Interior Lighting for Designers

Square Law Dimming Curve
The manner in which light output responds
to changes in the control setting is called the
dimming curve. If a change in the setting of
the dimming control, from full bright to full
dim, approximates the change in the
amount of electricity allowed to reach the
light source, the dimmer is said to have a
linear curve.
The eye is more sensitive to changes in
low intensities of light than to changes in
high intensities. This relationship between
light perceived and light measured is called
the “square law” curve (figure 11.8).
Electric lamps also respond in a nonlin-
ear way: at 81 percent of the voltage, the

light output is 50 percent. If the electrical

output of a dimmer changes in a linear

manner, then a light source will appear to

dim faster at low intensities and slower at

high intensities.

To correct this, good-quality dimmers

feature a “square law” dimming curve. Here

the dimmer control moves at constant

speed, but causes the light to dim faster at

high intensities and slower at low intensities.

To the eye, the result is a consistent rate of

change in the light intensity.

Incandescent Lamps

Dimming incandescent sources increases

the life of the lamp. Yet both incandescent

ELECTRICITY

Figure 11.8“Square law” curve: the relationship between perceived illuminance and measured illuminance.