A "Foot Candle" is a standard unit, established as reference,
that is used when measuring quantity of light. As lighting professionals
we need to know "how much" light we are working with. Like an
inch on a ruler, a reference unit to measure physical objects, the foot
candle is a reference unit for a non-physical element - light.
One "Foot Candle"
equals the total intensity of light that falls upon a one square foot surface
that is placed one foot away from a point source of light that equals 1
candle power.
Note: The term "Candela"
also refers to candle power. 1 Candela = 1 candle power.
Figure 1: This
illustrates the 1 square foot surface area of illumination that is created
by a light source that equals 1 candle power at a 1 foot distance. The total
illumination that falls on this one square foot surface equals 1 foot candle.
Figure 2: The
1 square foot surface area is exactly 1 foot in distance from the light
source. The surface area is actually semi-spherical to keep all sections
1 foot in distance from the light source.
A light meter is used to measure the foot candle level of a scene. A light
meter incorporates a photo-sensitive cell that creates an electrical current
when light falls upon it.
The current increases as the level of light falling on it is increased.
The meter will display the foot candle level in either a digital form or
as a needle moving along an incremental scale. The meter uses the electrical
current level as a reference to the level of foot candles that is falling
onto the photo-sensitive cell. The higher the current, the higher the foot
candle reading.
The metric level for light level is known as "Lux".
One lux equals the total intensity of light that falls on a 1 square
metre surface that is placed 1 metre away from a point source of light that
equals 1 candle power.
