What is color luminance?
Luminance is a measure of how much visible light is emitted, reflected, or transmitted from a surface in a particular direction. In simple terms, it describes how bright an object appears to the human eye. Luminance is commonly measured in candelas per square meter (cd/m²), often called “nits” when referring to displays such as televisions, monitors, and smartphones. Higher luminance values generally make an image appear brighter, while lower values make it appear dimmer. It is an important concept in fields such as photography, display technology, lighting design, and vision science.
In color science, luminance refers to the physical, measurable amount of light associated with a color, weighted according to the human eye’s sensitivity to different wavelengths. It is still measured in units such as cd/m² when describing a surface or display.
Luminance in physics?
In physics, luminance describes the amount of visible light emitted, reflected, or transmitted by a surface in a specific direction. It is an objective and measurable property of light that quantifies how concentrated the light is over an area and viewing angle. Because luminance accounts for the sensitivity of the human visual system to different wavelengths, it provides a useful link between the physical characteristics of light and human visual perception. As a result, luminance is widely used in optics, lighting engineering, display technology, and vision science to evaluate and compare the visual intensity of different light sources and surfaces.
Luminance in color theory?
In color theory, luminance represents the lightness component of a color independent of its hue and saturation. It describes how much light a color contributes to a visual scene and plays a crucial role in determining contrast, readability, and visual hierarchy. Colors with different hues can have the same luminance and therefore appear equally light or dark, even though they are perceived as different colors. Understanding luminance is essential in image processing, digital graphics, and display design, where it is used to create balanced compositions, maintain visual clarity, and ensure accurate reproduction of colors across different devices.
Luminance vs brightness
Luminance itself is a pure physical concept, but is sometimes used interchangeably with brightness when talking about colors. Also when the topic is a mixture of physics, vision, and color theory, it can easily be misused. But there is a very clear difference between luminance as a pure physical concept and brightness as an eye-perceived property of a color. Here, we’ll have another concept, which is a modeled relative luminance based on color models, which we talk about when talking about luminance in color theory, not pure physics. The image below can describe it better:
Read how we calculate the luminance in Negarity Color library.
How to calculate luminance?
How to Calculate Luminance in Color Theory
In color theory, luminance is calculated from a color’s red, green, and blue (RGB) components using a weighted sum that reflects the varying sensitivity of the human eye to different wavelengths of light. Since the eye is most sensitive to green light and less sensitive to red and blue light, each color channel contributes differently to the final luminance value.
For colors represented in the sRGB color space, the relative luminance (Y) is calculated as:
(\(Y = 0.2126R + 0.7152G + 0.0722B\) )
where (R), (G), and (B) are the linearized red, green, and blue color components, each normalized to the range ([0,1]). The coefficients indicate the relative contribution of each channel to the perceived lightness of the color.
As an example, consider a pure red color ((R,G,B) = (1,0,0)). Its luminance is (Y = 0.2126). In contrast, a pure green color ((0,1,0)) has a luminance of (Y = 0.7152), showing that green appears significantly brighter than red despite having the same maximum channel intensity. This luminance value is commonly used in image processing, computer graphics, and accessibility standards to measure contrast and ensure accurate visual representation of colors.
References
- https://en.wikipedia.org/wiki/Luminance
- https://developer.mozilla.org/en-US/docs/Web/Accessibility/Guides/Colors_and_Luminance