Dimmed vs. perceived light

This article aims to illuminate a few important matters about dimming that may be of interest to those who work in fields where lighting plays an important role. It is by no means a comprehensive analysis of all dimming types, but will help you to determine what questions to ask when you are faced with decisions around what light sources to use in a situation where dimming may be required.

As the light available to us gets lower, our pupils expand, letting us take in more light. This means the ratio between the measurable amount of light and how much light we perceive isn’t one-to-one. For example, when a light is dimmed to 50 percent, things appear only about 30 percent dimmer. Even when a light is dimmed to just 1 percent, it can seem to us like it is at 10 percent. This can be problematic in spaces where we want very low light levels.

Not all light sources follow the same rules when it comes to dimming. Incandescent lamps offer the smoothest transition: you can go from full brightness down to nothing with no perceivable flicker or harsh “pop” off.

In contrast, not all fluorescent lights can be dimmed. Those that can have no trouble dimming down to about 10 percent of the maximum. Because of the difference between measured and perceived lighting, that translates into an apparent light level of about 30 percent. Beyond that, things get more difficult. Although manufacturers have made progress in improving the ability of fluorescents to dim, even under the best circumstances they can still go down only to about 1 percent and then “pop” off. This is inescapable due to the physics of the lamp.

LED sources have similar limitations. As with fluorescents, they can dim to about 10 percent easily. LEDs that can dim to 1 percent have become much more commonplace. The most advanced LED systems can dim down to anywhere between .7% and .1%. These “dim-to-dark” sources are dependent on high quality drivers to keep the source from flickering and the “popping” off.

None of this matters much in spaces that are typically well lit, such as office buildings and schools. But in places like homes, theaters, and houses of worship, where dimming is more desirable to very low levels, it becomes critical. If you need LEDs to dim as smoothly as incandescent bulbs, you must make sure that both the fixture and the control device are capable of doing so. This requires tight control on the drivers used and verifying that the selected driver has been tested and will function as expected with the control system you are using.

It is highly recommended that you test your fixtures to see how well they function as they dim before you buy them. Take note of how bright materials and the space they occupy appear and what “temperature” the light seems to be. Because incandescent light shifts toward the red end of the visible light spectrum as it dims, most people are used to dimmer light feeling “warmer.” Fluorescents, however, shift to green/blue (“cooler”) end of the visible light spectrum. LEDs remain fairly stable as they dim from high to low, maintaining a constant color temperature. Again, this can be disconcerting if a client expects the light to get warmer as it dims. Some LED products on the market have the ability to “warm up” as they dim, thus appearing more in line with our previous experiences. Even though all of this is measurable and controllable with the right equipment, the simple truth is that it all comes down to human preference and perception. If it doesn’t look right, then it probably isn’t.