Integrating sphere #3 (portable and overly stuffed with features)

I was wondering why you re-callibrated for every flashlight. Wow! That’s mind-boggling.

Thanks for the detailed write-up and explanation. :nerd_face:

Hi djozz,

I was just wandering, is there a reason you use the lux setting on your meter over the foot candle one?

The reason is that I did not come across any practical light measurements before I started measuring flashlights. And since my original background is (or was, quite some time ago already) science, the logical unit would be the standard SI-unit which is the lux.
Isn’t the foot candle more an american than a european unit?

I’m not sure. I have always used the foot candle setting for lumen measurements. I use the lux setting when I take what I called “lux” readings. IE the thing where you measure distance by firing directly into the meter and the multiply by the cube root.

I had never noticed that you and some others I see are using lux. I did a few tests just now and so far the only difference I can tell is that a slightly higher multiplier is derived. When I measure lights I seem to get roughly the same result.

Using the FC setting a light that I get 4,335 lumens with renders like 4,458 ish when I do the conversion from lux. My system is not as accurate as yours though, I can get a variance like that repeating the same test twice.

EDIT: Also I was wanting to ask you. When using the small sphere you made where the sensor just attaches to the outside of the ball do you have to recalibrate in order to measure a bare LED? When I do it in mine the recalibration is vast. Multiplier for flashlights is typically 3.4, for doing bare LEDs it its 4.3.

Lux meters measure of intensity of light at a single point or a small area. The result can be converted to/from foot-candles by using a factor of 10.764, but lux is the SI standard unit. We can also determine candelas (intensity per angle) with a lux meter, by carefully controlling the distance from the light source.

The integrating sphere is supposed to spread all the photons evenly so that a single-point measurement will be representative of the total amount of light, regardless of how that light was initially distributed. An ideal sphere won’t really care if the light source is a bulb or a laser. So, if you get massively different results for a reflector-based light versus a bare emitter, the sphere isn’t doing its job very well. At minimum, you might need to add a baffle between the entry hole and the sensor.

There can be several reasons for a different multiplier:

1) integration is poor
As Toykeeper pointed out, the integration could be limited, due to the box being not spherical (I still think that a box vs a sphere should not matter that much for integration, but never measured it), or to the absence of a well positioned baffle blocking light from the source going directly to the light sensor. Like the bended pipe device that has limited integration, all works acceptably well as long as reflector flashlights are measured with similar light distribution (beam pattern) that are pointed in the same direction all the time, and if the calibration was done with such flashlights as well. But a bare led is an extremely different beam, getting accurate results for both a directed source like a flashlight and a spread-out source like a bare led, requires much better integration.

To check for integration quality of a measuring sphere/box/pipe, you can use a small zoomie (small to minimise entrance hole effects, see number 2 ) in spot modus, on low setting so that the output remains more or less the same (you can check for output variation of the zoomie before starting the next step). Then hold it in the same position in the entrance hole and write down the reading when shining it in several directions into the box (straight ahead, 30 degrees in different directions, 45 degrees in different directions. If you find a variation in your values of more than a few percent, the integration of the device is limited, different light distributions will affect the reading. If I remember well, my 46cm sphere gives variations of 3% maximum, with still only 10% difference (lower) when shining the spot directly on the baffle.

2) entrance hole effects.
the total reflectivity of your sphere, which is a measure for your multiplier, is also influenced by the reflectivity of the entrance hole. The reflectivity of the entrance hole varies with what object is in there. A flashlight with its (large or small) reflector will always cause a higher reflectivity than a bare led (or an empty hole), so your mulitplier will be lower for a flashlight than for a bare led, but how much? You can minimise this effect by using a low hole/inner surface ratio (big sphere, small hole), or you can every time correct your multiplier for ‘hole effects’ (3 of my spheres have such a correction option, it is using an build-in constant light source in the sphere). There is a very stubborn misconception on BLF btw that the sphere’s reflectivity must be maximised at all times for the most accurate results (people are using inserts that alter entrance hole size, or white disks around the flashlight), but what is really needed is that the reflectivity is kept constant, and you have no way to know (measure) how such an insert influences the reflectivity (and thus multiplier). A high reflectivity is good, is needed, but keeping it constant is way more important, that gives you the non-changing multiplier.

You can get a feeling for your entrance hole effect by fabricating an aluminium foil round around your bare led of the size of the entrance hole of the measuring device (a flat round of alu-foil, not conus shaped like a flashlight reflector), this mimics the reflectivity of a flashlight. Then measure the led at the same current with and without the aluminium round in place.

3) factory specs may differ from real world results.
I’m not sure how your calibration for bare leds was done. I assume you used the factory specs for a number of leds and figured an average multiplier out of that? It could be that how the factory measures the led (for milli-seconds is what I have read somewhere) is different and gives higher values than real world measurements with the led continuously lighted. This is not something I know a lot of btw, I just pose an assumption here.

I hope this makes a little sense to you to and I hope you will find out where the multiplier difference comes from. And if not, just use the different multipliers for different light sources and live happily everafter, being obsessive about accuracy (and never really get there :frowning: ) like me may not be very health anyway :slight_smile: :slight_smile:

:+1:

I am sure that the reason my setup has to be recalibrated is because of lack of integration. When I build that setup years ago I had a $0 budget and needed it mostly for A B comparison.

The main reason I asked is too see how much of the recaibration could be eliminated by going with a better one.

After reading a few articles (including TKs link) I don’t think that having used the FC setting really hurt me any. My results have been reasonably consistent with what others have posted anyway. Moving forward I may join the band though.

I think also what I really really need is a true constant source for calibration. I am thinking maybe something that plugs into the wall vs runs on batteries. Like maybe an MR16 LED bulb or something.

If you write a PM to texaspyro, he uses MR16 bulbs as a constant source for his sphere, he will know which one will give consistent output.

As an alternative, my constant output lightsource for checking my multiplier is a Sunwayman D40A. The high (not turbo) setting has a really consistent output, even at partly drained batteries. You could scan through your high end flashlight collection (SWM, Nitecore, Zebralight) for a good one. If the light uses current control for the lower modes (not PWM), there is a good chance that the light output on a medium setting will be nicely constant.

I’ve found Zebralights make a good constant output source for calibration purposes, as long as they’re only on a medium setting and have reasonably full batteries. I’m using selfbuilt’s data to choose reference lights though, since he has such nice runtime output graphs.

Here’s a Zebralight on two medium modes:

TK,

That helps in ways you cant imagine. I’ve had a Zebralight sc52 since like last year. Mine is the XM-L version. I love the light because its AA and Type III HA natural. But I was never able to get anywhere near a factory rating on output with it. Worse still there was no change to the turbo mode even using a 14500. Max output was like 154 lumens. Seriously I thought it was broke until just now. When I saw that M2 thing, my hopes are what lit up. I picked mine up second had so it never occurred to me to look at the instructions. Fortunately the new version has the same UI… :slight_smile:

I keep my D40A as new, and solely for checking my multiplier, keep it in its holster so not to damage the anti-reflection coating (which would alter the OTF output), it only comes out when a reference source is needed. When using it, I keep the batteries more than half full, switch it to turbo (~900lm) for 10 seconds to warm the light up, then switch back to high (~550 lumen according to the manual) which is then very nicely constant (in my integrating sphere #II it measures the same everytime within 0.5%, which maintains my confidence in both my D40A and my integrating sphere :slight_smile: ) . My ‘djozz-lumen’ is based on this high setting of my D40A, assuming that the 550 lumen is correct. It is not really important for what I do that this is really 550 lumen or a bit off (although I want to have it really measured sometime), what is important to me that it is exactly the same output every time, so that all my light measurements can be compared to each other.

I’m nowhere near that careful. :stuck_out_tongue_winking_eye:

I’d like to be, but I haven’t been doing much where accurate consistent measurements are very important. So, take my ideas with a grain of salt. Djozz knows what he’s doing. :slight_smile:

Right now I am leaning toward djs 2 ball design. I am also considering doing a coat of barium sulfate on the inside. I noticed that pro grade sphere use this so I assume there must be some benefit. I found a place that sells it premixed in polyvinyl alcohol.

I am hoping it can be applied directly to the foam using a sprayer. Still waiting for them to contact me back.

Vestureofblood, you know I'm a big fan of yours. Is this a project you are going to possibly bring to life? If so I would like to know more about it. I'm assuming it will do large flashlights too?

Thanks jhalb. I would like to make a new setup. I have been using a pretty rough light box for years. The other day I made one of those lumen tubes just for fun, but am not all that impressed with it. I was thinking for the new sphere I will probly cut the hole for the size of a maglite head since I work with them a lot. I will keep the lumen tube for measuring larger lights as it was made from 4” pipe.

I have another question for the pros… What method are you guys using to derive your multiplier?

I have always done it like this.

1. Take know source ( in my case 875 lumen maglite measured by another member).
2. Divide lumen output by lux reading.

Example 875 lumen light produces 2550 lux. So 875 divided by 2550= 0.343

So from hence forth all measurements from other lights are multiplied by .343

However just now reading posts by others I see that Match appears to be doing the opposite.

A quote from his post.

but the most accurate one I tried was to simply take a light of known value, test it to get the lux reading, and then divide that reading by it’s alleged lumen rating.

Then here I looked and found yet an even more obscure method. Something about measuring the hole??

How are you doing it?

I think that match did the same as you do but he wrote it up the wrong way around.

The ‘obscure method’ takes a careful performance to do it accurate, but a drawback is that to get the correct number, it is essential that your luxmeter is correctly calibrated, if the luxmeter is off, the multiplier will be off. Your current method does not care about the calibration of your luxmeter, the calibration comes from the reference flashlights.

I’m interested in the coating stuff from Sphereoptics, my own DIY BaSO4 coatings did not have that high reflectivivty that this stuff claims. The website does not mention a price or retailer. If you are able to obtain it, would you mind letting me know where, how and how much?

Ya, I will report back once I hear from them. If I don’t get a response via email soon I will try to reach out to one of our German speaking members and see if they will give em a call…

We may be out of lux on that premix even if they do contact me back. I just found this on another site $500-$1200?
http://www.labspherestore.com/SearchResults.asp?Search=6080&Search.x=8&Search.y=15

I am thinking if that is the case I may try to make some. Here is where I found out that it was made from.

PVA is not that expensive, and Bariumsulfate is not that expensive, so I guess they just tack on an extra $450 buck for adding the word “Labsphere”

Maybe, thanks for finding that abstract. I have some BaSO4 leftover and just ordered some PVA. Experiments are to come :slight_smile: