Reflectance/Transmission Accessory

A project log for Optronic Laboratories 740A/D Spectrometer

Getting a 1990 vintage spectrometer running on something more modern than MS-DOS

Jerry BiehlerJerry Biehler 01/07/2021 at 07:000 Comments

To make this thing useful for what I want to do I needed a reflectance and transmission accessory, this uses mirrors to bounce the beam from the monochromator through or against a sample at a set angle and then use a sensor to pick up what's left over. A friend actually has just what I needed, it was some sort of reflectance accessory that had been heavily modified for something with two spherical mirrors for handling the beam, one fixed and one on a circular translation that orbits the rotatable sample holder in the center. 

I removed the small mirror which was on the orbiting bit and replaced that with a piece of black delrin I machined to hold different sensors. I removed the rest of the parts on the accessory that were no longer useful and machined a slot to hold the large mirror at a spot that would focus the beam on the sensor. 

This all drops into a box I made out of some black scrap plastic that I had laying around. Had just enough to make the bottom and the sides, ill have to get some more to make a lid. The box slides over the output aperture of the monochromator and has feet that hold it up. The accessory drops in and lock in position with a couple pre-existing dowel pins in the bottom of the plate. 

I also made a sample holder out of delrin that will allow me to mount mirrors with the first surface at the center of rotation so I can do reflectance measurements at various angles of incidence. You can see it below with one of the IR filters I test next. 

For a first run I tested some IR filters I use for photography, I was curious if they actually different filters or the same glass put into different housings. 

I tested three filters, one was an older C&G branded filter I got years ago when I modified one of my first cameras, I thought I had lost it until I stumbled upon it thinking it was one of my others I had bought for my modified Canon 5D Mark II. The other two are made by Zomei and I got these off ebay. The C&G and one of the zomei filters are supposed to be a 850nm long pass filter and the other Zomei is a 950nm long pass. 

Looking at the resulting chart above I found several things. First, the UDT 261 germanium sensor acts really funny with the picoammeter are near zero light levels, the ammeter goes crazy. Once it gets the tiniest amount of light it settles down. This behavior has not been seen in any of the silicon or PMT sensors I have used. I will try to add a burden resistor across the diode to see if that helps quiet things down. 

Second, the two 850nm filters are identical. They almost perfectly overlap. They seem to be about what they say they are.

Third, I must of gotten a bit of bad data at about the 900nm mark, there is a bump across all three sample runs, if it was a sensor or beam path issue it should have been subtracted out. 

Lastly, that 950nm filter SUCKS. What a mess. With that slow slope I am losing a lot of light which means I need to run a higher ISO which means more noise, so I will probably stick with the 850 until I find a better filter.