There are two separate systems to measure the intensity of light. The first is a physical system that measures energy or power transferred. The second system measures the effect of light on the eye— in other words, how bright we see the light. You are familiar with the watt. It’s the rate at which energy is transferred. The equivalent unit for light, the luminous power, is the lumen. If you look at the box a lamp comes in you’ll find both the electric power it dissipates in watts (W), and the luminous power in lumens (lm). For example, a 25-W clear bulb emits 200 lm. A 100-W lamp emits 1,720 lm. A 60-W halogen lamp emits 1,080 lm. Compact fluorescent lamps produce more light for the same power: a 25-W lamp is rated at 1,600 lm.

The intensity of light depends on the degree to which the lamp spreads or focuses the luminous power. If the light goes into all directions it won’t be as intense as it would be from a reflector spot lamp that reflects light to form a narrow beam. The unit in which light intensity is measured is the candela (cd). If a 100-W, 1,720-lm lamp could spread the light into all directions it would have an intensity of 137 cd. But, if the same lamp were a spotlight, concentrating all the light into a 30° angle, then the intensity would be about 640 cd.

As the distance between the lamp and the surface illuminated increases, the illuminance provided by the lamp decreases according to the inverse square law. Suppose you have a light luminous power of 1,000 lm 1 meter (3.2 feet) away from the surface. If you now moved the lamp to 2 meters (6.5 feet) away, the illuminance would be 1,000 lm divided by the distance squared, or ½². That is, ¼ of the illumination at 1 meter. If you moved the lamp to 3 meters (9.8 feet) away, the illuminance would decrease to one-ninth the original illuminance.