The Twisted Road of Dimming Curves

Guest post by Kevin Willmorth.

Superficially, dimming seems to be a simple operation. Turn a knob, slide a bar, or touch a screen to change light levels – done and done. This simplistic form of dimming was once applicable to resistance loads (incandescent lamps) using rheostat controls. The combination creates a naturally feeling dimming profile, as the light emitted by the lamps is reduced exponentially. In other words, incandescent lamps dim on a curve. As the dimmer is moved from high to low, the light output is reduced in a non-linear relationship to the movement of the control, until the heat in the lamp is reduced to a point so low, that no light is emitted (but still consuming energy).

Electronic controls and loads are not as simple. There are numerous factors involved that demand the use of programmed control responses to dimming input that can deliver any dimming profile one might want. This includes a straight linear response to dimming input, or log scale curves of any shape. An analogy to this is found in audio control. While the 0-10 scale on the knob appears to indicate a linear relationship, in fact, audio controls are based on a curve reflecting the non-linear human response to sound. We are more sensitive to changes at low sound levels than high, so to make controls more natural feeling, an exponential formula is applied to deliver sound level changes that feel more natural. Response to light is also non-linear. We respond to small changes in low light levels more than at high levels. The pupil in the eye opens at low light levels, reducing the perception of dimming as light levels are reduced. This requires illuminance to be reduced at a greater rate to produce a desired perceived brightness change. At the high end of the brightness scale, the pupil closes, requiring more light be added to produce a perceived increase in light level. How much consideration this is given in dimming application depends on specific application intention.

There are two primary reasons to dim light sources. The first is to create a mood or effect; The second is to save energy. Within these, there are two dimming effects. Static changes made to set a mood or respond to natural light present, or other control sensor for energy saving, and dynamic control changes are used to create an effect where the transition from one state to another is purposefully visible to the observer, such as used in theatrical lighting.

For energy-saving applications responding to automatic controls, exponential dimming curves are irrelevant as long as the control selected can attain the minimum and maximum intended power levels to attain energy targets. For effects that are activated using pre-set controls, with no requirement for active dimming by occupants, whether dimming follows a curve or liner response relationship is not important. The visual effect of transitions and fades from one dim setting to another is not changed by the control-dim response curve relationship. However, the relationship between the control response and perceived light level is applicable in reaching very low light levels. Generally, linear controls response does not provide the fidelity required to reach below perceived light levels of 32% (10% measured illuminance). For this reason, to achieve the range of dimming desired may dictate the use of dimming components, drivers and ballasts using shaped dimming profiles.

For applications where end-user operation of dimmers is a dynamic part of the controls design, dimming curves that closely relate to perceived light response is essential. Further, in applications where low perceived light levels are a priority, controlled products capable of delivering light levels of 1% or less (10% perceived brightness). Not all electronically ballasted (HID or Fluorescent) or driven (LED) products can deliver sub 10% power levels, creating a floor of 32% perceived light. To achieve low perceive light levels demands greater fidelity at low dimmer settings than can be achieved with linear dimming response.

Dimming controls are not the only determinant of dimming response. LED drivers and fluorescent ballasts have built-in control response curves designed around assumptions of what controls they are connected to. A mismatch between the dimming control and the dimmed electronic components can result unpredictably. A common result is truncated control function, and erratic dimming reaction to control input. For this reason, it is critical to verify that a selected control is compatible with the selected luminaire electronics. Unfortunately, this information can be difficult to obtain. This leaves two options: 1) select drivers and ballasts matched to controls; or 2) test the combination of control and electronics to verify the desired performance is achieved.

Human perception of light is non-linear, resulting in a miss-match between delivered illuminance changes and perceived light levels. In addition to understanding the dimming behavior of a product, when a manufacturer promises their product delivers 1% dimming, it is important to know whether this is 1% of light output (10% perceived brightness) or 1% actual perceived light (which is 0.05% of light output).

Linear dimming creates an exaggeration of the miss-match between actual illuminance and perceived light in the behavior of controls – particularly in the relationship between control movement and change in light level. Application of a logarithmic, or shaped dimmer response curve addresses this issue, providing a dimmer control that “feels” more natural and intuitive. Further, by using more control movement for the lowest settings increases fidelity where small changes are perceived the most.