Beam Splitter Optical Glass: NOS. I opened one to photograph it and they do have some haze from sitting around for decades, that cleans off easily with glass cleaner. At an angle, some of the light goes through and some is reflected like a mirror. There's no documentation on the ratio of light transmitted to reflected, but the reflection is 1 2/3 stops darker than the light source. So that seems like 70T to 30R. These were for an old industrial color enlarger. Made by Pako Corporation. 1mm thick.