I wonder why the slight difference between yellow/white?
Perhaps the yellow pigment is on a layer above the actual reflectors, so it may absorb some light both coming and going, so to speak, making it a bit less reflective.
The interesting thing is the way they refer to their figures for the retroreflectivity.
Table A refers to the retroreflection as:
Minimum Coefficient of Retroreflection Candlepower per foot candle per Square Foot Candelas per Lux per Square Meter (0.2° Observation Angle).
With this footnote:
1Observation (Divergence) Angle — The angle between the illumination axis and the observation axis.
To me, the implication is that they expect you to be illuminating the retroreflector at an angle not more than 0.2° away from the observation angle. Or at least that's the angle they're using when testing to give the ratings. It would be interesting to see a graph of retroreflectivity versus "observation angle".
They do give some data points for the viewing angle with respect to the surface, but they don't say anything about how rapidly the reflectivity drops off as you move the light source away from the observation axis. They test with the angle between the two at 0.2°, though, which is pretty much straight on.
Without any test data for that, I have to think that viewing more or less straight down the illumination line will always be the best for this product.
It's probably somewhat forgiving, though. After all, a car's headlights are never too near to a driver's eyes. But what we do probably see is that when you're far away from a license plate, you get pretty good reflection because the angle between your eyes and your headlights is shallow, but as you approach the car, the angle becomes larger but at the same time, you're getting closer, so the brightness of the illumination gets better, too. Perhaps offsetting the reduced reflectivity in a rather pleasing way.
And the same may go for an LPR setup. You might actually do a bit better to place the illuminator slightly away from the camera so that when a car is distant, you have a smaller Observation Angle and better reflection, and then, as the car gets closer, the observation angle gets worse, but the intensity of your illumination is better because the car IS closer to the light source. And THAT might make the exposure to the camera remain somewhat constant, making exposure settings more forgiving. Sort of an unintended benefit!
But you'd probably want to optimize this for a particular type of retroreflector. Some may be very forgiving of Observation Angle, while others may be more "strict".
