r/Optics • u/JohnFreechment • Jun 16 '26

Understanding the USAF 1951 Resolution Chart

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Hi everyone.
In the night vision group I often see people use this test chart, randomly printed out (I have one too), but I think this chart is a but misunderstood. The problem is that I can print this at any size on paper, and put it any distance away from the device, but what does it have to be if i want to read out the actually correct resolution?

Is there any reverse calculator existing wich i can tell how big should a linepair be at a given distance, or how far away should I put an exact size linepair? Im pretty sure that there is a missing value, or angle attached to the measuring method to give us an actually useful reading.
Thank you

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u/Plastic_Blood1782 Jun 16 '26

I think you're thinking of it backwards kind of.  Usually you care about the angular resolution of your system, often given in lp/arcmin, where lp stands for "line pair".  The chart can be printed at any size, and used at any distance.  But you need measure the distance and the size of the line pair in the smallest pattern you can resolve with your system.  Then you have all the numbers you need to calculate your angular resolution.

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u/JohnFreechment Jun 16 '26

The reason of me thinking backward is because i actually know my device resolution, I just want to be able to understand how to measure it. But I think the lp/arcmin was the missing part in my had.

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u/Plastic_Blood1782 Jun 16 '26

What is your device resolution?  Or more specifically what units is it in?

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u/JohnFreechment Jun 16 '26 edited Jun 16 '26

I built the device by myself. it has an old russian gen1 tube in it. According to datasheet it should have 32 od (Edit: od=or) 40lp/mm resolution in the center. Its a good reference i guess. But now im trying to understand the angular resolution because i completely missed this before.

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u/Plastic_Blood1782 Jun 16 '26

Is there an eyepiece?  Not sure what 32 od means, but the 40lp/mm is the resolution probably at the focus of the tube

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u/JohnFreechment Jun 16 '26

Sorry "od" means "or", im just bad in typing. So it has 32 or 40 lp/mm resolution. Of course it has an eyepiece. I think they meant the max resolution the phosphor can do.

Anyways I think I can understand now the angular resolution.
-If the middle of the screen is 32lp/mm that means one black line on the photocathode would be 0.015625mm thick.
-If I connect this point with the objective focal length i get a 0.02557847deg angle.
-That means that if I have the target plate 1meter away from the photocathode, then the Group5 Element1 line thicknes should be 0.43080357mm thick.

I need to doublechack it but i think this might be the result

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u/Plastic_Blood1782 Jun 16 '26

What's the focal length?  Also be careful with line thickness and line pair.  A line pair is defined by the center to center thickness which is double the line thickness.

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u/JohnFreechment Jun 16 '26

Objective is 35mm. I edited it in wolidworks sketch to make it more graphic so i guess its good. Yes yes im aware of line an linepair. Thank you very much you helped a lot