A gray card is a middle gray reference, typically used together with a reflective light meter, as a way to produce consistent image exposure and/or color in film and photography.
A gray card is a flat object of a neutral gray color that derives from a flat reflectance spectrum. A typical example is the Kodak R-27 set, which contains two 8×10″ cards and one 4×5″ card which have 18% reflectance across the visible spectrum, and a white reverse side which has 90% reflectance. Note that flat spectral reflectance is a stronger condition than simply appearing neutral; this flatness ensures that the card appears neutral under any illuminant.
A major use of gray cards is to provide a standard reference object for exposure determination in photography. A gray card is an (approximate) realization of a Lambertian scatterer; its apparent brightness (and exposure determination) therefore depends only on its orientation relative to the light source. By placing a gray card in the scene to be photographed, oriented toward the direction of the incident light, and taking a reading from it with a reflected light meter, the photographer can be assured of consistent exposures across their photographs. This technique is similar to using an incident meter, as it depends on the illuminance but not the reflectivity of the subject. (Of course taking photographs with side lighting or back lighting implies that the gray card should be oriented toward the camera instead.)
In addition to providing a means for measuring exposure, a gray card provides a convenient reference for white balance, or color balance, allowing the camera to compensate for the illuminant color in a scene.
Gray cards can be used for in-camera white balance or post-processing white balance. Many digital cameras have a custom white balance feature. A photo of the gray card is taken and used to set white balance for a sequence of photos. For post-processing white balance, a photo of the gray card in the scene is taken, and the image processing software uses the data from the pixels in the gray card area of the photo to set the white balance point for the whole image.
Most digital cameras do a reasonable job of controlling color but they might get it wrong. For the casual user, a gray card is mostly unnecessary. Many serious photographers or hobbyists consider gray cards an essential part of the digital photography process.
Gray cards are made of a variety of materials including plastic, paper, and foam. Some photographers hold that any neutral 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 tend to not be very spectrally neutral. Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.
Almost all light meters are calibrated to provide settings that apply to a scene with average brightness distribution. The brightness distribution in the motif always determines an integral value.
However, if a subject is not average (eg white rabbit in the snow) or “an almost black object against a black background”, the values have to be corrected, otherwise there will be incorrect measurements and the resulting photo does not correspond to the light situation of the scene. In the case of the white rabbit in the snow, the exposure would be underexposed since the light meter will reduce the exposure to a value for a scene of medium brightness (the result would be a gray rabbit in front of gray snow).
By means of a gray card, this error can be compensated by placing it as close as possible to the object and measuring the card in full format with the light meter (object measurement). This measurement should be diffuse. This can be achieved by disarming. Alternatively, exposure measurement by light metering (handheld exposure meter with attached (diffuser) cap towards the camera or “away from the subject”) would provide nearly the same exposure value. It would stay with the “white rabbit in the snow”.
Another tool for very precise metering is a spot meter, with measuring angles – depending on the design – between 1 ° and 10 °.
In the first picture you can see a milk carton with distorted colors that was taken under artificial light. In the second image, a gray card for manual white balance was positioned in front of the milk carton in the same light situation. After the camera has been calibrated accordingly, in the third picture a correct color can be seen.
White balance and color balance
In digital photography, the gray card can also be used for white balance when the gray card is coated with a metameric-free color; d. h., It must always remain neutral in color with different light types or colors. Any light situation recorded with such a gray card can be reworked very well with regard to the manual white balance. The map serves as a reference for the image processing software to determine the color temperature of the light.
Color gray card
In the color gray card, two additional density fields and six color fields with defined minimum deviating color densities (0.05 D) are applied. As a result, the card looks like a traffic light. This facilitates the color cast assessment in color vision defects, printing, computer-aided image processing or z. As in fatigue in the laboratory. If, on the one hand, two fields are not so well recognizable, then the opposite field is to be seen more strongly, or vice versa.
All fields must always be equally visible. If not, there is a color cast or the receiving film has a sensitizing gap and is unsuitable.
To adjust screens, a monitor calibration is performed with a so-called “target”.
A gray card is useful for setting or correcting the balance of neutral colors, as well as for exposure. Other charts, such as various color charts, provide standard reference patterns with calibrated reflectance spectrum and color coordinates, for use in adjusting color rendering in a larger range of situations.
Some people consider 13% of reflectance more appropriate than the traditional 18% for photographs under sunlight. Even the most frosted surfaces tend to reflect the light with more intensity towards the virtual axis of reflection. In addition the actual surfaces tend to be curved, which increases the shape contrast of the objects. Also the inclusion of shadows contributes to the decline of the exposure meter.
Kodak continues to recommend the value of 18% for a technically correct exposure value, which is the most commonly used value for studio photography, including still photography.
18% gray card and “digital” gray cards
Since the market for digital photographic material has been offering a new gray card (the “gray” card) specific to color balance, the medium gray card has been used solely to calibrate the exposure value (EV) of digital cameras.
The new cards feature a gray to a point and a half lighter than the gray 18%.
On a sRGB ashes scale that can be seen on a common monitor, the relative gray 18% gray corresponds to the gray 50% positioned in the center of the scale. The gray gray cards have a gray around 70%, corresponding more or less to a relative brightness of 50%.
SRGB Gray Level Scale
0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
A “gray” gray card is too light compared to gray 18% to be used as a standard to set a correct exposure for shooting. But light gray usually has fewer electronic “noises” than a darker gray like 18% gray. Moreover, the fact that it is noticeably clearer does not disturb the white balance on digital cameras, and it also does not disrupt subsequent processing in graphic editors as they compensate for brightness automatically.
Source From Wikipedia