Middle gray
In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about halfway between black and white on a lightness scale;[1] in photography and printing, it is typically defined as 18% reflectance in visible light.[2][3]
Light meters, cameras and pictures are often calibrated using an 18% gray card[4][5][6] or a color reference card such as a ColorChecker.[7][8][9] On the assumption that 18% is similar to the average reflectance of a scene, a grey card can be used to estimate the required exposure of film.[5]
History
Following the Weber–Fechner law, at the start of the 20th century human lightness perception was assumed to be logarithmic. In 1903, The New International Encyclopædia illustrated this concept by stating that given a black and a white with a luminance ratio of 1 : 60 ( : ) the geometric mean had to be used to find the middle gray.[10] That is equivalent to a relative luminance of 12.9% ( ).
When Munsell was developing his colour system, he tried to ascertain the relation between luminance and perceived lightness. As early as 1906, he wrote: 'Should we use logarithmic curve or curve of squares?'[11] In 1920 Priest, Gibson and McNicholas showed using both a König-Martens spectrophotometer and an apparatus designed by Gibson where the outputs of two photocells are balanced[12] that the shades in Munsell's 1915 atlas followed a square root curve, which was later confirmed by extensive experiments.[13][14] In Munsell's system, the shades of neutral gray were labelled N1 to N9, with N5 in the middle and 0 and 10 denoting the unachievable ideal black and perfect white.[15]
From the 1930s onward various lightness curves were proposed, but halfway the 20th century the proposals settled on cube-root based curves. In 1976 the International Commission on Illumination defined the CIELAB color space,[16] an extension of which[17] would become the standard for the coming decades in a variety of applications.[18] Following research in the 80s, in the 90s more and more advanced models of colour vision were developed, the first major step being CIECAM97s; see Lightness § 1933 for details.
Table of middle grays
Below are various "middle" grays as based on various criteria. The shades in the rightmost column will only be correct if viewed on a calibrated monitor; you can check if the 50% luminance level of your monitor is correct by viewing the checkered image in the center of the 50% Luminance gray on the rightmost column. If the checkered image does not appear to be of the same brightness, then the "middle grays" rendered in the table are not correctly displayed on your screen.
Note that LCD screens, even when correctly calibrated, often have a brightness that varies considerably depending on the viewing angle. When using the luminance level checker above, try stepping back and changing your position until the checkered image in the center of the gray field appears to dissolve into the background.
| Middle gray as defined by | CIEXYZ relative luminance | sRGB brightness | CIELAB lightness | sRGB coordinates |
Appearance |
|---|---|---|---|---|---|
| Geomean of 60:1 | 12.91% | 39.46% | 42.63% | rgb(101,101,101) or #656565 | |
| 18% gray card | 18% | 46% | 49% | rgb(118,118,118) or #767676 | |
| L*a*b* | 18.42% | 46.63% | 50.00% | rgb(119,119,119) or #777777 | |
| sRGB | 21.40% | 50.00% | 53.39% | rgb(128,128,128) or #808080 | |
| Munsell's N5 | 25.00% | 53.71% | 57.08% | rgb(137,137,137) or #898989 | |
| Mac, pre-OS X 10.6 | ≈ 30% | ≈ 58% | ≈ 62% | ≈ rgb(149,149,149) or #959595 | |
| CIECAM02 lightness | Context-dependent; an estimate is displayed here taking the environment where it appears in the article into account and assuming a neutrally lit surround of 200 cd/m2. |
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| CIECAM97s lightness | Context-dependent; an estimate is displayed here taking the environment where it appears in the article into account and assuming a neutrally lit surround of 200 cd/m2. |
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| 50% Luminance | 50.00% | 73.54% | 76.07% | rgb(188,188,188) or #BCBCBC | 
|
References
- ^ Stephen Quiller (1999). Painter's Guide to Color: Includes the New Quiller Color Wheel. Watson-Guptill. ISBN 0-8230-3913-7.
- ^ Blain Brown (2002). Cinematography: Theory and Practice : Imagemaking for Cinematographers, Directors, and Videographers. Focal Press. ISBN 0-240-80500-3.
- ^ Woods, Mark. How to Effectively Use the Gray Card Archived 2014-11-29 at the Wayback Machine. cameraguild.com
- ^ Steven Barclay (1999). The Motion Picture Image: From Film to Digital. Focal Press. ISBN 0-240-80390-6.
- ^ a b Film and Its Techniques by Raymond Spottiswoode
- ^ Gevaert Manual of Photography, ed. A.H.S. Craeybeckx
- ^ C. S. McCamy, H. Marcus, and J. G. Davidson (1976). "A Color-Rendition Chart". Journal of Applied Photographic Engineering 2(3). 95–99.
- ^ Shoptalk: a new tool for checking color photography by Norman Goldberg
- ^ How to Photograph Landscapes & Scenic Views by David Brooks
- ^ Daniel Coit Gilman; Harry Thurston Peck & Frank Moore Colby (1903). The New International Encyclopædia. Dodd, Mead and Company.
- ^ Kuehni, Rolf G. (February 2002). "The early development of the Munsell system". Color Research & Application. 27 (1): 20–27. doi:10.1002/col.10002.
- ^ First the photocells are illuminated by lamps and their outputs are balanced without the sample present; then the sample is placed at one cell and the lamp illuminating the other cell is moved further away until the outputs rebalance. For a more detailed description see K.S. Gibson, Photoelectric spectrophotometry using the null method.
- ^ Priest, Irwin G.; Gibson, K.S.; McNicholas, H.J. (September 1920). "An examination of the Munsell color system. I: Spectral and total reflection and the Munsell scale of Value". Technical Paper 167 (3). United States Bureau of Standards: 27.
- ^ Munsell AEO, Sloan LL, Godlove IH. Neutral value scales I, Munsell neutral value scale. J Opt Soc Am 1933;23:394 – 401
- ^ A.H. Munsell, Atlas of the Munsell color system, 1915
- ^ CIE Colorimetry–Part 4: 1976 L* a* b* Colour Space
- ^ H. Pauli, Proposed extension of the CIE recommendation on "Uniform color spaces, color difference equations, and metric color terms"
- ^ Kenichiro Masaoka, Fu Jiang, Mark D. Fairchild & Rodney L. Heckaman, Analysis of color volume of multi-chromatic displays using gamut rings