76
Soleil-PJ1Training
Slide 76
Terms and Definitions - 5
Lux, Lumen, Candela
The brightness of a lamp is generally measured in lumens, and
ambient brightness and light reflected from a screen is generally
expressed in lux.
Lumen
» This ANSI unit is used to measure the amount of light emitted from a
light source. In the ANSI procedure for measuring the brightness of a
source, the light is directed onto a screen. The area of the screen is
divided by 9, and the brightness at the center of each of these is
measured. The average of these is taken to be the brightness of the
source in lumens.
Lux (lx)
» This is a measure of the intensity of illumination. It can be expressed
as lumens/square meter.
» Outside on a sunny day, the intensity is 15,000 to 20,000 lux.
» Inside a room with electric light, it is 1,500 lux.
» In a room lit by candlelight, it is 2 lux.
Candela
» This is often used for projectors with a built-in light source, such as a
television. It is normally expressed as candelas per square meter. The
candela is a measure of the strength of a light source, and cd/m2
expresses its brightness.
» cd/m2 = (lumens x screen gain)/(screen area m2 x
π
)
» Screen gain: Brightness increases with screen size.
Example of conversion between lumen, lux, candela
¾
1. From lumen (lm) to lux (lx)
For a 40-inch screen with a 4:3 aspect ratio, the area is about 0.5
m2.
For 1000 ANSI lumens, the amount in lux is: =1000/0.5, = 2000 lux
¾
2. From lumen (lm) to candelas (cd/m2)
For a 40-inch projector with a screen gain of 2 (double that of an
OHP projector screen), cd/m2 is:
(1000x2)/(0.5
×
3.14) = 2000/1.57 = 1274 cd/m2
