Light – Units of Measure
When people first set out to quantify visible light they chose as their standard a source that was familiar and common to all of them: a candle. It had to be a specifically sized candle, made of a specific material and molded in a specific way, but an ordinary candle nonetheless.
The amount of light emitted from such a candle became our first and most fundamental unit of brightness. It is called 1 candlepower.
If we visualize such a candle lighted at a center of a darkened room, we can see that its energy is radiating equally in all directions and the farther we retreat from its flame the less light it appears to be shedding. To establish a unit of emitted light on a surface 1 foot-candle was introduced.
The noonday sun can deliver 10,000 foot-candles.The full moon can deliver only 0.02 foot- candles.Workspaces in offices and operating rooms have 15 or more foot-candles.For comfortably sustained reading it’s nice to have 10.The next step was deciding exactly what fraction of the energy in 1 candlepower is expanded in the production of 1 foot-candle. We need it this unit because when we come to consider light sources other than candles. We recognize that some of them, video projectors for example, do not radiate spherically but beam all their output in just one specific direction. The new unit is 1 Lumen.
The light coming from 1 square foot radiating 1 lumen is producing 1 foot-Lambert.
The intensity of 1 lumen/square foot equals 1 foot-candle.
1 foot-Lambert is the unit of luminance equal to 1 lumen/square foot.
Metric people use as their basic unit as 1 lumen/steradian (square meter) or 1 candela.
- 1 lumen/square meter = 1 lux
- 1 lumen/square cm = 1 phot
- 1 candela/square meter is also known as a nit.
Dictionary:
- Candle (cd) - the basic unit of luminous intensity equal to 1/60 of the luminous intensity per square centimeter of a black body radiating at the temperature of 2,046 degrees Kelvin.
- Foot Candle (fc) -a unit of illuminance on a surface that is everywhere 1 foot from a point source of 1 candle.
- Lambert (L) - a cgs unit of illumination equal to the brightness of a perfectly diffusing surface that emits or reflects one lumen per square centimeter.
- Lux (lx) - a unit of illumination equal to 1 lumen per square meter; 0.0929 foot candle.
- Lumen (lm) - the unit of luminous flux equal to the amount of light given out through a solid angle of 1 square foot by a point source of 1 candlepower intensity radiating uniformly in all directions.
ELECTRICAL FORMULAS
- Lighting System Efficacy (Lumens per Watt or LPW) = System Lumen Output ÷ Input Wattage
- Watts (W) = Volts (V) x Current in Amperes (A) x Power Factor (PF)
- Voltage (V) = Current in Amperes (A) x Impedance (Ohms) [Ohm's Law]
- Demand for Power (kW) = System Input Wattage (W) ÷ 1,000
- Energy Consumption (kWh) = System Input Wattage (kW) x Hours of Operation/Year
- Unit Power Density (W/sq.ft.) = Total System Input Wattage (W) ÷ Total Area (Square Feet)
DESIGN FORMULAS
- Foot-candles (fc) = Total Lumens (lm) ÷ Area in Square Feet
- Lux (lx) = 1 Foot-candle (fc) x 10.76
- Lux = Total Lumens ÷ Area in Square Meters
