lightintensity

Light intensity, in photometry, refers to luminous intensity, the directional measure of how much light a source emits in a given direction. It describes the strength of light per unit solid angle and is inherently directional: a source can have different intensities in different directions. The quantity is quantified in candelas (cd). Luminous intensity is a property of the source, not of a surface.

Luminous intensity is distinct from illuminance and irradiance. Illuminance measures the amount of light arriving at a surface, measured in lux (lumens per square meter), while irradiance measures radiant power per unit area, in W/m^2. For a point source with luminous intensity I, the irradiance on a surface at distance r, oriented perpendicular to the ray, is E = I / r^2. If the surface is tilted by angle θ, E = I cos θ / r^2. These relationships connect a directional source property to the light that actually reaches surfaces.

The candela is the SI unit of luminous intensity. It is defined by a reference light source emitting monochromatic radiation at 555 nanometers with a radiant intensity of 1/683 watt per steradian in a specified direction. Luminous intensity is weighted by the eye’s sensitivity, reflected in lumen and lux when converting to luminous flux and surface illumination. The lumen equals candela multiplied by the solid angle (lm = cd × sr), linking intensity to total emitted light.

In practice, lighting design uses intensity distributions to describe how brightness varies with angle. Measurements with goniophotometers characterize isotropic or anisotropic sources, such as lamps and LEDs. In everyday language, “light intensity” is often used to mean brightness, which aligns more closely with luminance or illuminance than with luminous intensity.