Brightness
The perceived luminous intensity of each pixel. A fundamental quantity used in display calibration and image tonal adjustments.
Brightness (or luminance) quantifies the intensity of light emitted or reflected from a surface in a given direction. In imaging, it represents the perceptual lightness of a pixel. The SI unit is candela per square metre (cd/m²), commonly seen in display specifications such as "300 cd/m² peak brightness."
Several models define brightness in image processing:
- Relative Luminance: Defined by ITU-R BT.709 as
Y = 0.2126R + 0.7152G + 0.0722B. The weighting reflects human visual sensitivity, with green contributing most - HSB/HSV Brightness: Simply the maximum of the RGB channels:
V = max(R, G, B) - CIE L*: A perceptually uniform lightness derived from a non-linear transform of luminance, used in Lab colour space
Basic brightness adjustment adds or subtracts a constant from all pixel values. However, naive addition clips highlights (blowout) and subtraction crushes shadows. Professional tools use tone curves or gamma correction to redistribute tonal range while preserving detail in both extremes.
In web development, CSS filter: brightness() dynamically adjusts element brightness. A value of 1.0 is unchanged, 0 produces black, and 2.0 doubles brightness. This is widely used for dark-mode adaptations, hover effects, and image overlays without modifying source files.