Illuminance: The amount of light incident on a surface, measured in terms of flux per unit area.

Light Intensity: The amount of light emitted by a source in a specific direction.

Symbol and Units

Illuminance

• Lux (lx): The standard unit for illuminance, representing one lumen per square meter.
• Foot-Candle (fc): A unit of illuminance used mainly in the United States, where one foot-candle is approximately 10.764 lux.
• Phot (ph): An older unit of illuminance, where one phot is equivalent to one lumen per square centimeter.

Light Intensity

• Candela (cd): The standard unit for light intensity, representing the amount of light emitted by a source in a specific direction.
• Lumen per Steradian (lm/sr): Represents the luminous intensity of a source in a specific direction, where one lumen per steradian equals one candela.
• Watt per Steradian (W/sr): A unit used in radiometry to describe the radiant intensity of a source, representing the power radiated in a specific direction.

Formula

Illuminance (E): \( E = \frac{\Phi}{A} \), where \( \Phi \) is the luminous flux and \( A \) is the area.

Light Intensity (I): \( I = \frac{\Phi}{\Omega} \), where \( \Phi \) is the luminous flux and \( \Omega \) is the solid angle in steradians.

Key Points

• Illuminance measures the amount of light incident on a surface, while light intensity measures the amount of light emitted in a particular direction.
• Lux is the SI unit for illuminance, and foot-candles are commonly used in the US. Phot is largely obsolete.
• Candela is the SI unit for light intensity, with lumen per steradian being equivalent to candela, and watt per steradian used for radiometric measurements.

Applications

• Illuminance: Lighting design, photography, and safety standards.
• Light Intensity: Measurement of light sources, display technology, and lighting engineering.

Material Variance

• Different surfaces reflect light differently, affecting the measured illuminance.
• Light intensity measurements can vary based on the angle and distance from the source.

Examples

Example 1: Convert 500 lux to foot-candles:

500 lux = 500 / 10.764 ≈ 46.45 foot-candles

Example 2: Convert 100 cd to lumen per steradian:

100 cd = 100 lm/sr