# Lighting Units (Candela vs Lumens vs Lux)

Flux (Power) Watts Power/Wavelength Luminous Flux (Lumen)
Flux Per Area Intensity Spectral Intensity Intensity

Candela:

• Measures Luminous intensity aka luminous power in a given direction per unit solid angle

Lumens:

• Measures Luminous flux
• Photometric so it's radiant flux (power) weighted according to a luminosity function (modeling human eye's sensitivity to various wavelengths)
• $1 \operatorname{lm}=1 \ \mathrm{cd} \cdot \mathrm{sr}$

Lux:

• Measures illuminance aka luminous flux per unit area or how much light is falling on a surface
• Can also be used for luminous emittance which is emitted lumens (note: this is different from luminance)
• $1 \operatorname{lx}=1 \ \mathrm{lm} / \mathrm{m}^{2}=1 \mathrm{cd} \cdot \mathrm{sr} / \mathrm{m}^{2}$

Luminance:

• Measures luminous intensity reflected per unit area in specific direction
• $\mathrm{cd} / \mathrm{m}^{2}$

Luminous efficacy:

• measures efficacy of a light turning watts into lumens
• $\mathrm{lm} / \mathrm{W}$
Illuminance (lux) Surfaces illuminated by
0.0001 Moonless, overcast night sky (starlight)[3]
0.002 Moonless clear night sky with airglow[3]
0.05–0.3 Full moon on a clear night[4]
3.4 Dark limit of civil twilight under a clear sky[5]
20–50 Public areas with dark surroundings[6]
50 Family living room lights (Australia, 1998)[7]
80 Office building hallway/toilet lighting[8][9]
100 Very dark overcast day[3]
150 Train station platforms[10]
320–500 Office lighting[7][11][12][13]
400 Sunrise or sunset on a clear day.
1000 Overcast day;[3] typical TV studio lighting
10,000–25,000 Full daylight (not direct sun)[3]
32,000–100,000 Direct sunlight