...a slow going thread, but this work has no urgent practical use anyway :-) ....
A good integrating sphere integrates so well because the light is reflected multiple times with minimal loss on the inner surface of the sphere, so that the initial beam pattern of the light source 'drowns' in all subsequent reflected light. If on the inner surface the reflected percentage of the light is not the same for all wavelengths but slightly different, there is a problem that builds up, with every reflection the difference is increased, and if you realise that for the sphere's output 10+ reflections have a measurable impact, you understand that the reflection of the inner surface should be very close to the same for all (visible at least) wavelengths, or the sphere will 'favor' certain wavelengths. The crappier your sphere, the less this is a problem of course . But does a coated sphere do that any better than a bare styrofoam one? (to be honest: I can not measure 'better', I measure 'different', but that is already good information to me)
this weekend I did runtime measurements of six 'constant output' flashlights in the new sphere. I did this for two reasons, 1)to see how constant the outputs were (quite, but not really, but it is repeatable, that is even more important), 2)I will do the same measurements again when the inner surface of the sphere is coated, comparison will hopefully give a clue about what the difference is of the sphere's spectral output after integration, with the bare styrofoam, and after coating (does coating make a difference?).
To see a maximum difference of the sphere's 'response' for different parts of the spectrum I made a red, green and blue flashlight (host 602C, 1x7135-driver, XP-E2-leds), and to see how much of the (if any) differences are leftover in normal flashlight situations I made a cool white (Uniquefire S10 host, XM-L2 0D, 2x7135), neutral white (Fandyfire A10B host, XM-L2 3B, 3x7135), and warm white (Uniquefire S10 host, XM-L2 7D3, 2x7135) flashlight. All leds were on a DTP copper board and in case of a brass pill (Uniquefire and Fandyfire hosts) they were soldered to it. Batteries were freshly charged IMR batteries.
Here are the outputs of these six lights in the uncoated sphere, coating will be done later and then this will be repeated and compared:
So when made this way with good heat-paths to the shell and with only few 7135 current regulators, the flashlights have a fairly stable output but not 100%. For practical purpose, if these flashlights are used between 5 and 10 minutes, the output stays within 2%, some within 1%, for more constant output you can wait longer. BTW, I used no active cooling of the lights, they were left flat on the wooden sphere-box at room temperature (see picture below), the output may have been flatter with fan-cooling but that would make the lights very unconvenient to use. The clear winner btw is the blue flashlight, the output dropped just 2% for the entire runtime of 40 minutes, while for the red flashlight it was 10%. Differences are caused by battery voltage (even though they are supposed to deliver constant current, 7135 chips I found are slightly sensitive to input voltage) and by led temperature, while some leds are more temperature sensitive than others. . But for me the most interesting is that these runtimes are very repeateable (I showed that for the cfc-light in post #16), if I cool the lights down and recharge the batteries, these runtime graphs will be exactly reproduced.
An illustration of the problems you get when trying to measure in the 1% region is when I measured the red flashlight after 40 minutes, I noticed that during the measurement the output started rising again; it appeared that cooling of the light by touching it for the measurent was the cause: to check, I held it fully enclosed by my hand for ten seconds and that resulted in a 2% output increase, something very significant to account for when using this type of 'constant' lightsource . I have the impression though that the red XP-E2 led is more sensitive to temperature than the other leds.
2bcontinued..