grey sky | Strolls with my Dog

As we have seen in the previous post, knowing the characteristics of light at the scene is critical to be able to determine the color transform that will allow captured raw data to be naturally displayed from an output color space like ubiquitous sRGB.

White Point

The light source Spectral Power Distribution (SPD) corresponds to a unique White Point, namely a set of coordinates in the $XYZ$ color space, obtained by multiplying wavelength-by-wavelength its SPD (the blue curve below) by the Color Matching Functions of a Standard Observer ( $\hat{x},\hat{y},\hat{z}$ )

Figure 1. Spectral Power Distribution of Standard Daylight Illuminant D5300 with a Correlated Color Temperature of 5300 deg. K; and CIE (2012) 2-deg XYZ “physiologically relevant” Color Matching Functions from cvrl.org.

Adding (integrating) the three resulting curves up we get three values that represent the illuminant’s coordinates in the $XYZ$ color space. The White Point is then obtained by dividing these coordinates by the $Y$ value to normalize it to 1.

The White Point is then seen to be independent of the intensity of the arriving light, as $Y$ represents Luminance from the scene. For instance a Standard Daylight Illuminant with a Correlated Color Temperature of 5300k has a White Point of^[1]

$XYZn$ = [0.9593 1.0000 0.8833] Continue reading White Point, CCT and Tint →

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