Saturday, October 26, 2013

Spectrometer for LED Bulb Testing

So when I started out looking at LED bulbs I was interested in some text the vendor slap onto their boxes.  The acronym "CRI" shows up a lot as if it's something important... but in a clueless marketing fashion there is no explanation as to why that's good or bad. (numbers > 80 are good, > 92 are excellent.... but you need to pull out your smart phone to figure that out).

Turns out that this stands for color rendering index and one tool you need to figure it out is a spectrometer... but that sounded expensive.

A bit of Googling, however, drops you into this web page (  It's a web based program... nothing needs to be installed, other than a Chrome browser and a USB webcam

This is a web based program that allows you to get power-spectral-densities (sounds fancy!  but it's just a program which looks at a spectral line and computes its intensity).

The spectrometer, however, is just created out of a paper template and it is very hard to get repeatable results.  A better approach is to use something a bit more stable... in this case wood.

A spectrometer is nothing more than a tiny window slit, a diffraction grating and a view port in a box.  The diffraction grating can be made from a DVD.  The slit can be made with some metal tape, and the view port can be a USB web cam.

After building a simple box (dimensions don't seem to be too critical, mine is 8" x 2.5" x 2.5"). A slit on one end needs to be created with metal tape (it's in the furnace repair section of a hardware store):

The camera needs to be a close focus.  I bought a bore scope (design for inspecting plumbing and such on ebay).  Other option is to buy a web cam and take it apart.  The lens needs to be refocused for close in work.  Google "endoscope usb" and don't pay more that $16.00! (some come with 15 meters of cable which is not needed)

Next challenge is to understand how the specctra relates to CRI


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