Gray card
Encyclopedia
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 8x10" cards and one 4x5" 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 (see metamerism
).
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 camera
s have a custom white balance feature. A photo of the gray card is taken and used to set white balace 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. For the casual user, a gray card is 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.
s, provide standard reference patterns with calibrated reflectance spectrum and color coordinates, for use in adjusting color rendering in a larger range of situations.
Middle gray
In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about half way between black and white on a lightness scale; in photography, it is typically defined as 18% reflectance in visible light....
reference, typically used together with a reflective light meter
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
, as a way to produce consistent image exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
and/or color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
in film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
and photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
.
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 8x10" cards and one 4x5" 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 (see metamerism
Metamerism (color)
In colorimetry, metamerism is the matching of apparent color of objects with different spectral power distributions. Colors that match this way are called metamers....
).
Application
A major use of gray cards is to provide a standard reference object for exposure determination in photography. A gray card is an (approximate) realisation of a Lambertian scatterer; its apparent brightness (and exposure determination) therefore does not depend on its orientation relative to the light source. By placing a gray card in the scene to be photographed, oriented at a defined angle relative to 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.In addition to providing a means for measuring exposure, a gray card provides a convenient reference for white balance, or color balance
Color balance
In photography and image processing, color balance is the global adjustment of the intensities of the colors . An important goal of this adjustment is to render specific colors – particularly neutral colors – correctly; hence, the general method is sometimes called gray balance, neutral 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 camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
s have a custom white balance feature. A photo of the gray card is taken and used to set white balace 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. For the casual user, a gray card is 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.
Limitations
A gray card is only useful for setting or correcting the balance of neutral colors. Other charts, such as various color chartColor chart
In color-related fields, a color chart is a flat, physical object colored with an arrangement of standardized color samples, used for color comparisons and measurements such as checking the color reproduction of an imaging system...
s, provide standard reference patterns with calibrated reflectance spectrum and color coordinates, for use in adjusting color rendering in a larger range of situations.
External links
- Contrary to common belief, light meters are engineered to expose for 12% gray. See the article "Meters Don't See 18% Gray" for more.