Hello Unreal Rendering specialists!

I was doing a little personal research about Johann Heinrich Lambert, because I enjoy reading about the history of optics, and computer science and stuff like that. Lambert materials in 3D applications are named after JH Lambert for his historic contributions to the field of photometry, the science of measuring light, a lot of which has gone into calculating and simulating realistic lighting effects in 3D applications. I am by no means a computer scientist, and at best an intermediate-advanced 3ds Max and Unreal Engine 4 user, so I was hoping someone around here might help me understand this diagram from JH Lambert’s important book, “Photometria,” which if you’re interested is available to read in Latin on Google books. Attached is a jpg of the diagram. I find the diagram pretty compelling and would like to create a model for some of my students to follow and learn from and recreate.

The diagram is an example of visual photometry. The vertical screen (H) produces field (EFDC) illuminated by the single candle (N) and adjacent field (GFDB) illuminated by two candles (A). The candle distances are changed until the brightness on either side of FD is the same. The relative illuminating power can then be determined from the candle distances. I can understand the diagram pretty well, but what I am wondering about the diagram is what exactly L and M are supposed to be, and in what way they affect the experiment (The two vertical stands with window like shapes on them). I think these might be objects to look through, framing the color of the wall in such a way as to easily compare the colors of the two walls, but I just wanted to run it by some of you who might have seen this image before to confirm or correct this assumption. The position of L in particular is throwing me off. I have been unable to find a description of this diagram anywhere that describes L and M, so I thought I’d reach out to this bright community (no pun intended).

I apologize if this is a strange and/or stupid question, but I hope it might also be interesting for some of you to think and perhaps chat about.

Best,

c5ly