I took the liberty of overlaying Draper's own traces of their Radiant & Contrast Radiant gain vs. viewing angle curves. I then changed the colors of the traces/viewing angle diagrams for easy viewing:Draper Radiant vs. Contrast Radiant
Now this is more like what I expect out of a white retroreflective vs. grey retroreflective screen, as long as each screen received roughly the same depth/number of reflective beads. The viewing cone should really only change (get narrower) if the grey retroreflective screen got more beads or has a higher gain coating over the grey.
The Draper screens look like they have about the same coating, just with a white or grey base (the gain curves literally only/mostly shift up/down; the cone remains relatively unchanged). The viewing cone only changes from 34º to 30º going from white to grey screens.
Da-Lite lists their viewing cones as 60º and 40º for HP and HCHP, respectively.
I'm not sure how these numbers work out, though, since Da-Lite lists their half-gain (2.4-->1.2) happening at 20º for HCHP, while Draper lists their half-gain as 15º (2.7-->1.2, yeah slightly less than half-gain) for their Contrast Radiant. Yet, when I go far off-angle, the Draper Contrast Radiant has at least as much, if not more, gain than the HCHP.
Hence, I went with my own measurements, and am swapping out my Da-Lite HCHP for a Draper Contrast Radiant (fixed frame, permanently tensioned: Onyx line). Guess I'll be the guinea pig here!
Since I did the experiment of brightness vs. viewing angle for a patch of Draper Contrast Radiant over the HCHP screen, after I get the Draper screen, I'll do the reverse experiment: a patch of HCHP on top of the Draper screen
To once again summarize the results of my Draper Contrast Radiant patch over the HCHP screen experiment, if I calculate the fall-off of each screen relative to the original 5º angle brightness, and normalize the numbers such that the 5º viewing angle shot of the HCHP has a gain of 2.2 (which is what I consistently measure for my setup when comparing the HCHP vs. a matte white screen), then these are the gain curves I get for HCHP & Draper Contrast Radiant:
As you can see, in my setup, the Draper Contrast Radiant at least holds up as well to viewing angle as the HCHP; in fact, it holds up better. Finally, once again, I don't know if this is because my HCHP screen is dirty or something...
Actually, rather than speculate, I'll just redo the experiment tonight with a patch of HCHP sample to see if the HCHP patch holds up better than my 110" HCHP screen.