PWM-dimming of RGB LEDs


Here I have tested a single individual lamp, of a single brand and type. From the ubiquity of identical or very similar lamps sold under different brand names by numerous sellers in eBay and Aliexpress, this seems to be a “typical” or popular type. The intention is for it to serve as an example of one specific technology for adjustment of the light output of LED lamps. This is not a comparative review.


If you haven’t yet read the post on dimming of LEDs, I recomend that you read it before continuing.


In recent times cheap RGBW LED bulbs with four colour channels and their own small remote control have become widely available, both from well known brand names, and cheaper local and Chinese sellers. I tested one branded LEDSAVERs RGB, from a Swedish supplier. Although labelled RGB, the lamp has four independent colour channels red, gren, blue and white (RGBW).

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For the test shown below in a video, I used a PicoScope 2204A USB oscilloscope with the PicoScope 6 software running under Windows 10. These oscilloscope has two channels. I connected to each channel a TSL251 light-to-voltage optical sensor, one filtered with red acrylic (Plexiglas ) and the other with blue acrylic (Plexiglas ). I powered the sensors with a stabilised 5V power supply.

The test includes an initial section showing dimming of the white channel of the lamp. Later sections show how different colours are created by varying the length of pulses from pairs of channels. In the video only colours formed by mixing red and blue are shown, as well as how dimming in this case.

In the video the red oscilloscope trace, channel B of the oscilloscope, corresponds to the red-filtered sensor, while the blue trace, channel A, corresponds to the blue-filtered sensor.

At the lower edge of the window appear live measurements of some parameters: pulse frequency is approximately constant at 164 Hz, and duty cycle changes in steps of approximately 10%, with a minimum of 10%. (To see this number you will need to set the video to full screen and HD resolution in YouTube.)

From the trace in the video we can see that one cycle lasts for 6 ms or 1/166 s of a second. At 50% duty cycle, we have periods of 3 ms or 1/333 s light followed by 3 ms or 1/333 s darkness. At 10% duty cycle, we have 0.6 ms or 1/1666 s followed by 5.4 ms darkness. We do not see flicker at this rate, but a camera will see the pulses. With fast shutter speeds we should get a whole assortment of over- and under exposed images and differences in colour, plus broad bands. Even mechanical shutters do not expose the whole sensor exactly at the same time. For shutter speeds somehow slower than the length of the cycle, there will be some variation from frame to frame, unless the shutter speed used is a multiple of the cycle period.


Be aware that what is shown in the images below is completely invisible to the human eye. The wall looked as having a constant and even colour at each of the settings used.


With 50 Hz or 60 Hz AC line frequency or 100 Hz or 120 Hz light pulsing most digital cameras will adjust the sampling rate of the live-view screen and electronic viewfinder (EVF) to prevent flicker. This LED lamp, however, uses a different frequency for dimming and resulted in very strong flicker on both the screen and EVF. 


Compared to the usual oscillation in light output as a result of AC mains power, these square pulses and dark periods are more disturbing for photography. Here is a white wall illuminated with red + blue mix, dimmed down. Each photograph in the series was taken with a different shutter speed using the “silent” shutter of the olympus E-M1 camera.

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No dimming, mixed blue + red at a different %.

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In the case of this lamp, because of the way the colours are mixed, when photographing under a mixed colour, the colour will vary from frame to frame. Now using the mechanical shutter, a burst of 24 images all using a shutter speed of  1/1000 s. This demonstrates that even with the mechanical shutter not all the frame is exposed simultaneously.

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Even when using only the white channel of the lamp, dimming to about 50%, resulted in artefacts, and variation in exposure and uneven exposure.

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This lamp reaches a maximum duty cycle of 100% at full output, giving under this setting perfectly steady light output. This is better than in the case of many non dimmable LED bulbs whose output fluctuates to some extent at twice the mains AC frequency. Consequently, if one uses one pure colour, R, G, B, or W buttons, and full output, exposure will be even at any shutter speed, with either type of shutter.

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A similar, but weaker problem can affect other types of lamps whose light output varies at twice the frequency of mains AC line frequency, see the post on shutter speed and light flickr.

A caveat is that modern dimming driver circuits for LEDs not always  use PWM. In some cases they use constant current dimming, which is an alternative technology that completely avoids light output fluctuations.

Some LED lights meant for use as light sources in photography, avoid the effect of output fluctuations by using PWD dimming at a much higher frequency than in the LEDSAVERs  lamp described and tested here. As an example, I measured dimming of an Amaran AL-H9 LED light source. 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 the same approach as in the LEDSAVERs lamp. However, by using a frequency of 40 kHz (240 times faster) most of the problems of PWD 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.


Those readers interested in photobiology, or in growing plants, may find interesting that the effect of pulsed light and its frequency is a question of practical importance for which we do not yet have good answers. Furthermore, how plants respond to alternating pulses of light of different wavelengths, their frequency, relative duty cycles and phase shifts in mixed illumination remains as a big unknown. An unknown related to a type of illumination that has no relation to that available in the natural habitat, but which is becoming important for plant cultivation indoors.


As a conclusion, a photograph of of a white wall evenly illuminated with yellow light from the LEDSAVERs lamp, dimmed to about 50%. “Silent shutter” at 1/320 s.

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Yellow = Red + Green, dimmed.

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

(c) 2017 Pedro J. Aphalo


 

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Macro extension tubes (description)


This is the first of three instalments on the comparison of three sets of macro extension tubes for MFT cameras: Macro extension tubes (description)Macro extension tubes (glare), and Macro extension tubes (lens mount).


Disclaimer: I have no connection to any of the suppliers of the items compared in this test. I bought them from different on-line sellers. Although some of the products I bought have serious design flaws I have tested only one copy of each, bought in July 2017 (Kenko), June 2017 (COMIX), October 2015 (PIXCO). The items in production at the time you read this post may be of an updated design and different quality. It is also necessary to be aware that in the case of some Chinese brands, cheap and expensive versions of an item may exist.


Neither Olympus nor Panasonic sell macro extension tubes for Micro Four Thirds lens mount. There are several third party alternatives. I have bought three of them, I will describe here my experience using or trying to use them. In eBay photographs they all look rather similar, and the three sets I bought consisted of two tubes, with lengths of 10 mm and 16 mm. As I will describe bellow, there are major differences. The sets I own are Kenko “Extension Tube Set DG for Micro 4/3” (100 € to 140 €, Kenko Tokina, Japan), COMIX CM-ME-AFMM (35 € to 55 €, Commlite, China), PIXCO “Extension Tube Set DG  for Micro 4/3” (15 € to 20 €, PIXCO, China).

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The three sets of macro extension tubes.

One thing worth noting is that the set from Kenko is the only one of the three that displays the official Micro Four Thirds logo printed on the tubes and in the packaging and manual. This should ensure that the mount dimensions and placement of electrical contacts are within specifications, and the tubes widely compatible with Micro Four Thirds cameras and objectives. The body and lens/camera mounts are made of metal, and the inner surfaces are flat black, giving rise to very weak reflections. The set is rather hefty weighting 116 g. The mounts have very slight rotational play, but almost none along the lens axis.

The cheapest set, the one from PIXCO has a plastic body and metal lens/camera mounts. It is very light weight at 50 g. The inside shows an attempt to reduce reflections by means of a surface with fine ribs similar to those in the Kenko, the the surface is not as matt, giving rise to some diffuse reflections. The mounts have a bit more rotational play than in the Kenko, but no major problem with axial play. The location of electrical contacts is good enough and the rotational play small enough to make the set of tubes fully functional. However, what I dislike is that the metal used in the lens and camera mounts seems to be rather soft, and its surface rough. Mounting or dismounting a lens produces a grinding sound, which quickly results in visible mark on the lens mounts of the tubes. So, it is of rather poor quality as one could expect from its price, but is fully functional (at least under light use).

What could seem like a bargain, the COMIX tubes, very nicely packaged, including a nice small pouch for storage, turned out to be those of worse in use. Looked at casually they seem identical to the Kenko ones. The body of the tubes is plastic, but mounts are made from metal with a nice and smooth finish. Even the location of screws and the shape of the levers used for unlocking the mounted lens are almost the same as in the Kenko. On inspection they are light, weighting 59 g. On looking at the inside, I was surprised to see that the surface is smooth and shinny, just plain black plastic. On use, another unwelcome surprise: there is considerable rotational play together with a very important axial play. Electrical contacts do work in spite of axial play, but axial play as a result of the weight of the attached lens should cause under normal use a misalignment of the focus plane on the sensor (like in a tilt lens or a tilt adaptor). The dimensions of the mount are clearly not within reasonable tolerances and the metal springs way too weak to keep even light weight objectives in the required position and alignment. In other words, a nice looking set of tubes that is in practice non-functional.

Why is a matt black ribbed interior surface important? By changing the objective to sensor distance we project the image from further away. Consequently part of the light hits the inner walls of the tube. Reflections cause flare and can drastically decrease image contrast. Being macro extension tubes just tubes, one could think that as long as the camera and lens remain connected, the same lens using difference tubes should result in images of the same quality. This is not necessarily true, because both misalignment and  internal reflections can deteriorate image quality.

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The three sets of extension tubes, from left to right, Kenko, PIXCO and COMIX. The secondary light source was behind the tubes and slightly above, to avoid artefacts a large source was used: a 27″ computer monitor. The secondary light source used to test for reflections was at a shallow angle trying to simulate possible reflections in actual use.

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

(c) 2017 Pedro J. Aphalo