Luminance and Photometric Measurement

Luminance quantifies the perceived brightness of a light source or a reflecting surface. Its measurement involves considering both the radiant intensity of the light and the sensitivity of the human eye to different wavelengths.

Photometric Units

• Candela (cd): The base unit of luminous intensity, representing the light emitted by a candle. It measures the power emitted in a specific direction.
• Lumen (lm): A measure of luminous flux, the total amount of visible light emitted by a source in all directions. One lumen is equal to one candela steradian.
• Lux (lx): A measure of illuminance, which describes the amount of luminous flux incident on a surface per unit area (lumens per square meter). It represents the intensity of light falling on a surface.
• Nit (nt): A measure of luminance, indicating the luminous intensity per unit area of a light-emitting surface (candela per square meter). This is often used to specify the brightness of a display screen.

Calculating Luminance

The calculation of luminance depends on the context. For a point light source, it involves the luminous intensity and the solid angle subtended by the observer. For a diffuse reflecting surface, it involves the illuminance and the reflectance properties of the surface. Specialized instruments, such as photometers and luminance meters, are used for accurate measurements.

Factors Affecting Luminance Perception

• Spectral Power Distribution (SPD): The relative distribution of light power across different wavelengths affects the perceived brightness. The eye is most sensitive to green light.
• Adaptation: The human eye adapts to different levels of ambient light, affecting the perceived brightness.
• Contrast: The perceived brightness of an object is influenced by the brightness of its surroundings.