tvodrd said:
(Jar- give 'em hell!)
Larry
Well, I'm a working man, and I've gotta head out early in the morning, so I will try and keep things very brief.
For now, IMHO, the generic 65% is a great way to hit a rough approximation.
If someone wants to use another number, I do feel that an integrating sphere is going to be the only real legitimate way of claiming otherwise.
Don't forget that AR coated lenses range from 0.2 to 2% reflectivity on each surface (front and rear), depending on whether it is a single coat, or a multilayer broad band coat. The skill of the chamber technician will affect the final outcome. You also incur losses in the glass lens, depending on whether it is common soda lime, "water white" (soda lime with reduced iron), borofloat, or various other types of glass. There are also two common types of AR coatings, Magnesium Fluoride and Silicon Dioxide. The Magnesium Fluoride coating typically will have a lower reflection amount, but the Silicon Dioxide is a bit tougher. Usually a good Silicon Dioxide AR coating will look green when done correctly, and you can judge the quality of the Mangesium Fluoride coating, as it should have a purple hue or tint in the reflection. Having the coating thicknesses done just right, means it is centered in the visible band, and you get some reflection on the red side and some off the blue side, with the least in the center (green). Thus the purple hue.
*BUT* AR coatings do not work as well when the light doesn't hit the surface perpendicular. As the angle increases, the percentage of reflection increases very significantly. This is something to take in account for if the diameter of the opening increases without increasing the depth at the same time.
As far as the effect of the rear hole, the reflector focal point would have a good amount of effect. In most reflectors, the bulb itself takes up a considerable portion of that "escape" area anyhow. If that escape area is further filled with a mirror, or even a white ceramic or whatever, it's contribution will be even less. Though one would really have to wonder, if it wasn't directing the light out the front, what real good those few lumens would be. Consider the area of the gap, or the "escape area", it's distance from the filament, create a sphere around the filament at that distance, and compare the area of the sphere to the gap/escape area (and remember not to include the area the bulb itself blocks)...
It would be a pretty trivial task to stick all this into a decent raytracing program and assign all the surface properties, and then re-run it with new propertes. It just takes time, which I do not have at the moment.
The reflector coatings are an important piece of the whole situation. Though I don't know if I'd personally buy just an Enhanced Aluminum, or Enhanced Silver. I'd be more apt to lean towards a protected enhanced aluminum or protected enhanced Silver. I did several posts on these coatings in the 2003-2004 timeframe, complete with quite a few links to references.
Boy, I sure wish the search function still worked, there is an awful lot of wonderful information we built up over the years. These special reflector coatings are nothing new, and we have discussed them several times before.
Unfortunately, standard Vacuum Deposited Aluminum is not quite the same as Enhanced Aluminum, though it usually is better than no coating. Additionally, I've seen multilayer stackups of various materials, that can attain +98% reflectivity, even with the Silicon Dioxide protective overcoat. Think of it kinda like a dichroic reflector, but used to further enhance the coatings even more.
Again, IMHO, the 65% is a *darn* good number to start with, until a given item is actually tested with an integrating sphere. At that point, with real numbers in hand, then one would be able to legitimately claim otherwise for that particular setup.
Oh, and on the ohmic thing. Doesn't the formula only figure for a filament in free space? One would have to consider the contribution of putting it in glass envelope, pumping it full of a gas, and the heat it would contain, as well as the contribution of cooling since there is a gas in there, and the air currents. One might want to also figure in the effect of the stand off wires to the whole situation. I've never seen the source for the formula, nor bothered to look up how it was produced. I'd also figure that how the filament was wound, single turn loop, single turn loop then wrapped in a larger loop, and the proximity of the other turns in the loop, as they'd radiate into each other. Also, something that comes to mind, is the halogen cycle... Now, once you contain all this in another housing, such as a reflector with a cover glass on the front, you in essence have formed yet another thermal containment chamber of sorts.
Just my quick thoughts on things, I really gotta hit the sack now, you guys have a great night, and even a better tomorrow.