Luminance: The amount of light emitted from a surface in a particular direction per unit area.

Brightness: The perceived intensity of light from a surface as observed by the human eye.

Symbol and Units

Luminance

• Candela per Square Meter (cd/m²): The SI unit of luminance, representing the amount of light emitted from a surface per unit area.
• Nits: Another term for candela per square meter, commonly used in display technology.

Brightness

• Foot-Lamberts (fL): A unit of brightness used in the United States, representing the amount of light reflected off a surface.
• Lumens per Square Meter (lm/m²): A unit for brightness, less common than candela per square meter for luminance.

Formula

Luminance (L): \( L = \frac{I}{A} \), where \( I \) is the intensity of light and \( A \) is the area.

Brightness (B): \( B = \frac{L}{\text{Reflectance}} \), where \( L \) is the luminance and Reflectance is the ratio of reflected light to incident light.

Key Points

• Luminance measures the amount of light emitted from a surface, while brightness is the perceived intensity of that light.
• Candela per square meter (cd/m²) is the standard unit for luminance, whereas nits are commonly used in displays.
• Foot-lamberts (fL) and lumens per square meter (lm/m²) are units used for brightness, with foot-lamberts being more common in the US.

Applications

• Display Technology: Measurement of screen brightness and clarity.
• Photography: Adjusting exposure settings to achieve desired brightness levels.
• Lighting Design: Ensuring adequate illumination in work and living spaces.

Material Variance

• Different materials reflect light differently, affecting the perceived brightness.
• High reflectance materials will appear brighter under the same lighting conditions compared to low reflectance materials.

Examples

Example 1: Convert 300 cd/m² to nits:

300 cd/m² = 300 nits

Example 2: Convert 50 fL to lm/m²:

50 fL = 50 × 3.426 × 10¹ lm/m² = 1713 lm/m²