CHROMATICITY ANALYSIS OF REFLECTED SPECTRA DUE TO COLOR TEMPERATURE VARIATION OF LED LAMPS ON THE CHANGE OF OBJECT’S COLOR APPEARANCE

• Main Author: Revantino
• Format: Dissertations
• Language: Indonesia
• Subjects: Teknik (Rekayasa, enjinering dan kegiatan berkaitan)
• Online Access: https://digilib.itb.ac.id/gdl/view/80023
• Institution: Institut Teknologi Bandung

The use of light-emitting diode (LED) as light sources has grown rapidly in the last decade. This lighting system, besides more excellence on economic aspects, also has flexibility in generating of spectrum and color. This flexibility opened the way for research about the effect of color parameter variation of light sources towards the appearance of colored objects. Studies about this topic were usually conducted with subjective approach, which certainly affected by the visual abilities and preferences. On the other side, the proper metrics for assessing the color quality of LED lamps were also developed by some studies. However the variation of used object’s color samples has not cover all of the color space yet, especially the intense colors. Meanwhile the conventional color rendering index (Ra) is still dependent on the similarity of color temperature (Tc) between the light sources to the standard illuminants. Whereas the LED lamps can produce the different Tc according the user’s preference. The problem occurred when the used Tc can bring the color shifting of object’s appearance. The shifting occurred shall not be the same toward all colors. It is based on the interaction between the LED lamp’s spectrum to the characterictis of object’s spectral reflectance or ?(?). Therefore, a color could have the best appearance on certain Tc, which is different to the other colors. The hypotheses would be proved in this study, with the quantitative apporach through the measurement and simulation towards the various color samples. This study is started with the psychophysical experiment by a group of subjects, using the LED lamp samples with variation of lighting parameters, namely: illuminance, Tc, and Ra. This experiment aimed to determine whether parameter which affected most to the color aspects, namely the appearance and warm/cool impression. The assessment is conducted in Likert scale, with object in the form of Macbeth color checker for color appearance, while the color impression is assessed from the ambient. Data processing is conducted with analysis of variance method towards the three lighting parameters. The Tc variation evidently does not only affect on the color impression, but also towards the color appearance compared to the other two parameters. The Tukey method is used to determine the significant different between Tc, which occurred between 3,000 K to 6,500 K and 8,000 K; but not on the two latter Tc. Therefore, the next stage is focused on Tc parameter, which more detail variation between 3,000 K ~ 6,500 K. The spectrum model for each Tc is obtained from commercial tunable LED lamp. The measurement is conducted to ensure the obtained spectrum lied in tolerance limit of chromaticity (?u'v') ? 0.0055 towards the reference value at each Tc. As its specification, this LED lamp can produce the Tc from 2,000K ~ 6,500 K; so those are 10 spectrum models obtained to be used on the next experiment, namely the assessment of ?(?). This experiment is conducted with the methods of integrating sphere and dark booth, using the color samples variation in combination of RGB values: 0, 127, and 255. The chromaticty analysis then is conducted towards the reflected spectrum of color samples on each Tc, which is compared towards the chromaticity of its ?(?) under the standard illuminant type E or equal energy. The color group of red-orange has the reflected chromaticity which most approximated to its ?(?) on Tc = 4,000 K and 4,500 K, while the yellowish colors on 4,500 K and 5,000 K. Hereafter the colors of bluish and purplish are on 5,000 K and 5,500 K, while the greenish colors on the range of 5,000 K ~ 6,500 K. It showed that every group of colors has the best chromaticity on different Tc. The biggest chromaticity shifting as consequence of Tc variation occurred on the color group of bluish and purplish, while the smallest shifting occurred on the greenish colors. It is related to the blue color component on LED lamp’s spectrum, which continues to increase with the Tc increment. Meanwhile the green color component keep to be constant relatively, so the best chromaticity of this color groups lied in the wider Tc range. The simulation of color appearance shifting is conducted with enlarged color samples variation in parameters of lightness, hue, and color intensity. Most of the color appearance on the three lowest Tc, namely: 2,000 K, 2,500 K, and 3,000 K; are significantly different toward their reference. It is caused by the spectral contents of LED lamp on those Tc tend to monochromatic, so the object’s color intensity appeared paler. Meanwhile the object’ lightness is getting increase on the lower Tc. It is related to the Helmholtz-Kohlrausch effect which occurred along the light source’s chromaticity shifting away from the white color. Overall, the Tc variation of LED lamp affected more on the colors with high intensity/saturation. It is caused by the spectral interaction which occurred on the tighter range compared to the dull colors, so the different of color appearance between Tc become larger. The results of this study can be used on interior lighting application with colored objects and background. The users still have freedom to choose the preferred LED lamp’s Tc, nevertheless it needs to be consider that the using of improper Tc affected towards the color appearance. The using of appropriate Tc certainly can make a color’s appearance become properly. The used spectral interaction method in this study, other than more basic, also suitable towards any possibilities of pairing spectrum between the light source and colored object.