Units of Light

 

NOTE: Please check units for luminance before using in critical applications.

 

Light Parameter	Term	SI Unit
light source	light source	lumen, candela steradian, candle
light incident on a surface	illuminance	lux
light reflected from a surface	luminance	lambert

(from Table 6.1, Miller and Sarmiento, 1999)

 

 

Light Source

 

Lumen (lm): The luminous flux through a unit-solid angle (steradian) from a uniform point source of one candle

(1) 1 lumen = 1 candela (cd) steradian = 1 candle

 

Illuminance

 

Lux (lx): 1 lumen per square meter

(1) 1 lumen per square meter = 1 meter-candle = 1 lux

(2) 1 lumen per square meter = 0.0929 foot-candle

 

Foot-candle:

(1) 1 foot-candle = 1 lumen per square foot = 10.76 meter-candle

 

Phot:

(1) 1 phot = 1 lumen per square centimeter

(2) 1 lumen per square meter = 0.0001076 lumen per square centimeter

 

	Output
Input (below)	lm per sq meter	lm per sq cm	lm per sq foot
lm per sq meter	1	10^(-4)	0.0929
lm per sq cm	10^4	1	929
lm per sq foot	10.76 = 1/0.0929	1/929	1

 

Luminance

 

Lambert:

(1) 1 lambert = (1/π) * (1 candle per square centimeter) = 0.3183 candles per square centimeter

(2) 1 lambert = 3183 candles per square meter

(3) 1 lambert = 295.4 candles per square foot

(4) 1 lambert = 2.05 candles per square inch

 

Foot-lambert:

(1) 1 foot-lambert is the luminance of a perfectly diffusing and reflecting surface illuminated by 1 candle at a distance of 1 foot.

(2) 1 foot-lambert = 0.3183 candles per square foot

(3) 1 foot-lambert = 0.00221 candles per square inch

(4) 1 foot-lambert = 0.0003426 candles per square centimeter

(5) 1 foot-lambert = 0.001076 lamberts

(6) 1 foot-lambert = 3.425 candles per square meter

 

where:

• 1 meter = 3.28 feet; 1 square meter = 10.76 square feet; 1 / (square meter) = 0.0929 / (square feet)

• 1 square meter = 10^4 square centimeters

• 0.3183 = 1 / π

• 0.2919 = 1/ (3.426)

• 1 square foot = 144 square inches

• 1 square inch = 6.45 square centimeters

 

	Output
Input (below)	lambert	foot lambert	candle per sq inch
lambert	1	1/(0.001076)	2.05
foot lambert	0.001076	1	0.00221
candle per sq inch	1 / 2.05	1 / (0.00221)	1