Wednesday, August 4, 2010

Sony DSC S50 Response for Normal Exposure

The image of the spectrum for incandescent light looks closer to normal if no exposure bias is used. The camera's RGB color model was designed to work this way. The colors missing from the spectrum are on the left and right ends. The eye see a faint violet at about 400 nm and a richer red about 750 nm. To me there appears to be less yellow and cyan in the true spectrum. The CIE 1931 Color Space which models a standard observer shows more of a "red response" to blue light which seems to be missing from the camera's response.

Note that the RGB response curves are clipped near the peaks giving a flatter response.

The result is that the intensity has two peaks in it corresponding to yellow and cyan which are brighter by nature. The colors red, green and blue are the corners of the color cube which are closer to black while yellow, cyan and magenta are corners closer to white and hence brighter. Physically the energy density at each wavelength in incandescent light is about the same for thermal radiation at 3000 °K and one would expect a similar response at all frequencies. The eye is most sensitive to green and one would expect a stronger response there.

The conclusion is that the camera does not respond in the same way to light that is underexposed as it does to a normal exposure. The colors in shadows may be shifted somewhat. One does not see the colors yellow or cyan in the underexposed spectrum. The underexposed images give a better indication on how the RGB filters work.

edit: Photopsins are the protein-pigment complexes which determine the human eye's response to light.

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