From my experience where I work, and three Opt Comp's on the floor: 1- CCP CC-14 "green screen), 2 - old Deltronic (don't remember the model), 3 - old Deltronic (don't remember the model.
Both the Deltronics are white light and black lines on the viewing plate. The CCP is Green light on black lines.
Features / factors that have been points of discussion, either good or bad, in no particular order:
White light or green light? Many of us here are able to see the demarcation of the black scale lines scribed on the plate better with the green screen. Some of us it doesn't matter.
Magnification: The CCP has a built in flippable lever where we can instantly swap between 10x and 20x magnification. This is super useful if we have to check a .500 radius: flip the lever to 10x (20x is the baseline) and check to the .250 radius line (scale maxes at .375 I think it is). Then just flip back to 20x.
The Deltronics require a lens swap out: unscrew the lens sticking out at chest level, put the new on in, etc.
However, we are looking at a new comparator right now, another "green screen" CCP/OGP and there's been focussed discussion on what magnification we want. 30x, 50x?
Surface Illumination. The Deltronics have better/clearer surface illumination, and some measurements require it. The problem is, those use the flexible little "robot arms" with the light at the tips you have to pull out, bend around, and try to position right where you need them to get the image on the screen. The green screen apparently has an "integrated" surface illumination without arms, but it's pretty weak. You can never get a solid image. Bottom Line: if you want surface illumination, pay attention to exactly how it does it, and if you can test it out before buying.
Auto-Edge Detection. Different units do it different ways. I like the CC-14 method because there's no ring/probe pasted to the front of the plate screen. It's a super useful feature, and removes human error/deviation on measurments it can deal with. CAVEAT: Calibrate to a certified pin/block before each use to double check auto-edge is working as intended. Deviations we've seen: 1) If the internal mirrors are dirty, it will affect auto-edge accuracy. Screen illumination has to be at 100%. If illumination is (accidentally, usually always set to 100%)at say 70% or less, auto-edge accuracy is affected.
You are going to have to learn where you can, and cannot trust auto-edge detection on your machine, if it can be calibrated to "zero", or if you manually offset the measurement by an amount identified when you calibrate to a pin/block close to the size in question.
Also on auto edge detection, check it in both the X direction and Y direction. We've had instances of the Y direction being fairly accurate, with the X direction being off by .0001 to .0002 from Y, on the same gage pin.
Templates / glass plate scales. Select the one that's going to serve your needs the most, and/or get multiple. For example: The CC-14 green screen has a single template on it with the standard cross-hairs, 45-deg line, and a 30-degree line, with 1/2 of the plate having radiuses starting at .005,incrementing every .005 out to .375. One of the deltronics has all four quadrants between the cross hairs filled with different increments of radiuses, such as .002, .004 .006, as well as .003, .005, .007, etc.
Bottom line: Consider what you want in the glass plate scale/templates.
Work Holding: Because the CC-14 gets 80% of the use, and is over in our precision Dedtru area, someone here manufactured a super basic, but incredibly useful sliding plate that slots into the dovetail milled into the Comparator's work table. It's about, oh, maybe .300 thick, 3-4" wide, and however deep the table is, with a polished surface. When slid onto the worktable is precisely square to the table. Throw a V-block on the table, nudge it flush to the edge of that plate, dead square. Need to fiddle around with other types of work holding and need the part aligned square/tru, now you have something true to the table, with a polished true surface to set it on, and a true edge to square things up with.
It's likely the OEM for whatever you are looking for has workholding options for their Comparator tables, just outlining what was done here and stood the test of time.
Best of luck, just usability thoughts from where I work.
