Dimming of LEDs

If you are interested in photography, and take photographs under illumination from LEDs, you need to be aware of how the dimming of LED lamps works. LEDs are becoming very popular, and dimmers are quite frequently used to adjust the light level. This applies to households, offices, commercial spaces, and the now ubiquitous special LED lamps sold for studio and on location video and photography.

There are two main approaches: constant current (CC) dimming and pulse width modulation (PWM) dimming. The later approach is much more frequently used, due to technical reasons.

Pulse width modulation, in everyday words means that the LEDs are very rapidly switched on and off. They alternate between maximum output and no output. When we plot the light output it is a train of peaks and valleys, and the average amount of light depends on the ratio between the light periods and the whole cycle period. The frequency remains constant, what changes is the width of the “square-wave” shaped peaks. Hence, pulse width modulation. The frequency used can vary very widely, from  100 Hz or so, the minimum that humans will not perceive as flicker, to much higher frequencies.

The figure below shows the irradiance variation in time, measured with a light sensor connected to an oscilloscope. (Irradiance is the amount of radiation energy per unit of receiving area and unit time. The relative units in the figure correlate with illumination.)


The time axis is in milliseconds, and each cycle takes about 4 ms, corresponding to a frequency of 250 Hz.

Const current dimming, just limits the electric current driving the LED to a value that remains constant in time. By reducing the current flow, the irradiance is decreased.

This second figure, shows measurements from the same LED, and using the same measuring setup, but with the LED connected to a different driver circuit.


Some LED lights meant for use as light sources in photography, avoid the effect of output fluctuations by using PWM an example, I measured dimming of an Amaran AL-H9 LED light source from Aputure. The manufacturer advertises it as using a type of dimming that maintains constant illumination. This is not completely true, as dimming used in this light source is based on PWM. However, by using a frequency of 40 kHz (0.025 ms) most of the problems of PWM are avoided. This is so because it is unlikely that anybody will use a shutter speed close to 1/40000 s, with a light source that has rather weak light output and still use the dimming function.

Caveat: An additional approach, mostly used when retrofitting LEDs for living and working space illumination, is the use of a phase shift dimmer, as earlier used for incandescent lamps, in the circuit that feeds the electronics driving the LEDs. I have not measured light in any installation using this approach.

All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo



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