# Need creative ideas for measuring LED intensity



## blasterman (Dec 3, 2011)

Since I don't have a spectrometer I've been trying to come up with a way to measure relative intensity levels between specific LEDs that has little bias in regards to spectrum as possible, and drawing a blank. Dealing with a lot of large reef tank builds and the sheer volume of offbrand LEDs I need some way to measure the relative intensity of LEDs. I can keep it as simple as using a Cree at a specific current, and then determining what intensity level 'X' LED is when using the same config given that 99% of everything else is going to be less efficient anyways. I really don't care about color readings - I want to determine how much less energy 'X' LED is emitting compared to the benchmark. That has to make things easier.

I thought about using a trick from the laser geeks and using a small solar panel, and using a multimeter to take voltage / current readings off the panel with bare LED's illuminating them at close range.

Problem with this is I need to take readings from both royals and various whites. Depending on composition solar cells start to get terribly non-linear around 460nm. When I start throwing in that math, along with actually trying to determine the offset I simply have too many varibles to keep things straight.

Any ideas appreciated.


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## idleprocess (Dec 3, 2011)

I'm not an engineer, nor do I play one on youtube docu-dramas ... but if you had some color filters and a light meter, perhaps you could get some approximate readings on intensity at whatever wavelengths the filters work at then do some fuzzy sum-integration of said plots?


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## saabluster (Dec 3, 2011)

blasterman said:


> Since I don't have a spectrometer I've been trying to come up with a way to measure relative intensity levels between specific LEDs that has little bias in regards to spectrum as possible, and drawing a blank. Dealing with a lot of large reef tank builds and the sheer volume of offbrand LEDs I need some way to measure the relative intensity of LEDs. I can keep it as simple as using a Cree at a specific current, and then determining what intensity level 'X' LED is when using the same config given that 99% of everything else is going to be less efficient anyways. I really don't care about color readings - I want to determine how much less energy 'X' LED is emitting compared to the benchmark. That has to make things easier.
> 
> I thought about using a trick from the laser geeks and using a small solar panel, and using a multimeter to take voltage / current readings off the panel with bare LED's illuminating them at close range.
> 
> ...



Maybe take one of the other tricks from laser geeks and use a TEC. Coat it with flat black BBQ grill paint and mount it to a heatsink. Then measure the voltage off that.


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## slebans (Dec 3, 2011)

I remember seeing apps/add-ons for the IPhone and other smart phones that would be suitable for your application.

Stephen Lebans


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## MikeAusC (Dec 3, 2011)

blasterman said:


> Since I don't have a spectrometer I've been trying to come up with a way to measure relative intensity levels between specific LEDs that has little bias in regards to spectrum as possible, and drawing a blank. Dealing with a lot of large reef tank builds and the sheer volume of offbrand LEDs I need some way to measure the relative intensity of LEDs. . . . . . . I really don't care about color readings - I want to determine how much less energy 'X' LED is emitting compared to the benchmark. . . . . .



So you want Radiometric measurements that measure the total radiant energy, without weighting according to the eyes' sensitivity vs colour ?


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## Kinnza (Dec 3, 2011)

Before having the spectroradiometer, I used a cheap spectroscope (you can use a difraction grating directly aswell). You take a pic of the spectrum and process it with sofware developed for this task, available for free on the net. It basically consist on process the pic (TIFF or RAW formats, so camera corrections and white balance can be discounted) to black/white image, so absolute scale can be inferred from gray scale. Then this info is plotted as a SPD graph.

The main difficulty with this method is to calibrate the wavelength of the pic. You can overcome it measuring with a back lighting of known spectrum which provides the calibration info for knowing the peaks, but which is discounted from the last process stage or well used only to determine the peaks wavelength and later process a pic of the SPD of interest alone, but now having the info about wl peaks).

Check this page, it has all the info required to do it, along with the free sofware or links to it. Very in deep analysis of how to get SPDs from pics.

I dont know of any built in sofware for Ipads or so that does it as Stephen says, but since I got the spectrometer I didnt search for it anymore. I wouldnt surprised if somebody has came with such solution. Would worth to search deep about it.


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## Kinnza (Dec 3, 2011)

Oh, forget to add how to know absolute radiance once you know the SPD. You need to have an integrating sphere or a goniometer, measure total flux and correct it for the LER of the SPD. For royal blues, it may become inaccurate when using standard light meters, which often are not well calibrated for wl below 460nm.

But if you only want comparative results, as the response is the same with the same sensor, you can get good enough data easily.


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## Kinnza (Dec 3, 2011)

http://www.audela.org/dokuwiki/doku.php/en/start


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## Kinnza (Dec 3, 2011)

I read your first post again, blasterman, and I noticed you are not interested on the total radiometric flux, but on intensity (mW/sr). In order to compare such magnitude, you just need to measure at a given distance of the optical axis with a light meter (for royal blue, better a good one) and correct it by its LER (obtained by its SPD). 

Likely some software for digital camera or add-on for phone with camera could do it, but the latter probably would be limited by the sensibility on the blue range for short wl.


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## David_Campen (Dec 3, 2011)

Take a metal cylinder that is closed at both ends. 
Paint the inside flat black. 
Surround the cylinder with good thermal insulation.
Place a temperature measuring device underneath the insulation, mounted so that it is thermally well connected to the cylinder.
Make a hole in the cylinder so that the LED can be placed there emitting all of its light into the cylinder but with the LED thermally isolated from the cylinder.

The rate of change of the temperature of the cylinder will be proportional to the intensity of the light being directed into the cylinder. Compare an unkown LED against your reference LED and the ratio of the rate of change of cylinder temperature gives the ratio of the light intensities.


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## blasterman (Dec 3, 2011)

> you just need to measure at a given distance of the optical axis with a light meter (for royal blue, better a good one) and correct it by its LER (obtained by its SPD).



That's problem - how do I* measure *it? While there are many interesting suggestions here, most rely on CCD type photon counters, and these simply won't work because color CCDs use color cutoffs on their sensors. Digital camera sensors for instance typically have a 455nm blue filter, which means a light source at 455nm is going to read more accurate than one at 450nm or 460nm. I've already run into this problem when using 10watt 440nm LEDs given they don't record properly as standard ~455nm royals with a dSLR because the 455nm cutoff filter can't distinguish color/intensity below 455nm. So, any device using color CCD / CMOS is out. Not sure what the spectral weighting is for monochrome CCDs, at least one affordable for this project.

If I had a small solar cell that was fairly linear between 420-530nm I'd be happy, and that would work, but all the charts I've seen show they aren't. So, I need to start with a photon detector that doesn't care about color in the first place, at least in blue-green, and work from there.



> The rate of change of the temperature of the cylinder will be proportional to the intensity of the light being directed into the cylinder



Very innovative, but I would think that temperature changes would be slight, and or take a very long time (kinetic transfer of energy, correct?). Plus, the assembly would have to be in a vacuum otheriwse ambient temperature influences would screw it up. (??)


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## MikeAusC (Dec 3, 2011)

David_Campen said:


> . . . . The rate of change of the temperature of the cylinder will be proportional to the intensity of the light being directed into the cylinder. . . . .



This will also respond to Infrared radiation from the LED - Warm White XP-Gs have 28% of the peak radiation at 700nm.


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## MikeAusC (Dec 3, 2011)

The "Clear" channel of the TCS3200 Colour Light to Frequency Converter has a response at 350nm that's 28% of its peak at 720nm. The sensitivity rises roughly linearly from 350 to 720nm.

You also have R, G and B channels available so you can make an assessment of the spectral distribution. The colour channel to be measured is set by applying DC to two input pins. The sensor area is 1mm x 1mm,

It has internal scalers so you can set the maximum to to 600, 120 or 12kHz. The Dark output is <2Hz. Many Multimeters have a frequency measurement capability.

It draws 2mA at 5 volts.

And all of this costs . . . . . . $6 !!!!!!!


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## MikeAusC (Dec 3, 2011)

The TEMD5510 is just a Photodiode that has a response from 400 to 700nm, but between 400 and 520nm its repsonse is higher than the eye's.

The VTB8440 is just a photodiode that has a response from 330 to 720nm, but the spectral response is not published.


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## David_Campen (Dec 3, 2011)

> The rate of change of the temperature of the cylinder will be proportional to the intensity of the light being directed into the cylinder





> Very innovative, but I would think that temperature changes would be slight, and or take a very long time (kinetic transfer of energy, correct?). Plus, the assembly would have to be in a vacuum otheriwse ambient temperature influences would screw it up. (??)



Rate of change of temperature would not be slight if the cylinder is matched to the power of the LED.

Say the cylinder was 1" diameter X 1" long and constructed of 6.75 mil copper foil. The cylinder (with end caps) would weigh 4.6 grams. Copper has a heat capacity of 0.38 watt-second/gram-degC so if you put 2 watts into this cylinder the rate of change of temperature would be 1.1 degC/second.

Phenolic foam insulation has a thermal conductivity of 0.5 milliwatt/inch-degC. So if you insulated the cylinder with 1 inch thick phenolic foam the rate of heat loss would be 2.3 milliwatt at a 1 degC temperature differential.


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## slebans (Dec 3, 2011)

Kinnza said:


> I read your first post again, blasterman, and I noticed you are not interested on the total radiometric flux, but on intensity (mW/sr). In order to compare such magnitude, you just need to measure at a given distance of the optical axis with a light meter (for royal blue, better a good one) and correct it by its LER (obtained by its SPD).
> 
> Likely some software for digital camera or add-on for phone with camera could do it, but the latter probably would be limited by the sensibility on the blue range for short wl.



Kinnza, I just found the lowest price spectrometer I have ever seen. Generally, I have found pricing between $1499 and $2499. This one is just $749 and it originates from Canada - of all places. If you get a minute can you have a look and see what you think.
http://www.aseq-instruments.com/index.html

Stephen Lebans


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## MikeAusC (Dec 3, 2011)

slebans said:


> Kinnza, I just found the lowest price spectrometer I have ever seen. Generally, I have found pricing between $1499 and $2499. This one is just $749 and it originates from Canada - of all places. If you get a minute can you have a look and see what you think.
> http://www.aseq-instruments.com/index.html
> 
> Stephen Lebans



Interesting - Toshiba don't specify the spectral response of the TCD1304 sensor below 400nm at all, although ASEQ can apply a custom calibration curve for each sensor to give reasonable accuracy.


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## slebans (Dec 4, 2011)

MikeAusC said:


> Interesting - Toshiba don't specify the spectral response of the TCD1304 sensor below 400nm at all, although ASEQ can apply a custom calibration curve for each sensor to give reasonable accuracy.



This sensor is also used in the Ocean Optics' newer HR4000 models. 

Stephen Lebans


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## SemiMan (Dec 4, 2011)

Very cool spectrometers.

They even have low cost cosine corrected adapters. Would be pretty easy to whip up a basic goniophotometer.

Semiman


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## SemiMan (Dec 4, 2011)

If I am interpreting what you are trying to do, then all you are looking at measuring is radiometric output of the LED?

1) You need some way of diffusing the light source to even out the distribution. You can make a poor mans integrating sphere with a box or sphere and some really good pure white flat paint. Even better, paint for the inside of signs ... >95% reflective and highly diffuse.

2) You need a somewhat linear light sensor. A wide bandwidth photo-diode. Do not operate it forward biased as a current source, but back-biased as a light based current modulator. It will be more linear in this mode. You may even be able to do it with a monocrystalline solar cell (just need a piece of one).

Semiman


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## Kinnza (Dec 5, 2011)

I think the way suggested by David Campen is by far the easiest setup, and should work fine when you just want to perform comparative measurements. Calibrating it would be difficult, you could know from it very well the rank of outputs compared, but saying what is the actual difference between each rank would be more complex. As it is easy to build, it may worth the try.

A copper piece for the end of copper tubing might do the job well. Put the LED on the open end with several layers of kapton tape for insulating thermally LED from cooper piece and it may work fine enough for your purpose (you could build a reflective piece for the open side with just the aperture for LED dome).

For measuring the way I suggested, you need a photodiode well matched with photopic response, usually found on good light meters (cheap ones usually are not well matched on the short blue range). But you could use any as far as you know accurately its response, so you can calculate its measured LER, being able to find radiometric output from the lm output. The key of all is having the SPD.

I have processed royal blue spectrums using digital camera and it works fine. When you use RAW or TIFF formats, data includes the actual response of the CCD array to each wavelength, so still when response is lower on the short blue range, it is detectable and mensurable. Just Signal to Noise ratio becomes lower, but not a problem for your purpose.

On the other hand, some years ago I was searching for photodiodes with flat response. I wanted to build a good light detector using 3 photodiodes (RGB) with flat response. I found two but not the third, so I give up the project. I did not remember if it was the Blue or the Green the one I couldnt find with a flat response, but I believe it was the G. Anyway, maybe now there are more photodiodes out there. Unfortunately, I dont have the datasheets or reference of the models I found, but you can search for it. Finding a photodiode with flat response in the B would simplify your task a lot and make it very straightforward.


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## Kinnza (Dec 5, 2011)

slebans said:


> Kinnza, I just found the lowest price spectrometer I have ever seen. Generally, I have found pricing between $1499 and $2499. This one is just $749 and it originates from Canada - of all places. If you get a minute can you have a look and see what you think.
> http://www.aseq-instruments.com/index.html
> 
> Stephen Lebans



WOW! Really cheap for its performance.

I think it should work perfect for lighting measurements. We are not performing Raman spectroscopy. The spectrometer itself looks good, compared with similar ones cost 1/3.

But I miss more detailed info on the page. Clearly they are new, and want to enter the market by offering low prices. Sofware included seems little developed. Probably they will improve it with time, but for now, it has some shortcomings. Specifically, it dont have an automatic function for calibrating. Calibrating is done by introducing manually three coefficients for a 2nd order polynomial fitting. I doubt anybody does it. So actually, recalibrating it would be near impossible, still having a NIST certified calibrated lamp.

Anyway, it is a powerful spectrometer for very cheap. The initial calibration should be enough for lighting measurements, where we dont need extra accurate results. Still not being for the moment as tuned as other brand spectrometers, usually we dont need such fine tuning, and I would expect they improve the software with time. Spectral range for the standard configuration is impressive, using a very good CCD array, for 200-1100nm. Probably S/N ratio on the UV range wont be good, but at least it measures it. Configuration A, with range 300-1000nm and resolution below 3nm, S/N 300:1 looks very good for a general purpose spectrometer.


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## Kinnza (Dec 5, 2011)

SemiMan said:


> If I am interpreting what you are trying to do, then all you are looking at measuring is radiometric output of the LED?
> 
> 1) You need some way of diffusing the light source to even out the distribution. You can make a poor mans integrating sphere with a box or sphere and some really good pure white flat paint. Even better, paint for the inside of signs ... >95% reflective and highly diffuse.
> 
> ...



Could a royal blue LED be used as photodiode that way?


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## slebans (Dec 5, 2011)

Kinnza said:


> WOW! Really cheap for its performance.
> 
> I think it should work perfect for lighting measurements. We are not performing Raman spectroscopy. The spectrometer itself looks good, compared with similar ones cost 1/3.
> 
> But I miss more detailed info on the page. Clearly they are new, and want to enter the market by offering low prices. Sofware included seems little developed. Probably they will improve it with time, but for now, it has some shortcomings. Specifically, it dont have an automatic function for calibrating. Calibrating is done by introducing manually three coefficients for a 2nd order polynomial fitting. I doubt anybody does it. So actually, recalibrating it would be near impossible, still having a NIST certified calibrated lamp.



Thanks for looking at this, Kinnza.
The software and lack of automatic calibration were the two issues that bothered me as well. Still, the price is incredible compared to what is currently on the market. I think we need to buy one and test it out.

Stephen Lebans


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## SemiMan (Dec 6, 2011)

Kinnza said:


> Could a royal blue LED be used as photodiode that way?




My guess, and it's only a guess is that the lack of optimization of the LED to act as a broadband photodiode would negate its use. Solar cells, especially back-contact monocrystalline solar cells have reasonably flat response across the visible range even into the blue.

Check out this paper http://tayloredge.com/reference/Electronics/Photonics/HighEfficiencySolarCells.pdf

There is a response on the right side of page 2 for the Sunpower back contact cell. The external quantum efficiency looks to be within about 5% across the visible spectrum. The bigger issue will be the lack of linearity though I believe for Sunpower cells it is not bad. Reverse biasing it will improve the response linearity.


On another note, I am thinking of asking that company for a loan of their spectrometer. Serious consideration of buying it if it works and I would be willing to write them a review. I do consulting on LED fixture design and have been putting off building a goniophotometer ... can't justify the cost of buying one. This would be a good additional piece. I only need something good enough for first pass testing .. before spending on lab testing. In theory optical simulations should be enough, but alas .... the real world like to interfere with that.


Semiman


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## MikeAusC (Dec 6, 2011)

You can get a 3 colour channel + white sensor with documented response for $5 - why use undocumented sensors ?



MikeAusC said:


> The "Clear" channel of the TCS3200 Colour Light to Frequency Converter has a response at 350nm that's 28% of its peak at 720nm. The sensitivity rises roughly linearly from 350 to 720nm.
> 
> You also have R, G and B channels available so you can make an assessment of the spectral distribution. The colour channel to be measured is set by applying DC to two input pins. The sensor area is 1mm x 1mm,
> 
> ...


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## SemiMan (Dec 7, 2011)

MikeAusC said:


> You can get a 3 colour channel + white sensor with documented response for $5 - why use undocumented sensors ?



-- The original poster was looking for radiometric output, not photopic output, hence a visual response sensor will not cut it.

-- On second thought though, a photodiode and/or solar cell will not work either. While the QE is very linear, the power response is not and that is what is needed.


..... back to the optically black tube ... though it is tough to make such measurements.

Semiman


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## bshanahan14rulz (Dec 7, 2011)

So why not use a TEC? they even make some tecs with special coatings for absorbing energy. Saabluster mentioned it up in post 3...


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## MikeAusC (Dec 7, 2011)

blasterman said:


> . . . . Problem with this is I need to take readings from both royals and various whites. Depending on composition solar cells start to get terribly non-linear around 460nm. . . . .



Why not just paint a Temperature Sensor (Glass Thermometer - Thermocouple - Semiconductor Sensor) with a non reflective coating on the side facing the LEDs and cover the other side with an insulator. Measure the rate of temperature rise.

If you want to maximise the radiant energy absorption from one direction, put the sensor at the focal point of a parabolic reflector. Metallic reflectors should have a flat spectral response.

The response time will be much faster than heating a pipe or TEC and you will have a narrow zone of absorption.


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## MikeAusC (Dec 7, 2011)

Do you really want to measure Infrared radiation > 700nm - I can't imagine this helps with Reeftank lighting.

An XP-G Warm White has radiant energy at 700nm that's 28% of its radiant peak at 610nm.

One low-cost way to reduce IR sensitivity is to use dichroic filters that are used in halogen downlights. From the front they look like they're solid silvered, but if you look at the back of the reflector when the light is on, you can see redddish light.

Halogen downlights must use dichroic reflectors, otherwise a 50 watt downlight would focus 45 watts of heat on the subject !

If you remove the halogen globe and put a temperature sensor at the focal point of a 10 degree, 50 or 20mm halogen downlight, you will have a very directional radiant energy sensor with reduced IR sensitivity. The IR component will pass through the dichroic reflector.

Hopefully someone with a spectrometer can measure the response of these readily available dichroic reflectors.


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## saabluster (Dec 8, 2011)

bshanahan14rulz said:


> So why not use a TEC? they even make some tecs with special coatings for absorbing energy. Saabluster mentioned it up in post 3...


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