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]

50% CIELAB lightness
(Y = 18.4%)

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 the 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 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 Albert Henry 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]

In 1933, Alexander Ector Orr Munsell (Albert Henry Munsell's son) found "that a series of neutral reflecting surfaces whose reflectances are...18.0...and 100.0 percent...form for the average observer a series of equal differences in value."[16]

From the 1930s onward various lightness curves were proposed, but halfway through the 20th century, the proposals settled on cube-root-based curves. In 1976 the International Commission on Illumination defined the CIELAB color space,[17] an extension of which would become the standard for the coming decades in a variety of applications.[18][19] Following research in the 80s and 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 based on various criteria.

The shades in the rightmost column will only be correct if viewed on a calibrated monitor.

Note that LCD screens, even when correctly calibrated, often have a brightness that varies considerably depending on the viewing angle.

Middle gray as defined byYearCIEXYZ relative luminance (Y)Appearance
Geomean of 60:1190312.91%
Munsell, Sloan & Godlove193318%
L*a*b*197618.42%
sRGB199621.40%
Munsell's original N5190625.00%
NCS S 5000-N196426%
Mac, pre-OS X 10.61984≈ 30%
CIECAM02 lightness2002*
CIECAM97s lightness1997*
50% Luminance193150.00%

Quarter luminance list (typical monitor calibrated to 2.2 gamma, 50% luminance gray has RGB hex 0xBA value, which is 0.5^(1/2.2)*255)

Middle gray as defined byYearCIEXYZ relative luminanceAppearance
100% Luminance1931100%
75% Luminance193175%
50% Luminance193150%
25% Luminance193125%
0% Luminance19310%

* 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.

References
  1. Stephen Quiller (1999). Painter's Guide to Color: Includes the New Quiller Color Wheel. Watson-Guptill. ISBN 0-8230-3913-7.
  2. Blain Brown (2002). Cinematography: Theory and Practice : Imagemaking for Cinematographers, Directors, and Videographers. Focal Press. ISBN 0-240-80500-3.
  3. Woods, Mark. How to Effectively Use the Gray Card Archived 2014-11-29 at the Wayback Machine. cameraguild.com
  4. Steven Barclay (1999). The Motion Picture Image: From Film to Digital. Focal Press. ISBN 0-240-80390-6.
  5. Film and Its Techniques by Raymond Spottiswoode
  6. Gevaert Manual of Photography, ed. A.H.S. Craeybeckx
  7. C. S. McCamy, H. Marcus, and J. G. Davidson (1976). "A Color-Rendition Chart". Journal of Applied Photographic Engineering 2(3). 95–99.
  8. Shoptalk: a new tool for checking color photography by Norman Goldberg
  9. How to Photograph Landscapes & Scenic Views by David Brooks
  10. Daniel Coit Gilman; Harry Thurston Peck & Frank Moore Colby (1903). The New International Encyclopædia. Dodd, Mead and Company.
  11. Kuehni, Rolf G. (February 2002). "The early development of the Munsell system". Color Research & Application. 27 (1): 20–27. doi:10.1002/col.10002.
  12. 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.
  13. 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. United States Bureau of Standards (3): 27.
  14. Munsell AEO, Sloan LL, Godlove IH. Neutral value scales I, Munsell neutral value scale. J Opt Soc Am 1933;23:394 – 401
  15. 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. United States Bureau of Standards (3): 27.
  16. Munsell AEO, Sloan LL, Godlove IH. Neutral value scales I, Munsell neutral value scale. J Opt Soc Am 1933;23:394 – 401
  17. CIE Colorimetry–Part 4: 1976 L* a* b* Colour Space
  18. H. Pauli, Proposed extension of the CIE recommendation on "Uniform color spaces, color difference equations, and metric color terms"
  19. Kenichiro Masaoka, Fu Jiang, Mark D. Fairchild & Rodney L. Heckaman, Analysis of color volume of multi-chromatic displays using gamut rings
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