The Dimming Curve: Linear vs Logarithmic and Why Human Perception Decides It

A dimmer that reduces output in equal steps does not feel even to the eye. That is not a fault; it is physiology. The human visual system perceives brightness on an approximately logarithmic scale, so a small change near the bottom of the range is as visible as a much larger change near the top. The mapping between the control signal and the light output is called the dimming curve, and choosing it well is what makes a dimmer feel natural.

A linear dimming curve maps the control signal directly to output. At 50 percent control the light is at 50 percent output; at 10 percent control, 10 percent. This is simple to understand and useful where a control loop needs a predictable relationship, such as daylight harvesting, where a sensor expects the output to follow the command proportionally. The downside is that the user perceives the bottom half of the range as crowded: moving the slider from 20 percent to 10 percent looks like a big change, while moving from 100 percent to 90 percent looks like almost nothing.

A logarithmic (or gamma) curve compresses the bright end and expands the dark end, so that equal steps in the control signal produce roughly equal steps in perceived brightness. This is the curve that DALI standardises, and it is why a good driver's dimming feels smooth from bright to dark. The trade-off is that a logarithmic curve is not a simple proportion, which makes it harder to use in an open-loop control application that needs the output to track a sensor reading linearly.

The choice matters most at the low end. On a linear curve, getting from 10 percent to 1 percent requires the control signal to pass through a narrow band where every fraction of a volt matters. On a logarithmic curve, the same perceived progress is spread over a wider control range, so the driver's low-end regulation has more room and the transition feels steadier. This is one reason drivers that claim smooth 100-to-1 percent dimming usually implement a non-linear curve.

For a triac dimmer the curve is partly imposed by the phase angle. The relationship between the triac's firing angle and the RMS voltage delivered is itself non-linear, so the driver has to correct for it to produce the intended curve. A well-designed driver measures the phase angle, maps it through the chosen curve, and drives the output accordingly, which is why two drivers on the same dimmer can feel very different even though the dimmer is identical.

The specifier's question is not which curve is better, but which the application needs. For simple user-facing dimming in a room, the logarithmic curve feels right. For a closed-loop control or a test that needs proportionality, the linear curve is the safer choice. The datasheet should state which curve the driver uses, and the right driver for the job is the one whose curve matches the way the control will be used.

The low end of the range deserves special attention when selecting a curve. On a linear curve, the difference between 5 percent and 2 percent output is a small control-signal step, and if the driver's resolution is coarse, the user sees visible jumps near the bottom. On a logarithmic curve, the same range of output is spread over a wider portion of the control travel, so a coarse resolution produces smaller perceived steps. This is why a driver advertised with smooth 100-to-1 percent dimming and a logarithmic curve is easier to trust than one with a linear curve and no stated resolution. The curve and the resolution together, not either alone, define how the low end feels.