Floating lumens II, my second integrating sphere in the make

Flashlight Modding and DIY Parts
41

Some details of the sphere, still before coating.

The sphere half with the holes and the baffles. You can see some repairs, the inner surface had notches and polystyrene bits sticking out, I cut bits away and filled holes with lightweight wall filler.

Detail of the glued in (foamboard) baffles. BTW, not all glue goes well with polystyrene.

Here is the luxmeter-sensor. While the sensor of match's sphere was easy to clamp into the sphere repeatedly in quite the same way, with this one it was not so easy. I made a mounting solution for it with a black metal ring I had leftover and that accidentally exactly would fit the sensor within a fraction of a mm, making it possible to repeatedly mount the sensor in exactly the same position.

The ring on the inside of the box. The hole in the sphere fits around it with no play.

On the outside, with the clamping mechanism.

The sensor in position.

And how it looks on the inside of the box.

42

Ah, thanks, the specs back this up, also no surprise that the blue one is so nicely constant :-)

43

I started on the coating. For the first layer I used plain latex wall paint, to create a stable substrate for the 60%BaSO4/40%latex paint mixture of the subsequent layers. The mixture is not very great as a paint but it does stick and does not readily crack away when the styrofoam is lightly deformed.

Only when I started painting I discovered that this paint is not really white but slightly off-white, to the yellow/rosy side, just a bit, not a real problem because there will be more layers. For the mixture with BaSo4 I will use the other pot of latex paint I have that is much whiter, actually looks as white as the BaSo4 powder itself (and the styrofoam by itself also looks surprisingly white btw).

The off-white paint of the first layer gives an opportunity to show the cumulative effect of an integrating sphere. In the first picture I shine a flashlight (Nichia 219B) through the hole of one of the sphere halves onto the other half. But because the halves are separated, multiple reflections are not possible. (the greenish tint is from my limited camera). You can see that the reflected light from the paint is (almost) the same tint as the light reflected from the bare styrofoam on the outside of the sphere.

But with the halves connected, multiple reflections are taking place, leading to a much different tint (as seen through the hole), the yellowing effect of the paint is multiplied! :

Off course I should have shown this for the bare styrofoam too, but I forgot. I will get me another styrofoam ball one of these days and do the experiment still.

44

Thanks for the update.

45

… What?

I can see PWM pretty well, but I’m not very good at small differences in tint, and even worse at brightness. Especially at the red end of the spectrum, since I seem to have a bit of a blue shift in my vision.

46

I finished the coating and tested the results. The news is bad and the news is good. First some pictures:

On the left is the wrong paint I used for the first layer on the polystyrene, on the right is the latex paint that I used for two more layers, mixed with BaSO4, it is visually the same white as the BaSO4 powder in the middle. I made the 60/40 BaSo4/latex-paint mixture much thicker than for my first sphere, so I sufficed with two layers, the combined thickness was more than what is inside my first sphere.(for a background why I use this mixture, see my first 'Floating lumens' thread)

A picture of the BaSO4 powder next to bare polystyrene, also pretty much the same white visually (part of the polystyrene can be seen painted with the off-white latex).

After finishing the coating:

The sphere was put inside the wooden box, the holes matching the holes in the wood and the sphere firmly clamped in position by some cushioning leftover from flashlight packaging.

Then the lid was screwed in place.

I let the paint dry for a day and measured the reflectivity (the output of the cfc-light and my SWMD40A on high setting), then again two weeks later. In those two weeks the reflectivity of the sphere went up 1.02%, I hope that was still the paint drying for the last bit. Now it is two days later again and the measurements are exactly the same as two days ago. I will confirm (I hope) that is still the same a month from now.

The reflectivity of the coated sphere compared to the bare sanded styrofoam had gone down significantly (like in my first sphere) , by a whopping 37%. This is not nice because that reduces the integrating properties of the sphere. Luckily all my observations suggest that the integration is still pretty good. Because the total reflectivity is a combined result of a large number of reflections taking place inside the sphere, the decrease in reflectivity of a single reflection is much less of course. To get an idea, a few months ago, to get a feeling for my first sphere, I calculated the combined reflectivity after 17 reflections (adding more reflections to the calculation does not change the number much anymore) for two efficiencies for single reflection: 88% and 90%. After 17 reflections it comes to a difference of 13.6%. I could try to calculate what the difference for single reflection would be for the combined difference of 37%, but being a bit lazy, I estimate that on 6%. Ok, a bit less lazy, I just calculated it for 90% and 84% efficiency for a single reflection, after 17 reflections the difference is 30%. So the loss in reflection efficiency is even a bit worse than 6% .

So, if not for the reflectivity of the sphere, the coating must be good for something, right? At least the reflectivity for different light colours must have become more constant. A good integrating sphere must have the same reflectivity for all colours. Unfortunately I can not measure reflectivity directly, what I can do is measure the luxvalues inside the sphere of different coloured constant light sources before and after coating. If the percentual difference in luxvalue before and after coating differs for the various colours (sorry for this horrific sentence, any virtuosity in the english language really leaves me here), the coating has had an influence on the respective reflectivities over the colour spectrum (improved or gone worse I can not tell).

So I did the runtime tests again for the constant output flashlight that I measured in the uncoated spere as well. To get an idea in what analog age I am stuck:

Then I made the exact same graph that I made before coating of the sphere (see post #38), I only altered the scaling of the vertical axis, to set the runtime curves of the neutral white flashlight before and after coating on exactly the same level. So then it is easy to see if any curve has shifted (this only works if your display is wide enough to show the two sets of graphs next to each other):

Hmm, that difference is not great, let's flip the uncoated graph, so that differences are more easy to spot:

Still looks very similar, but there are changes, of course most clearly for the single colours. After the coating as compared to before coating, if the neutral white curves are defined equal (just to have an off-set), blue measures (about) 10%lower, red 10%higher, green 3%lower, coolwhite 0.7%lower, warmwhite 1.5%higher. Am I impressed by the changes in reflectivity over the colour spectrum that the coating causes? Not really. The extreme colours blue and red do have a clearly altered reflectivity, but not many will want to measure them very accurately (maybe me ;-) ). What matters in the real tough flashlight world is the different whites, and going from 3000K to 6500K the difference uncoated vs coated is only 2.2% (from neutral to cool only 0.7%).

I assume here, because I did some research on what substance to use, that this coating improves the constantness of reflectivity of the integrating sphere over the colour spectrum. But I did not find anywhere how good bare styrofoam already actually is in doing that (perhaps very good, who knows?). It is not unthinkable (I do not like that thought of course) that the difference that I find here are not an improvement but a change for worse. I just measure relative changes, not absolute reflectivities.

My conclusion about coating with BaSO4/latex mix? Because the effect is not that great, and because I do not even know for sure if that not so great effect is for the better, and because it is a messy and tedious job: just don't bother! And that is bad news because coating my spheres was apparently not really worth the effort, and that is good news because this makes all those uncoated styrofoam integrating spheres out there just a bit more trustworthy :-)

47

The loss in reflectivity caused by the coating should not have any effect on the ability of the sphere to integrate the readings (the shape of the sphere is what mostly determines that).

As far as wavelength dependent reflectivity/absorption goes, I have seen a slight effect of the styrofoam from an ideal reflector, but it is not much. I can operate my sphere in an ultra-high lumens mode by attaching the sensor to the outside wall of the sphere, using the styrofoam wall as a light attenuator. My sphere uses a TAOS RGBW color sensor for its measurements. I get minimal changes of the color values/calculated CCT between the interior/exterior light measurements (the biggest effect seems to be on the blue channel).

I came to the same conclusion as you… the coating is not really any better than just sanding the sheen off the sphere interior.

48

Amazing effort and research. How did you work out the baffle locations and size. If I've missed this somewhere sorry.

49

No, it does affect the integration a bit, this is why: There is an amount of light entering the sphere. Because of the baffle it can not directly illuminate the detector but light that has undergone a first reflection can. This first reflected light comes, depending on beam shape, from all sides to the detector or from a certain area in the sphere, so the angle from which it enters the detector is not random and the distance from which it comes is not average. So the reading of the first reflection is susceptible to limitations of the detector. Light that has undergone more than 1 reflection probably already is random in distance and angle from the detector. Now, the better the reflectivity, the higher the contribution of multiple reflected light to the lux-reading, and that is lowering the contribution of that first reflection of which the reading can not be fully trusted. (in case of 90% reflectivity of the combined inner surface of the sphere, which is already a challenging number for non-professional coatings and the presence of holes that fit flashlights, a quick and dirty calculation learns that light that has been reflected once contributes to about 11% of the light that reaches the detector, probably in reality this number is higher).

That is of course a quite qualitative test but it does give a good indication I guess. You do not know how many reflections the light that comes through the walls of the sphere have undergone compared to light that is bounced inside the sphere, but you do know for sure that unlike the light inside the sphere there is no contribution to the reading of the first couple (10? 20? I don't know how many reflections it takes to get through 2cm of styrofoam) of reflections.

50

Thanks Steve! :-)

The baffle location is the middle between two holes. The size is determined by trial and error: I made test-baffles out of cardboard, and while holding them in position I looked from one hole to the other to see if it is fully covered visually. then trim the cardboard a bit and check again, until the minimum size is reached (I added an extra 2mm to be sure).

51

I think I’ll be lucky if I ever get around to making a cheap papier mache sphere… and so far, I’ve been using an even less accurate integrating milk carton with a bottom-of-the-line HS1010A meter.

BTW, for maximizing reflectivity and integrating properties, would it be worth attempting to cover the inside with a mirror-like material? (and perhaps aim the light in at an angle so it won’t reflect directly back at itself) The idea would be to make the light sensor the only absorbent surface. Due to increased overall brightness though, it might also be necessary to put the sensor behind a sheet of foam to attenuate the signal. I have no idea if this would actually work though.

52

The integrating properties of such a sphere rely on the cosine law of diffuse reflection, a mirror-like surface would kill those properties. Great effort is made to get the surface as diffuse as possible.

53

HaHa! I just read this in your first integrating sphere build thread. :stuck_out_tongue:

54

I must re-read my threads more often, saves a lot of work and trouble ;-)

Ok, but now I have a sphere of which it is dubious if it is improved at all by the coating, but at least I extracted some data from the work. (I am especially fond of the quote in Bort's sig-line: "In God we trust, all others must bring data". The quote is attributed to William Edwards Deming, a thorough man, it seems)

55

Yes, the first time around you didn’t mention anything about measuring the differences in reflectivity for different wavelengths of light. This time around, you included that consideration in your testing the Styrofoam vs. the coating, so maybe when you get around to making your THIRD integrating sphere, it will be coating-free. :wink: Or maybe it will be coated with glass micro-beads suspended in clear epoxy, then wet-sanded. Wouldn’t that give you a better reflectivity of all wavelengths, while also diffusing the light source? Hmmm. Oh wait, epoxy yellows over time, doesn’t it?

56

Yes, it does, while my latex painted walls look as white as when I painted them. The glass beads must be lens-grade clear and I'm not sure if you can finish glass to such a matt that reflections are close to lambertian.

My third sphere (you hit the nail on the head here ;-) ) will indeed be uncoated (now for real), small, and easy to make. I'll post about it when time comes :-)

57

I was thinking about the hollow sphere glass micro-beads that are used in making reflective coatings on signs and such. The way they work is by reflecting the light off their interior concave surface. Surely, there is some clear coating that will not yellow over time. But, if a non-coated surface works, why bother anyway? I’m just trying to add a little silliness to your thread. :bigsmile:

58

I don’t think it makes any measurable difference. You have a zillion photon paths bouncing light around a zillion times and reaching the sensor from a zillion angles. It all averages out at the sensor to a constant value.

59

In more ways than one! :slight_smile:

I used the glass bead material for my photography lights. Not impressed. It works great on a white stripe on the road, even yellow, but questionably in a 15” reflector for a photography light stand.

60

… but those are specifically chosen for their ability to reflect light back toward its source, no matter where the source is. It’s like if you ever stood in front of two mirrors placed at a 90 degree angle to each other, it always reflects your image no matter where you are relative to the mirrors. Add a third mirror perpendicular to both, and it’ll do that from any angle… or, just use a concave half-sphere and it’ll reflect any light back to its source.

That seems like it would defeat the point of an integrating sphere, because the photons would be aimed to go directly back into the flashlight instead of into the light sensor.