This is a so-called "sodium D line light" for your gem refractometer. Normally a refractometer doesn't come with a light, or if they do it's a white light. The yellow colored sodium light at 590 nm is what is needed to accurately determine the refractive indexes of dual refractive index gems. (Ordinary and extraordinary indexes.)

A 590 nm light can be generated by a high voltage sodium vapor lamp, which is very hard to do and costly. Or you can use LEDs of 590 nm. Be sure to use "pure" LEDs of one frequency. Not the fancy RGB LEDs that mix frequencies to generate a color, that the humans eye will combine into an amber color. The ones I used were NTE part number 30067. Many other similar ones are out there.

This thingy is a little box to hold 2 large sized LEDs and the two associated resistors, plus a little wiring in a little snap-on rear compartment.

I printed this in orange PLA to show the details in these photos. But normally you would want this in black to eliminate any other colors or light entering your refractometer.

You do need to do a little math to figure out your resistors. See attached photo. For a source voltage of 6 VDC, and the LEDS with a forward voltage of 2.5 V, and a forward current of 25mA, that comes to using 140 ohms resistors (quarter watt is fine). Best to use a parallel circuit with each branch having it's own ballast resistor.

Add a little glue to the holes where the LEDs and resistors go thru the holes to hold them nicely in place. No glue required for the two parts of the case, they snap together. But you can add a little if you wish.

Enjoy!

Cheers, Michael