Color Charts for Digital Camera Profiling

A color chart is a necessary tool for any digital photographer involved in product photography, fine art reproduction, or any photographic endeavor where matching image colors to real world colors is important. A color chart with good gray patches allows the chart to be used for setting the neutral balance for cameras using spot balancing or with image processing programs, such as Photoshop. The color chart is also useful for creating a color management profile.

Requirements

When a color chart is used for neutral balancing it is subject to all the same requirements of a gray card. For color management profiling there are additional constraints. Here is a list of attributes necessary for a good color chart.

1. Grays with uniform spectral response. A uniform spectral response is when the reflectance from the gray patches is the same, regardless of the wavelength, or color, of the illumination. Having a uniform, or flat, spectral response means that as the lighting changes, the gray will continue to have a gray appearance, there will be no color cast.
2. Good color space sampling. The color space sampling helps determine the quality of a color profile. Too few colors means the profile has to estimate too much of the color space. Too many colors and the profile may be unnecessarily large without any extra benefit for the storage and processing expense.
3. No UV induced optical brighteners. Many paper substrates have optical brighteners added to increase their apparent whiteness. The cellulose fibers comprising paper have a natural yellow color that is bleached during manufacturing, but some slight yellow remains, or can return as the paper ages. To counteract this yellowing, a "bluing" agent is added to paper that converts invisible ultraviolet light to visible blue light, making the paper appear to be whiter. Any color chart with a fluorescent "bluing" agent will have its colors change appearance as the amount of ultraviolet light in the illumination changes. With time, these "bluing" agents can fade, letting the yellow paper color return and shortening the effective lifetime of the color chart.
4. Minimal specular reflections. One very important characteristic is the texture of the color chart surface. Glossy materials produce reflections of the surroundings that can change a patch's color. These reflections can be controlled in studio environments by adjusting the lighting or blocking the reflections, but this is not possible outdoors. Matte finish materials produce a uniform appearance without the reflectance issues of glossy colors.
5. No polarization. In many fine art reproduction situations, it is necessary to use polarized or cross-polarized techniques to remove unwanted specular reflections. Any color chart that polarizes the light will interact with the polarized lighting to result in colors that will change lightness as the chart is rotated. Polarizing color charts cannot be used with polarized lighting. However, a polarizing color chart may be perfectly useful in unpolarized situations.
6. Ultraviolet light stability. Natural sunlight and some types of artificial lighting, such as HMI, emit large amounts of ultraviolet radiation. Exposure to ultraviolet light can cause a breakdown of chemical bonds in the colorants and other materials in the chart. This can result in fading, color shifts, yellowing, or sometimes physical changes of the color chart.
7. Thermal color stability. Some colorants can change their appearance as the temperature changes. Since some heating of the chart is expected when it is being exposed on the set or outdoors, the chart colorants must not lighten, darken or become discolored with changes in temperature.
8. Thermal durability. Many photographic situations require hot lights for illumination or will be exposed in sunlight, thereby heating the color chart. When the chart must be used in a hot environment the material should be physically durable. Many charts are produced by applying the colorant to a thin substrate material to create a patch, then affixing the patches to a more durable mount. The glue used should not soften or release the patches when enduring photographic situations. The chart should not burn, melt, curl, or otherwise physically deform with normal heating.
9. Durability. Any chart must be able to withstand the rigors endured during photographic sessions. Charts that scratch easily, crease when slightly flexed, are easily dented, or otherwise effect the color patches during use will not last long. This would be, at the least, an annoyance, and possibly a financial burden to the photographer.

ColorCharts Tested

Updated 12.4.2007

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