This application note will show how all the colors within the color gamut formed by red, green, and blue constants in a CIE plot can be measured and mapped with an RGB color sensor. This RGB sensor can also monitor the color output of LEDs in a display and/or provide feedback to maintain a reference color. An RGB sensor will also be mapped to measure the color temperature of practical light sources.
The same process can be achieved more directly using the methods of the ICC color managment in which RGB sensor values are mapped to CIE XYZ by a 3x3 matrix as shown here and then by 1D LUTs to correct for the errors in the 3x3, which can be substantial.
In the description of color temperature the adjectives for the visual quality of the light are reversed. The solar disk + sky light, sometimes call noon daylight has a correlated color temperature of around 5000K and that is nearly spectrally neutral. Daylight with correlated color temperatures around 6500K will appear bluish and are often termed north sky light or overcast daylight. Daylight in shadow regions can often be recorded to have a CCT of 10,000K or more. This is the challenge of traffic safety sign manufacturers - maintaining the discernable color from sunset conditions (2300K) to shadow conditions (15,000K)
The problem is that if the light source spectrum changes with these RGB based sensors, which do not have an actual human eye response curve, the sensor looses accuracy, unless you re-calibrate and create another compensation matrix for each spectral source of light. This is a common problem with devices that use cheap RGB color filters which result in considerable inaccuracy in applications where it would see different spectrums. Even LEDs shift their spectral content with temperature, Red usually being among the worst.
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4 comments
write a commentPulk Posted Feb 16, 2013
Very great idea, hope they will apply this technology to new TV soon. Giuong go tu nhien
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ColorMan Posted Mar 13, 2013
The same process can be achieved more directly using the methods of the ICC color managment in which RGB sensor values are mapped to CIE XYZ by a 3x3 matrix as shown here and then by 1D LUTs to correct for the errors in the 3x3, which can be substantial. In the description of color temperature the adjectives for the visual quality of the light are reversed. The solar disk + sky light, sometimes call noon daylight has a correlated color temperature of around 5000K and that is nearly spectrally neutral. Daylight with correlated color temperatures around 6500K will appear bluish and are often termed north sky light or overcast daylight. Daylight in shadow regions can often be recorded to have a CCT of 10,000K or more. This is the challenge of traffic safety sign manufacturers - maintaining the discernable color from sunset conditions (2300K) to shadow conditions (15,000K)
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Doug.Leeper Posted Apr 3, 2013
The problem is that if the light source spectrum changes with these RGB based sensors, which do not have an actual human eye response curve, the sensor looses accuracy, unless you re-calibrate and create another compensation matrix for each spectral source of light. This is a common problem with devices that use cheap RGB color filters which result in considerable inaccuracy in applications where it would see different spectrums. Even LEDs shift their spectral content with temperature, Red usually being among the worst.
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joey899244 Posted Apr 10, 2013
it will appear bluish and are often termed north sky light or overcast daylight. AT90CAN128
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