Question:easy
What is the SI unit of brightness/illuminance?
What is the SI unit of brightness/illuminance?
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Recall which photometric unit measures lumens falling per square metre of surface area.
Updated On: Jun 23, 2026
- Candela
- Luminance
- Lumen
- Lux
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The Correct Option is D
Solution and Explanation
Photometry units -- distinguishing the options:
In photometry (the science of measuring light as perceived by humans), four key quantities exist:
Medical relevance: In hospital/workplace settings, recommended illuminance levels are expressed in lux. For example, operating theatres require 10,000-40,000 lux, general wards require 100-300 lux.
Since the question asks for the SI unit of illuminance (brightness of a surface), the answer is lux.
\[\boxed{\text{Lux (lx)}}\]
In photometry (the science of measuring light as perceived by humans), four key quantities exist:
- Luminous intensity: How bright a source is in a given direction -- unit = $\text{candela (cd)}$ (SI base unit)
- Luminous flux: Total light emitted in all directions per second -- unit = $\text{lumen (lm)}$
- Illuminance: Luminous flux received per unit surface area -- unit = $\text{lux (lx)} = \text{lm/m}^2$
- Luminance: Luminous intensity per unit projected area of source -- unit = $\text{cd/m}^2$
Medical relevance: In hospital/workplace settings, recommended illuminance levels are expressed in lux. For example, operating theatres require 10,000-40,000 lux, general wards require 100-300 lux.
Since the question asks for the SI unit of illuminance (brightness of a surface), the answer is lux.
\[\boxed{\text{Lux (lx)}}\]
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