27 s and 139 s for the print version and between 31 s and 186 s, when a monitor has been used. Therefore it might be concluded that the color vision impairment is independent of the presentation mode. Chart 1 in fig. 1 offered comparably more problems in order to become identified and resulted in somewhat larger identification times. Red and green blinding created about the same identification times and blue was responsible for the strongest impairment. In addition most of the subjects felt blue as the most uncomfortable color, namely during the stimulation and over the lasting period of the afterimage. This part of the study has shown that the color vision might be disturbed more or less during about 2 minutes and in particular cases even more than 3 minutes as a result of an irradiation from a HB-LED. Therefore certain colored objects might not be distinguished from an environment of a different color. 3.3 Color vision disturbance measured with color plates Subjective color vision disturbance as a result of temporary blinding has been carried out with color plates. The goal of this part of the study was to determine whether two or even more colors could be confused due to preceding glare. A total of 40 measurements have been performed and mean values have been formed from 5 individual measurements per color plate and glare incidence.


The description of colors like red, green, blue and yellow did not make problems, but magenta and cyan have been called pink, cyan, light blue or turquoise. Since the tested subjects reported the various perceived colors of the presented color plates during the afterimage duration a gross progression of the various phases could be derived and the respective process of color mixture identified. Therefore these results extend the ones achieved in a special study on the flight of colors, which is reported in the same issue of this proceedings9. In detail yellow color plates did show the least corruption with the exception of blue as a stimulating color. This might be easily explained in the concept of complementary colors and the respective afterimage formation. On the other hand the red color plate had the longest impairment time and showed several color phases during the identification process. Again the blue LED was connected with the longest interference of color vision, i.e. times up to 150 s were required in order to identify a red color plate correctly as red. This method showed relatively large variations as far as the respective observations of the different subjects are regarded. Therefore the results should be considered as qualitative contributions primarily. On the other hand it has been shown in this investigation that red color plates became mistaken as green for a while when green or yellow LEDs were applied as glare source. Similar results could not be observed with other complementary color pairs. Nevertheless this test has shown that certain colors presented as print plates or on a monitor might be perceived wrong or changed when a direct look into a bright LED precedes the inspection of a colored object.

4. CONCLUSIONS Temporary blinding from current available high-brightness LEDs may cause more or less serious impairment of color vision up to about 3 minutes and should thus be incorporated in an assessment of vision in general if irradiation from high brightness light sources might be expected. The presented results might be regarded as a contribution to a more trusted data base for further treatment of bright light sources as far as color dependent activities are concerned. The comparison of the 3 different test series show coincidently that blue HB-LED evidently produce the largest impact on color vision, especially concerning color discrimination and color contrast sensitivity, compared with other colored LEDs at the same optical power output and exposure duration. Since only blue radiation was able to produce relatively long disturbance as far as yellow and green color plates are concerned and in addition several color phases appeared, this might be the reason why longer identification times occurred in the respective pseudoisochromatic charts and its symbols. In order to clarify potentially interrelationships further investigations based on the already available data are recommended, especially in order to resolve questions regarding analogies between color discrimination and color identification. Up to now it seems that color differentiation is quicker than color identification. Based on the performed investigations temporary blinding from bright light sources like high-brightness LEDs might be perceived as very disturbing but after-effects beyond temporary afterimages and its influence, e.g. on color vision, have not been found so far. Only in very few cases minor symptoms like a vague feeling of pressure in the head was reported, which could not be described exactly and which did not last more than about an hour or two after the respective irradiation. Similar observations have been made and annotated in an internal report only in a study which encompassed about 2,500 subjects, which have been irradiated either with laser radiation from class 2 lasers or high-brightness LEDs in order to determine aversion responses including the blink reflex. In this cases the exposure duration was even shorter compared to the now reported results on temporary blinding. Therefore a precautionary approach is recommended, especially as far as multiple exposures are concerned and excessive strains from blinding bright light should be prevented as far as possible.

ACKNOWLEDGEMENT The funding from the Federal Institute of Occupational Safety and Health (FIOSH, BAuA) under contract No. F 1984 is gratefully acknowledged. The author wishes to thank Dipl.-Ing. Miriam Bischof and Dipl.-Ing. Sabine Peters for their valuable commitment and assistance carried out in time-consuming laboratory tests.


REFERENCES 1 2 3 4 5

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Commission Internationale de l’Eclairage, “CIE equations for disability glare”, CIE 146:2002. Commission Internationale de l’Eclairage, “Technical Report Discomfort Glare in Interior Lighting”, CIE 117: 1995. Commission Internationale de l’Eclairage (1987) International lighting vocabulary, 4th ed. CIE 17.4-1987. H.-D. Reidenbach “Local Susceptibility of the Retina, Formation and Duration of Afterimages in the Case of Class 1 Laser Products and Disability Glare Arising from HB-LEDs”; ILSC 2007, Conference Proceedings p. 102 – 111. H.-D. Reidenbach “Some quantitative aspects of temporary blinding from high brightness LEDs”; Proc. SPIE Vol. 6426 Ophthalmic Technologies XVII, Manns, F.; Söderberg, P. G.; Ho, A.; Stuck, B. E.; Belkin, M. (eds.), 2007, 642629-1 – 64629-10. ISO 8596:1994 “Ophthalmic optics -- Visual acuity testing -- Standard optotype and its presentation”. K. Velhagen and D. Broschmann (Eds.) Tafeln zur Prüfung des Farbensinnes; 33. Aufl.; Georg Thieme Verlag, Stuttgart, 2003. IEC 62471 (CIE S 009:2002): 2006-07 Ed. 1.0 “Photobiological safety of lamps and lamp systems”. H.-D. Reidenbach “Determination of the time dependence of colored afterimages”; Proc. SPIE this issue.


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contrast level Fig. 3. Identification time (mean values) as a function of contrast level for various colored HB-LEDs; black curve for reference without glare; vertical bars show the respective standard deviations.
