Quantitative Properties of Light: The Measurement of Light
In the study of light, it is important to understand basic physical concepts and terminology. These properties help in measuring and analyzing how light interacts with different surfaces and environments.
1. Luminous Flux (Φ)
Definition: The total quantity of light emitted by a source, regardless of direction or distribution.
Unit: Lumen (lm)
Analogy: Similar to the flow of water in plumbing.
Key Point:
A lamp receives watts and emits lumens.
Efficacy is the measure of efficiency in converting energy into light, expressed in lumens per watt (lm/W).
As light travels outward, it is reflected, transmitted, or absorbed by surfaces.
2. Illuminance (E) – Illumination
Definition: The density of luminous flux incident on a surface.
Unit:
Lux (lx) = lumens per square meter (lm/m²)
Footcandle = lumens per square foot (lm/ft²)
3. Luminous Intensity (I)
Definition: The amount of luminous flux emitted in a particular direction.
Unit: Candela (cd)
Key Point: A light source emits luminous flux in different directions and intensities, making candela (cd) the standard unit to measure its intensity in a specific direction.
4. Luminance (L) – Brightness
Definition: The perceived brightness of an illuminated or luminous surface.
Unit: Candelas per square meter (cd/m²)
Key Point: Luminance determines how bright a surface appears to the human eye after reflecting or emitting light.
5. Luminous Exitance (M)
Definition: The density of luminous flux leaving (reflected or transmitted) from a surface.
Unit: Lumens per square meter (lm/m²)