Yes, a Fresnel lens. But since magnification/projection is done by the mirror optics, part of this Fresnel's purpose is to direct the light out to the viewers (preventing the effect of looking down a tunnel where only one viewing position is good, or only part of the picture can be seen from any position). It controls the spread of the light such that multiple viewers can see it, yet it is not spread over a whole hemisphere to extreme angles where no viewers would be located.

I don't know how many layers the screen has and how much general diffusion is there in addition to the Fresnel. It would be interesting to check. Can you try looking at the picture from extreme right and left and extreme high and low, and see how far the viewing angle goes?

Bear in mind that there were ~3 different projection systems RCA, Philco, and Phillips Norelco (which this and a variety of later projection sets used). There are probably performance differences with each.

Very good point. I believe I saw an early Philco playing once and, although acceptable, it wasn't very impressive. It appears the Norelco system was superior.

The screen appears to be a single layer, with a pattern of light horizontal lines (like extremely fine sandpaper scratches) on its rear surface and the pattern of concentric grooves on the front.

I looked at the screen from various angles, and tried to take some photos, and even a video, but the pics don't show anything very quantifiable, beyond "Yup, looks brighter head-on."

The brightness and contrast are clearly the best when you are looking head-on. The image looks quite good if you remain within an angle of about 45 degrees from the left or the right (or up or down). Beyond that, the picture is still coherent, but shown in shades of gray rather than vivid white & black.

Regards,

Phil

The 45 degrees in all directions is the kind of thing you would expect from the circular pattern. Not sure what the horizontal lines are doing in your case.

Modern rear projectors (CRT and DLP) used very strong tiny horizontal prisms to get the light hitting the screen at a steep angle bent to horizontal and out the front. In the very shallow cabinet DLP sets, the horizontal prisms actually produced total internal reflection, so light entered from the bottom of the prisms, reflected backwards off the top, and then reflected off the back of the prisms, forward and out.

Modern screens typically also had vertical or circular fresnel elements on the opposite side to get the kind of directivity you are seeing, but it often was stronger in the vertical direction than the horizontal.

In an ARF discussion of this screen, it was suggested that the horizontal lines were a cheap way to make the panel translucent, and they might also slightly defocus the CRT's horizontal scan pattern.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html