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

What is a Spectrometer and why would you want one?

A Spectrometer, or to be precise in this case an optical Spectrometer, is a device that allows one to see and measure the color spectrum of a light source.

With the Spectrometer, it is possible to see the spectrum of colors of which the light is composed. Spectrometers have a vast variety 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 determining the CRI (Color Rendering Index) or the color temperature of a particular light source.

My Spectrometer, the Ocean Optics HR2000+, has a resolution of approximately 0.5nm over a range from 200nm to 1025nm. This covers everything 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 they are not, unsightly shadows are the result. For the lack of a better word, I call this property color accuracy.

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 they are not, unsightly shadows are the result. For the lack of a better word, I call this property color accuracy.

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 or 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 man's 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 characterization 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.

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