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Perfect pictures can only be shot with perfect exposure and color balance.
Both can only be achieved using a quality Gray Card with a uniform spectral reflectance, regardless of wavelength, color or intensity of the illumination.
Some hobby photographers hold that any white or gray surface, such as a white piece of paper, a concrete or stone wall, or a white shirt are suitable substitutes for a gray card; however, since bright white papers and clothing washed in typical detergents contain fluorescent whitening agents, they are never spectrally neutral. Using them for white balancing, you do not get color balance, but just another kind of color cast.
No matter how difficult the lighting situation is, with Digital Grey Cards you can determine the best exposure and white balance setting.
DGC-l large set of Digital Grey Cards:
1 card 6x8 inches (15x20 cm)
1 card 8x10 inches (20x25 cm)
DGC-l Small set of Digital Grey Cards:
1 card 4x6 inches (10x15 cm)
1 card 6x8 inches (15x20 cm)
Gray Cards have a defined, nearly perfectly flat spectral reflectance curve (the most relevant criterion of a Gray Card):
In the spectrophotometry curve above you can see, that the gray side of the cards has a constant reflectance of 18% across the whole spectrum of the visible light. The relatively low reflectance of only 18% makes sure, that there will be no clipping during white balancing, even in very bright lighting.
On the other hand, the white backside of the cards has a just as constant reflectance of 92% across the whole spectrum of the visible light. In poor lighting this side should be used for white balancing, to make sure that the threshold of the sensor for an optimal white balancing is reached.
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