Diffuse AND reflective? Isn't that a contradiction in terms?
Not at all. Best example I can think of of this technology is integrating spheres, which are used to accurately measure the number of lumens coming from a light source:
http://en.wikipedia.org/wiki/Integrating_sphere
Idea of an integrating sphere is to completely diffuse all the light that is pumped in without a significant amoutn being lost to absorption. The entire surface of the sphere will then be completely uniform and the brightness of light going out the exit port will only depend on the total number of lumens going in, not the beam distribution going in.
The white coating in remote phosphor bulbs is the same idea.
I think it's kind of like the difference between soft white and frosted bulbs. reflective meaning not absorptive, as opposed to specular reflections like mirrors do.
Perhaps the idea is to not have any "hotspots" of pump light hitting the phosphor, since heat can have a negative effect on downconversion in most phosphors. Like, to keep the average irradiance lowest at all points on the phosphor, it would be best to diffuse 100% of the pump light across 100% of the area of the phosphor. This would maximize conversion while keeping heat at the phosphor to a minimum at that output level.
I do have some questions in this regard, though. Are there some phosphors that are more resiliant to heat than others, or is this mainly a function of the material in which it is embedded?
The actual phosphor material itself is a powder suspended in plastic. Tolerance to heat has to do with the material it's encapsulated in. The Intematix produts from what I understand are polycarbonate and are not meant for operating at high temperatures, or high temperature gradients (can cause cracking) The phosphors that are adhered directly to the surfaces of LEDs are embedded in silicone, which is soft and can tolerate high temperatures (150C+) and significant temperature gradients.
And so far, all phosphor uses seem to be having the pump pass through one side, and the usable light coming out the other. What if for my application I wanted to pump the material from the front AND capture and use the light emitted from that same side? It would be a bit bluer than if I had the ability to pump through the entire thickness of the material?
As a thought experiment lets say you were to shine a blue laser beam, or collimated blue LED beam onto a phopshor panel. A few undesireable things will happen:
1) Unless you AR coat the phosphor plate, you will have a substantial specular reflection. This will be a source of loss. It will also be a source of glare -- an obnoxious (or in the case of a laser, potentially dangerous) blue beam of light will be visible from outside your device
2) A significant amount of the blue light (more than half) will pass through the phosphor unabsorbed.
3) the yellow light emitted by the phosphor will be emitted approximately equally forwards and backwards. Without some kind of reflector behind the panel, half the light emitted by the phosphor will go out the back.
To fix 2 and 3 you could try to put a reflector behind the phosphor. A mirror-like surface is not good though because even on a double pass, you'd get a significant amount of blue light that would pass through and not be absorbed (adding to the obnoxious glare problem). The ratio of blue light absorbed to yellow light emitted would also be totally out of whack unless you basically engineered your own phosphor panel from scratch with the intention of using it in a front-lit, rather than back-lit application.
In the case of back-lighting with a sealed mixing chamber, surface reflection is not a problem -- light that is reflected from the panel, or emitted backwards from the panel will get dumped back into the mixing chamber and will bounce off the walls multiple times being recycled over and over again until the light eventually makes it out. If you were to run a raytrace simulation, many of the rays that eventually make it out of the lamp undergo several "round trips" through the mixing chamber and phosphor before eventually getting out.
The thickness and absorption properties of the intematix panel is designed with these "round trips" in mind.
Because light often undergoes many many reflections inside the mixing chamber, it's important that the material be highly reflective. The magnesium oxide white coating in integrating sphere and remote phosphor lamps absorbs a fraction of 1% of the light on each reflection. Aluminum or chrome paint (as suggested in this thread) absorbs somewhere on the order of 10%+ percent which adds up to a huge amount of loss. Hobby applications for remote phosphor is not very easy unfortunately.
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