Tag Archives: gray card

Color, color science, correlated color temperature

White Point, CCT and Tint

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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 response of the retina of a typical viewer, otherwise known as the CIE Color Matching Functions of a Standard Observer (\hat{x},\hat{y},\hat{z} in the plot)

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 we get three values that represent the illuminant’s coordinates in the XYZ color space.  The White Point is obtained by dividing these coordinates by the Y value to normalize it to 1.

For example a Standard Daylight Illuminant with a Correlated Color Temperature of 5300 kelvins has a White Point of[1]

XYZn = [0.9593 1.0000 0.8833]

assuming CIE (2012) 2-deg XYZ “physiologically relevant” Color Matching Functions from cvrl.org. Continue reading White Point, CCT and Tint →

adobe camera raw, Color, LR, Processing, Sharpening

How Is a Raw Image Rendered?

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What are the basic low level steps involved in raw file conversion?  In this article I will discuss what happens under the hood of digital camera raw converters in order to turn raw file data into a viewable image, a process sometimes referred to as ‘rendering’.  We will use the following raw capture by a Nikon D610 to show how image information is transformed at every step along the way:

Nikon D610 with AF-S 24-120mm f/4 lens at 24mm f/8 ISO100, minimally rendered from raw as outlined in the article.
Figure 1. Nikon D610 with AF-S 24-120mm f/4 lens at 24mm f/8 ISO100, minimally rendered from raw by Octave/Matlab following the steps outlined in the article.

Rendering = Raw Conversion + Editing

Continue reading How Is a Raw Image Rendered? →