6.1: How Lighting is Measured

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When most people buy a light bulb, they look for watts (W). Recall that a watt is a unit of power, (i.e., the rate at which energy is consumed from the electricity supplier). It does not say anything about the light.

The most common measure of light output (or luminous flux) is the lumen. All lamps are rated in lumens, as shown in the figure below, and every bulb has 3 parameters listed on the package, as shown in Figure 6.1.1:

Lamp lumen output or light output
Power consumption in watts
Life of the bulb in hours

Figure 6.1.1. Parameters listed on light bulbs

Watch the video below to find out more about lumens.

Footcandles

A footcandle (fc) is the standard unit of measure for illumination on a surface. It is a lumen of light distributed over a 1-square-foot (0.09-square-meter) area, as depicted in Figure 6.1.2.

Figure 6.1.2. Footcandle

The average footcandle level on a square surface is equal to the amount of lumens striking the surface, divided by the area of the surface.

\[ FC = \dfrac{Lumens \, of \, Light}{Area (ft^2)} \]

Example 1

A 40 watt bulb produces about 505 lumens and has a life of about 1,000 hours. When this bulb is used to light a room of 10 x 10 feet, these 505 lumens are distributed over 100 square feet of floor area. What is the illumination?

Image of a 40 W bulb (505 lumnens) sitting in a 10ft by 10 ft room.

Answer: \[ Illumination = \dfrac{505 \, lumens}{100 ft^2} = 5.05 \, lumens/ft^2 = 5.05 \, fc \nonumber\]

Additional Information

A useful conversion is

\[ 1 \, fc = \dfrac{lumen}{ft^2} = 1 \, Lux \, (metric) \nonumber\]

The following video shows how a light meter is used to measure lumens per ft².