Neutral References for Digital Camera Calibration

Almost all photographers are very familiar with using an 18% reflectance gray card to set the camera's exposure. Usually these are used by placing the gray card into the scene and pointing the meter or camera at the card and setting the shutter speed and aperture for the exposure. The color balance needed to achieve neutral gray tones in the image is determined by the film manufacturer, set in the balance of the cyan, magenta and yellow dyes in the film. The closest comparison to the film emulsion in a digital camera is the sensor. However, unlike film, the digital sensor is not created with a particular color balance. The balance is determined by the camera's circuitry. Some digital cameras have predetermined color balances for different lighting situations such as direct daylight, indirect daylight, flash, tungsten or fluorescent illumination, or a method for automatically adjusting the color balance.

Most, if not all, of the professional digital cameras, and many of the high-end consumer (also known as "prosumer") digital cameras, have a method for setting a custom neutral balance. This allows the photographer to adjust to any lighting situation, not just the built-in ones. To make this color balance, also known as a white balance or gray balance, a neutral gray or white card is placed in the scene and the camera analyzes its measurement of the card to make the settings.

Some camera programs can use several measurements, either from a single gray or from a series of grays, to set the balance. Better Light ViewFinderT software is an example of a camera control program that can use up to 12 gray points to set the balance.

Since the image's tone is dependent on the card's reflected light, the selection of a neutral reference is very important. There is no standard for manufacturing these references. Each manufacturer is free to make the neutral colorant from whatever they want. Some of the neutral references sold for setting the balance are made with paints, dyes, inks, plastics and cloth. Each type of colorant and substrate has its advantages, and disadvantages, which will be examined.

Requirements

Neutral references have several requirements that can be used to evaluate their utility. They should work in a wide variety of lighting environments and still maintain their gray appearance. They must be physically durable, able to withstand the environmental and handling rigors normally associated with photography. They should last long enough to be economical. Here is a list of attributes necessary for a good neutral reference.

1. Uniform spectral response. A uniform spectral response is when the reflectance from the neutral material 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 material will continue to have a neutral appearance, there will be no color cast. This is the most important criteria for selecting a gray card.
   
2. Lightness. The gray should be in the correct reflectance range for the camera. Using very dark or almost black references can lead to erroneous camera settings due to inadequate light for the camera to get a good signal. Some cameras can use a white reference, some cannot. Some cameras can use either a gray or white reference. Check with the camera manual for the correct reflectance. Traditionally, film has used the 18% reflectance gray card since this approximately corresponds to the midpoint of the human visual response. Most digital cameras allow using these cards and this is a good point to start.
   
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 neutral reference with a fluorescent "bluing" agent will have its color 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 neutral reference.
   
4. Minimal specular reflections. One very important characteristic is the texture of the neutral reference surface. Glossy materials produce reflections of the surroundings that can influence the camera's balance. 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 gray 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 neutral reference that polarizes the light will interact with the polarized lighting to result in a change of lightness or color as the chart is rotated. Polarizing neutral references cannot be used with polarized lighting. However, a polarizing neutral reference 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 neutral reference.
   
7. Thermal color stability. Some colorants can change their appearance as the temperature changes. Since some heating of the neutral reference is expected when it is being exposed on the set or outdoors, the reference 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 neutral reference. When the reference must be used in a hot environment the material should be physically durable. Many products 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 neutral reference should not burn, melt, curl, or otherwise physically deform with normal heating.
   
9. Durability. Any neutral reference must be able to withstand the rigors endured during photographic sessions. References that scratch easily, crease when slightly flexed, or otherwise effect the gray surface during use will not last long. This would be, at the least, a bother and possibly a financial burden to the photographer.

Neutral References Tested

* The Controlcard has a gray and a white surface. The white surface does have an UV brightener.

Updated 17.5.2007

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