brightness


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brightness

The intensity of light or other radiation emitted from – absolute or intrinsic brightness – or received from – apparent brightness – a celestial body, the latter decreasing as the distance from the body increases. Intrinsic brightness is directly related to the luminosity of a body in a given spectral region. Apparent brightness is considered in terms of apparent magnitude: a star one magnitude less than another is about 2.5 times brighter. If two stars belong to the main sequence then the brighter star is the hotter of the two. See also radio brightness.

Brightness

The amount of light energy reflected from a surface. The degree of brightness depends on the color value and texture on its surface. The surface brightness of a task should be the same as its background or brighter. The maximum brightness ratio should be 3:1, and the brightness between the task and the darkest background area should be 5:1 to avoid objectionable glare.

Brightness

 

(also surface brightness), in astronomy, a characteristic of the emittance or reflectance of the surface of a celestial body. The brightness of faint celestial sources is expressed in terms of the number of stars of a given stellar magnitude in an area measuring 1 square second of arc (arcsec2), 1 square minute of arc (arcmin2), or 1 square degree (deg2). In other words, the illuminance from such an area is compared with the illuminance produced by a star of known stellar magnitude.

The brightness of the moonless night sky in clear weather, which is equal to 2 × 10–8 stilbs (sb), is characterized by one star of stellar magnitude 22.4 per arcsec2, or one star of stellar magnitude 4.61 per deg2. The brightness of an average nebula is equal to one star of stellar magnitude 19–20 per arcsec2. The brightness of Venus is equal to about one star of stellar magnitude 3 per arcsec2. The brightness of an area of 1 arcsec2 over which the light of a zero-magnitude star is distributed is equal to 9.25 sb. The brightness of the center of the solar disk is equal to 150,000 sb; that of the full moon, to 0.25 sb.

A surface for which the brightness does not depend on the angle of inclination of the area to the line of sight is said to be orthotropic. The flux emitted by such a surface behaves in accordance with Lambert’s law and is called the luminance. The unit of luminance is the lambert, which corresponds to a total flux of 1 lumen/cm2.

D. IA. MARTYNOV

brightness

[′brīt·nəs]
(optics)
The characteristic of light that gives a visual sensation of more or less light.

brightness

That attribute of visual perception in accordance with which a surface appears to emit more light or less light. Now called luminance.

Brightness

Alpha Centauri
brightest star in Centaurus constellation; closest star to Earth. [Astronomy: NCE, 74]
diamond
April birthstone, most reflective of gems. [Gem Symbolism: Jobes, 440–441]
North Star
bright star visible to naked eye and nearest to the north celestial pole. [Astronomy: EB, VIII: 79]
Sirius
dog star; brightest star in the heavens. [Astronomy: EB, IX: 238]
St. Elmo’s fire
glow of electrical discharge appearing on towers and ships’ masts. [Physics: EB, VIII: 780]
Venus
bright planet, second from the Sun. [Astronomy: EB, X: 392]

brightness

(graphics)
(Or "tone", "luminance", "value", "luminosity", "lightness") The coordinate in the HSB colour model that determines the total amount of light in the colour. Zero brightness is black and 100% is white, intermediate values are "light" or "dark" colours.

The other coordinates are hue and saturation.

brightness

The black level of a display screen. Although it may sound peculiar, the brightness adjusts the "black level" of the display system (how black the black is). See contrast.
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