RGB LED Color Mixing or The Journey to The Perfect White LED Light

Nov 4, 2017

In this article I will look at the difference between warm and colde white LEDs as well as the difference between RGB, RGBWW and RGBCW in Terms of their color mixing properties. The final goal is to build an LED Lighting System that can produce colorful effects as well as warm and cold white light.

To build a good lighting system I think it is important to gain a basic understanding of light and color mixing itself. For that purpose I purchased a Spectrometer a while ago.


What is a Spectrometer and why would you want one?

A Spectrometer, or to be prcice in this case a optical Spectrometer, is a device that allows to see and measur the color spectrum of a lightsurce.

With the Spectrometer it is possible to see the spectrum of colors of which the light is composed. Spectrometers have a huge variaty of applications in the medical field as well as in analytical chemistry. But it also allows for an objective rather than a subjective comparison of different light sources. Further than that, it allows to determine the CRI (Color Rendering Index) or the color temperature of a particular light source.

My Spectrometer, the Ocean Optics HR2000+ has an resulution of aproxematly 0.5nm over a range from 200nm to 1025nm. This covers everething from UV-B all the way up to IR light.

What is the "Perfect" White Light?

I think there are mainly two things that should be considered to achieve a good looking lighting solution. The first thing is of course the CRI or the color accuracy of a lighting system. The second thing is the color temperature.

Whit a low CRI everything looks just not quite right. So a high CRI is usually desirable. From a systems perspective, especially for colored lighting installations, it is also important, that the colors of different light sources that light the same object are accurate compared to each other. If theay are not, unsightly shadwows are the result. For the lack of a better word I call this property color accuracy.

To find the right color temperature for an application is a bit more difficult and strongly subjective. I think this question belongs in the same category as if white or beige colored walls or better. It only depends on your personal taste.

But for a more objective point of view, consider this: At day time, when lots of natural light is coming through the windows, a light source whit a low color temperature looks yellow and does not blend in with the natural sunlight. At this time a high color temperature of at least 4000k or higher is optimal. At night time, however, the same light with the same color temperature would look cold and unfriendly. Of course, this is still subjective, but I think most people would agree.

In summary:

A really good lighting solution should be optimized for the objects it has to light and adaptable to a certain situation. - It should be RGBW.


The First Measurements

I started to measure some LEDs as well as old incandescent light bulbs and compact fluorescent lights to get a feeling for their different looks and light spectrums.

Although it was not the first time I have used a spectrometer, it needed a bit of experimentation with the setup to get more ore less usable results.

The Measurement Setup

The measurement setup I used to measure the LEDs is pretty simple.

I laid the device under test, which I wanted to measure, flat on a white tabletop and bounced the light off of a sheet of white paper into the fiber leading to the spectrometer. This setup is a poor mans approach of creating something like an Ulbricht sphere (integrating sphere) to diffuse and mix the light of the two light sources before entering the fiber. In the picture you can see one warm white and one cold white LED-Bar as well as an RGB LED-Bar.

In the left top you can see the SMA connector of the fiber, which picks up the light from the LEDs. Of course, this setup is not perfect and by far not suitable for a real characterisation of the LEDs, since it does not really prevent ambient light from entering the system and I don't use a cosine corrector for the fiber. But for a first and quick comparison measurement it should be close enough.

The First Results

In the digramm below you can see the mesurment results of the different LEDs.

Warm White Cold White Red Green Blue Warm White Retrofit 60W Incandescent Compact Fluorescent Halogen