I’ve got a glass diopter. It goes into an optical relay and magnifies the image. Had it for decades, know it does the job. The person who added it to the kit is long dead.
Who do I approach who can measure this thing and give me numbers on magnification sufficient to allow me to track down more of the same?
It has nothing to do with eye glasses and I suspect heading to the local Lens Crafters won’t do it.
Where should I take this lens for measurement?
I am not sure what your application is and would appreciate if you elaborate.
In general, you can approximately measure the power (diopter) of a convex lens by just holding it in the sun (or any light source) and focusing on a plane. Measure the distance between the lens and the image at its sharpest (a point in sunlight). Convert the measurement to meters, then the inverse of the measurement is the diopter.
So suppose the distance measured above is 0.5 meters, your lens diopter is 2 (1/.5)
The terminology is strange to me: It sounds like what you have is a lens. Diopters are a unit for measuring lenses. What you’re saying sounds something like “I have a coil of nylon meter”, instead of saying “rope”.
am77494 covers the main idea.
Some caveats:
The source needs to be far off. The Sun is great in terms of distance but since it isn’t a point source you will get a circular image. Making the edge as sharp as possible may not be easy, esp. while also trying to measure.
Another way is inside a dark room with a window that has a view to scenery a bit off in the distance (a couple blocks is plenty). Focus the lens on a wall/screen to get a sharp image of the view. At nighttime you can use a streetlight but again you want an image of the light.
am77494 has it correct, and I agree with Chronos that your usage seems odd.
I would tell someone I had “a glass diopter” if I had one of those 1" diameter lenses with a metal handle that formed part of a set of standard lenses of different optical power, of the sort ophthalmologists and optometrists used in measuring the defects in people’s eyes. These were called a Diopter set, and it was common to call one element a diopter. These sets are getting harder and harder to ome by – they’ve been broken up in recent years as people sell them to steampunk costumers and the like. I’m not sure what your “optical relay” is.
If you’re looking for a set of such lenses, you can look on ebay or the like:
Magnifying lenses used in photography - especially for viewfinders or macro kits - are often (confusingly) called diopters. So the power of your diopter is measured in diopters. (:rolleyes:)
am77494 is right about how to get the (approximate) power. There are automated gizmos that can give you a more precise measurement, but you should be able to get in the ballpark pretty easily with some patience and a tapemeasure. That should be enough to narrow down the specific product you want, since they’re generally only made in a small number of discrete values.
It’s not so crazy. Candles and horses get the same deal. 
With the complication that a horse’s power is typically about two horsepower.
And the metric horsepower is less than the standard horsepower. This is just a ton (short ton not a metric ton) of BS really 
Allow me to address all of the queries by quoting this post.
Indeed, it is a photographic accessory. I am using the nomenclature used in the early 1980’s when this optical relay was first designed and built. When I bought it, I was informed that this item was the Diopter. The only other time I’d heard that word used in association with movie cameras was in a VERY closely related situation. Back in the day, camera operators would press their eye to the eyepiece of the camera. One could focus the eyepieces to the ground glass and in doing so be able to obtain focus in the shot itself. For people who wore eyeglasses, this was an onerous arrangement. Diopter were placed into the viewfinder assembly- just at the back, near one’s eyeball- that mimicked the adjustments to vision offered by one’s eyeglasses. Or rather, by the right eye lens. This allowed the camera operator to shoot without pressing the glasses against the eyepiece. The degree of discomfort AND real danger of flashing the footage with an eyepiece light leak varied by manufacturer. Arriflex eyepieces were notoriously difficult to remain “centered” on, and the angled eyecup sucked. One of Panavision’s lovely improvements was the cylindrical padded eyepiece end. Afforded full view when a degree or two off of center on the eyepiece, and avoided light flashing.
Now to the issue at hand. While I am tempted to suss this out myself as instructed above, I need to be able to go out on the open market with a set of measurements that are relatable to the industry.
Perhaps I should visit my local Lens Crafters and sit with the technician and ask them if they can measure the degree of magnification. My calipers will tell me the diameter.
If I get no love from folks like that, I will make a date with my measuring tape and with The Sun !!
Places like Lenscrafters and all optometrists/ophthalmologists have a gadget, often made by Bausch + Lomb, that can measure both spherical and cylindrical curvatures (and compounds) in diopters. These instruments are used to measure glasses, natch. The old instruments were entirely optical and mechanical; the new ones are all automatic with a computer interface. If it’s the value you want, they should be able to tell you what (I think) you are asking.
Thank you.