2013-06-11

What We Mean by Dimming

Lab-based testing of user preferences is behind these recommendations into how LEDs (or any other lights) should behave when dimmed.

What do we mean, exactly, when we talk about dimming lights? The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute has released a new "ASSIST Recommends..." publication that aims to answer that question in a technology-neutral way. (ASSIST is the Alliance for Solid-State Illumination Systems and Technologies, made up of researchers, manufacturers, and government agencies working in cooperation with the LRC.)

The 16-page publication, "ASSIST recommends... Dimming: A technology-neutral definition," is available from the LRC's website.

The LRC set out to determine all the factors and effects within the "user experience" of dimming a light, and through user testing to determine which ones are important -- and indeed which are even perceptible -- in order to make the dimming experience what users expect it to be. The figure below, adapted from the report, defines some of these factors.

The project took two years. Here are some of the findings and recommendations.

Dimming profile
Previous research had indicated that people tend to like a dimming profile based on a square law, possibly because it matches the logarithmic response of the human visual system. (The dimming profile is the curve of light output versus control setting.) But this research found that people don't have a statistically significant preference for one profile shape over another, as long as they are monotonic: no sudden jumps in brightness while dimming and vice versa.

Dimmest
The recommendation is that dimmers take the light level down to 5 percent (or less) of its maximum. The experiments carried out by the LRC found that the dimmest light people were looking for was about 14 lux -- a level not dependent on the application or on the brightness of lighting before the dimming commenced.

Brightest
Acknowledging that a dimmer will rob some efficiency from a lighting system, the LRC recommends that an arrangement of light-plus-dimmer produce not less than 90 percent of the light output of the light in isolation from the dimmer.

Flicker
An earlier publication had defined a way to measure flicker, and this study pointed to that earlier one with the recommendation that a dimmer's flicker achieve a non-zero desirability rating by its methodology.

Dead travel
This refers to movement of the control during which no perceptible change in light output results. LRC found that half of test subjects noticed when dead travel approached 20 percent of the dimming control's range. The recommendation is that dead travel be limited to 10 percent.

Color shift
Incandescents tend to shift redder when dimmed. The LRC does not recommend either mimicking this behavior in LED lighting systems, nor necessarily avoiding it. This research indicated that LED lamps shifting color when dimmed were equally rated as those that did not shift color. However, earlier research had established that shifting color in fluorescent light sources was not desired.

The table below, adapted from one in the report, summarizes these recommendations.

Criteria for acceptable dimming based on users' expectations

Feature	Characteristics
Dimming profile	No limitation on the detailed shape as long as the dimming profile is monotonic and meets the dead travel and range requirements.
Minimum light level	No more than 5 percent of the maximum light output.
Maximum light level	No less than 90 percent of full light output at rated voltage (i.e., not on a dimmer).
Dead travel	No more than 10 percent of dimmer travel at the top, bottom, or middle of the dimmer setting.
Flicker	For frequencies below 100 Hz, less than 10 percent flicker; for frequencies equal to or above 100 Hz, a calculated acceptability rating greater than zero, following the procedure put forward here.
Audible noise	Audible noise is an important parameter to consider in the definition of dimmable products; however, at the time of writing, no specific measurement method or noise criterion has been established.
System efficacy	To be measured at maximum output when operated on a compatible dimmer.

This community has discussed dimming in the past, and in particular has speculated about dimming profile and color shift. What are your reactions to the LRC's recommendations?

— Keith Dawson , Editor-in-Chief, All LED Lighting