Question

What is the SI unit of brightness/illuminance?

• A. Candela
• B. Luminance
• C. Lumen
• D. Lux

Hint:

Recall which photometric unit measures lumens falling per square metre of surface area.

Answer 1

The Correct Option is D

Step 1: Define the terms.

• Illuminance: The amount of luminous flux (light) falling on a unit area of surface. It measures how much light illuminates a surface.
• Luminous intensity: The power of light emitted per unit solid angle -- measured in candela (cd).
• Luminous flux: Total quantity of light emitted per unit time -- measured in lumen (lm).
• Illuminance: Luminous flux per unit area -- SI unit is lux (lx), where \(1 \text{ lux} = 1 \text{ lm/m}^2\).

Step 2: Identify the SI unit for illuminance. Lux (lx) is the SI derived unit of illuminance and luminous emittance. It is defined as lumens per square metre: \[1 \text{ lux} = \frac{1 \text{ lumen}}{1 \text{ m}^2}\]

Step 3: Eliminate distractors.

• Candela: SI unit of luminous intensity, not illuminance.
• Luminance: Photometric measure of luminous intensity per unit area -- unit is candela per square metre (cd/m2), not an SI base unit name.
• Lumen: SI unit of luminous flux, not illuminance.

Conclusion: The SI unit of illuminance (brightness of a surface) is Lux.