Fiber Optic Cuvette Holder (w/ focusing lens)

Improved insertion efficiency version of the GaudiLabs Open Fiber Spectrometer Cuvette Holder.
(https://www.gaudi.ch/GaudiLabs/?page_id=825).

I just extended the original design's LED tube. By using a 5mm plastic lens from a cheap $1 laser pointer (650nm, Dollar Tree), this design greatly increases the insertion efficiency of light into a standard fiber optic audio cable.

Paint all interior surfaces black and press the lens (convex side towards fiber optic) into the hole on the inside of the cuvette holder until it sits flush.

I used a flat-head white LED from a string of dollar-store Christmas lights as my light source. The LED was 5mm in diameter, allowing it to slide into the LED tube until the light beam focused tightly on the fiber optic input. Once aligned, I secured the LED in place with hot glue. If you want to use a standard LED, you can adjust the tube length to match the focal length of your lens (mine was 4mm).