This is why we suggest using the tone curve retrieved from the color profile of the source pictures instead of attempting to calculate a calibration curve. Since High Dynamic Range values represent the luminance of the scene, it is necessary to know the camera response curve so as to reconstruct an HDR image from photographs. For an RGB shade picture, 8-bit implies that every one of the three colour channels can be considered one of 256 levels of color. Since each pixel is represented by 3 colors in this case, 8-bit per color channel actually means 24-bit per pixel. There are numerous codecs obtainable to store HDR photographs, similar to Radiance RGBE (.hdr) and OpenEXR (.exr) among the many most commonly used. See Greg Ward’s HDR Image Encodings page for a superb overview of HDR formats.