Evaluating input targets

There are a variety of color profiling targets available for characterizing input targets, and it is sometimes hard to decide which one will be best for your particular purpose. Listed here are some criteria to evaluate:

Color gamut

The resulting profile will only be accurate between or near the color values contained on the chart. This means that if the colors you are actually processing with your device go outside the gamut of your test chart, the color values will have been extrapolated by the profile, and are therefore likely to be not very accurate.

Color Resolution

The profile will be most accurate for colors that are near those contained on the chart. This means that the more closely and evenly spaced within the color gamut the value of
the test chart are, the more accurate overall the chart will be. So typically the greater the number of test values, the better.

Dynamic range and White point

Similar to color gamut, the profile will only be accurate over the range of lightness levels exercised by the chart. At the dark end the ideal black test value will be a light trap. At the white end the ideal white test value would be the perfect 100% reflective diffuser. In practice there is another consideration, which is that by default the white point of the profile is set by the white value of the test chart, and any values over this may be clipped by the profile. So ideally the white patches should represent the white value of the work you will be using the input device for.

Spectral similarity

One of the fundamental problems with colorimetrically characterizing input devices, is that typically input devices don't have the same spectral sensitivity as a human observer. This means that it "sees" color differently, and that there is no way to perfectly compensate for this in a device profile. [There will be some spectral values that look the same to the device but appear different to us, and visa-versa.] A colorimetric profile will best compensate for such differences when the target test colors have the same spectral reflectance characteristics as the the intended work. What this translates to is that you will get best results when the test chart uses a similar printing process to whatever work you will be using the input device for. So if you are intending to scan photographic prints, you should use a photographic based test chart. If you were scanning artworks, then you should use a test chart that has pigments that are similar to paint used on such artworks, etc. When characterizing camera's, an additional source of spectral differences is the illuminating light source used. Once again, it will be best to choose a light source to characterize the camera that is going to be most similar to the light source you will typically shoot photographs under.