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Old 09-25-2013, 08:06 PM   #1
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PAR Meters, and what it takes to make one


The “gold standard” in PAR meters is the Li-Cor PAR meter. It, alone, accurately measures light intensity uniformly in the entire 400-700 nm spectral range that defines PAR (photosynthetically active radiation). The spectral response of a Li-Cor PAR meter looks like:

Clearly, a Li-Cor PAR sensor will measure light intensity for the entire PAR spectrum and do it very accurately. But, that accuracy comes at a price - thousands of dollars.

The next level of accuracy in PAR meters is the Apogee Quantum PAR meter, which measures light intensity much less uniformly and not over the entire PAR spectral range. The spectral response of an Apogee Quantum PAR meter looks like:

You can see that the Apogee Quantum PAR meter isn’t nearly as good as the Li-Cor meter, and it omits almost all light in the 660-700 nm spectral range and distorts the measurement of light in the 400-450 nm spectral range. Apogee compensates for the distorted spectral range by using correction factors for certain sources of light. But, this PAR meter only costs about $350.

In spite of the obvious inadequacies of the Apogee PAR meter, the lower cost has caused this to be the standard PAR meter in the aquarium hobby. And, it works very well for that use, since hobbyists don’t need, and can’t use any more accuracy than they get with the Apogee meter.

The Apogee Quantum PAR meter uses a silicon photodiode as the sensor, with filters to adjust its spectral response to that shown above. The readout for this sensor is a digital millivolt meter, which is calibrated to read in PAR units. If you own a good millivolt meter you can obtain the sensor alone for about $120, and use your millivolt meter and a calibration constant to measure PAR.

All silicon photodiodes have about the same spectral response, before filters are added. That typical spectral response looks like:

You can see that it takes a lot of filtration, especially infrared blocking, to make this work as a PAR sensor.

continued
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Old 09-25-2013, 08:13 PM   #2
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There are now some Chinese designed and manufactured lux meters being sold on Ebay and Amazon websites. These also use silicon photodiodes as a sensor, and they have a filter on the photodiode that changes the above spectral response to:

Because this meter is measuring light intensity in lux and not PAR units, it is measuring primarily green light, and the filter used to adjust the silicon photodiode spectral range largely eliminates all light below about 500 and above about 640 nm. This makes the photodiode plus filter unsuitable for use in a PAR meter, even with added filters. Infrared blocking filters are not readily available, are normally made of glass, and are expensive. A lux meter can be modified to read in PAR units, but to do it economically a new photodiode with a more appropriate spectral response is needed.

Fortunately, there is at least one silicon photodiode that does have a more appropriate spectral response, the Excelitas Tech VTB8441BH, costing only about $5-$6 depending on shipping costs and how many you buy. The spectral response of this photodiode looks like:

You can see that the Excelitas photodiode spectral response covers the entire 400-700nm range of the PAR spectrum. With proper filters it should be possible to convert that spectral response into something at least as good as the Apogee Quantum PAR meter spectral response.

Rosco is a company that manufactures gel filters, primarily for theatrical use, adjusting the colors of lighting used in stage productions, and for photography, to adjust he colors of studio lighting. The Rosco website, http://www.rosco.com/filters/roscolux.cfm is a great source of information about the filters they make, giving the spectral energy distribution curve (spectrum) for light transmission through each of the hundreds of filters they manufacture. You can use the data from those spectral energy distribution curves to calculate the spectral distribution of light transmitted through any combination of those filters and the spectral response of a photodiode which has those filters between the light and the diode. This is easy to do, using a spreadsheet and the tabulation of transmissivities for each increment of light wave length, which is also on the website. The data for a typical filter looks like:

A typical spreadsheet, using this data is:

After about a year of off and on calculating, with various filters and combinations of filters, this is the final result, showing the spectral response of the Excelitas photodiode plus 3 filters, #3313 (Tough 1/2 Minus green), #373 (Theatre booster 3) and #1995 (Thermashield):

Note that this duplicates the Apogee Quantum PAR meter response at the low end, and expands it just past 700 nm at the high end. A PAR meter based on this sensor should be at least as accurate as the Apogee meter.

Actually making a PAR meter, using this information, is a bit complicated, but I will post details about that in the DIY forum.
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Old 09-26-2013, 02:23 AM   #3
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Have you ever considered pulling the "filter sandwich" out of a digital camera?? A good source of IR cut filters and an anti aliasing one to smear the light a bit..
Wouldn't doubt that it also cut some UV/blue..
Then "all" you have to do is flatten the mid range..
Some reference:
http://www.rocklandastronomy.com/NEA...ap_Griffin.pdf
Or just use one of these..
http://www.highpointscientific.com/p...FTRo7AodS1AAEg
$67 not bad.. for what you get..
Quote:
Baader Planetarium UV IR Cut Telescope Filter 1.25" FUVIR-1

Baader Planetarium has introduced the world's finest UV/IR cut filter. This filter is a must for all digital imaging with digital cameras, CCD cameras and modified webcams. It's also useful for protecting valuable H-Alpha filters from heat stress and damaging IR (Daystar, etc). Due to their extremely high optical quality, these filters may be stacked and used far in front of the focal plane (necessary for imaging uses). Like the Baader Planetarium Contrast-Booster, the UV-IR-Cut filter uses the very latest coating technology to deliver the finest filtration quality, and lifetime durability.

Baader Planetarium UV IR Cut Filter Features:

Unmatched 98% Average Transmission! Freedom from ghost images (very low reflection multi-coating, Striae-free, and plane-parallel substrate).
Extremely Sharp Cutoff at 690nm (leaves all the visible red, including the important 656nm H-alpha emission line).
Sharp cutoff below 400nm. Eliminates violet halos around bright stars (more effectively than simpler so-called Minus Violet photographic filters).
True optical quality glass substrate and coatings. Why spend all your money on quality primary optic, only to place a lower grade filter optic in the path?
Combine with any of the other Baader filters to deliver the sharpest images possible.

$70.. kick tail PAR sensor..

Last edited by jeffkrol; 09-26-2013 at 02:36 AM.. Reason: chart
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Old 09-26-2013, 02:25 AM   #4
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Originally Posted by jeffkrol View Post
Have you ever considered pulling the "filter sandwich" out of a digital camera?? A good source of IR cut filters and an anti aliasing one to smear the light a bit..
No, I haven't considered that, but without knowing the characteristics of the filter I would have no way to know what effect I would get from it.
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Old 09-26-2013, 02:36 AM   #5
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No, I haven't considered that, but without knowing the characteristics of the filter I would have no way to know what effect I would get from it.
See above...
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Old 09-26-2013, 05:09 AM   #6
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You are right. That would work for a PAR meter, but the spectral response curve for a silicon photodiode combined with that filter would leave you with a response curve that has a big peak in the middle, because it has such a flat transmissivity curve. Another filter would be needed to flatten out that hump - roughly the same two filters I mentioned above. And, that filter would need to be cut down to the roughly 1/4" dia. size needed, or that size filter, as manufactured, would be needed. Do they come that small?

EDIT: Actually, if I had that filter available in a small size, I would just use the bare photodiode from the lux meter, and filtering to get rid of the remaining slope upward with wave length that would result from that. Without the added filtering the response would go from about .3 to .8 over the PAR range. I don't remember any Roscolux gel filter that has the needed transmissivity spectrum, so a combination of filters would be needed.
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Old 09-26-2013, 02:19 PM   #7
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Originally Posted by Hoppy View Post
You are right. That would work for a PAR meter, but the spectral response curve for a silicon photodiode combined with that filter would leave you with a response curve that has a big peak in the middle, because it has such a flat transmissivity curve. Another filter would be needed to flatten out that hump - roughly the same two filters I mentioned above. And, that filter would need to be cut down to the roughly 1/4" dia. size needed, or that size filter, as manufactured, would be needed. Do they come that small?

EDIT: Actually, if I had that filter available in a small size, I would just use the bare photodiode from the lux meter, and filtering to get rid of the remaining slope upward with wave length that would result from that. Without the added filtering the response would go from about .3 to .8 over the PAR range. I don't remember any Roscolux gel filter that has the needed transmissivity spectrum, so a combination of filters would be needed.
1 1/2 inches isn't too big... and w/ the orig photodiode.. it seems a simple slope correction factor (looks fairly linear) is all that is needed, without the need for filtering

One filter, 1 diffuser one math equation.. but that is only a guess..



On further thought.. probably not that simple..

3rd filter..


or daylight blue.. or..
Roscolux #4330: CalColor 30 Cyan or
?????

I combined the 2 graphs w/ transparency and some "squishing".. to compare slopes (Booster blue and diode response)


The more I look at this the more I believe you can make a better sensor than the Apogee AND close to what Li-Cor has at under $100 (more like $50)

I was also thinking about how you can test these sensors cheaply.. How about using daylight and splitting it through a prism.. then projecting that on a surface and running the sensor through the bands, monitoring output..



As to your findings.. I hope you will take this correctly ..but you still have too much attenuation in the important red and blue area.. But that is just a personal opinion w/ all due respect..

One more thing.. and I think you will understand this The slope of the photo diode is 600 while the slope of booster blue is -440 .. Which would come to a combined slope off the ideal of 0 at 26% I remember correctly..or am not just making things up.. Haven't really been using math much lately (like decades)

Last edited by jeffkrol; 09-26-2013 at 03:39 PM.. Reason: image
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Old 09-27-2013, 01:10 AM   #8
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The signal from the photodiode contains no information about what wave lengths of light are contributing to the signal, so you can't electronically correct the slope to a flat slope. Only a filter will do it. To calculate which filter, you can read the relative response of the diode at each wave length, then do the same for the transmissivity for the filter at each wave length. Multiply those two numbers, divide the result at each wave length by the highest number you got multiplying the two numbers from the charts, and you have the relative spectral sensitivity vs wave length.

I agree with you that a near Li-Cor accuracy PAR meter can be made using that infrared/UV blocking filter plus one or two colors of optical filters, and the original lux meter photodiode with its filter removed. The problem I ran into when trying to do this is finding a source of the infrared/UV blocking filters in a small diameter, 5/16" would be ideal. Trying to cut those from a bigger glass filter wouldn't be likely to work out. And, I couldn't find a company, using Google, that sells that size of filters. If anyone can find a source for those I would certainly try this idea.
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Old 09-27-2013, 02:22 AM   #9
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Quote:
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And, I couldn't find a company, using Google, that sells that size of filters. If anyone can find a source for those I would certainly try this idea.
how about one that is 9.5mm....
Fleabay:
Quote:
9.5mm Optical UV-IR CUT filter UV/IR Blocking filter for camera astronomy lens

Price:
US $5.65

$2.25 ePacket delivery from China

Tavg >96% @435-625nm
T =50% @650 +/- 10nm
Tavg <= 1% @ >= 700nm Tavg <= 0.5% @ <= 375nm
BTW: THAT price is for 2................
Baader is still better..
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Old 09-27-2013, 04:56 AM   #10
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Now, that is a great find. I plan to try them out. They won't give results as good as the Li-Cor PAR meter, because they start cutting out the IR at about 650nm, instead of 700nm. That is as good as the Apogee PAR meter does, but not the Li-Cor. I suspect that these may also affect the visible light too and not necessarily uniformly, but they are still cheaper than new photodiodes.
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Old 09-27-2013, 05:16 AM   #11
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Now, that is a great find. I plan to try them out. They won't give results as good as the Li-Cor PAR meter, because they start cutting out the IR at about 650nm, instead of 700nm. That is as good as the Apogee PAR meter does, but not the Li-Cor. I suspect that these may also affect the visible light too and not necessarily uniformly, but they are still cheaper than new photodiodes.
Does that filter filter specs and the Canon line on my chart above seem "close" to equivalent?..

My guess is those filters are the same as for digital cameras... at the least "why" they were designed and exist..
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Old 09-30-2013, 03:55 PM   #12
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I spent a lot of time trying to find a way to make that Ebay IR/UV blocking filter work for a PAR meter. It can't be done very well. The problem is at both the UV and IR ends of the spectrum. That blocking filter cuts off so that it eliminates most of the energy between 400 and 435 nm wave length, and between 650 and 700 nm. This keeps it from being used to make a PAR meter that compares at all to the Li-Cor meter, and even somewhat less accurate than the Apogee meter.

That blocking filter is good for a digital camera because the perceived color of the missing parts of the 400-700 nm spectrum is essentially black, so they are not necessary for accurate color in photographs. But, a blocking filter for a PAR meter should have a wider spectral range, covering the complete 400-700 nm spectrum.
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Old 09-30-2013, 06:35 PM   #13
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That blocking filter cuts off so that it eliminates most of the energy between 400 and 435 nm wave length, and between 650 and 700 nm. This keeps it from being used to make a PAR meter that compares at all to the Li-Cor meter, and even somewhat less accurate than the Apogee meter.

Well I did say the Baader was better..
Quote:
The absorption maxima of chlorophyll a are lambda= 430 and lambda= 662 nm, that of chlorophyll b are at 453 and 642 nm..
Not so worried about the cutoff at 700 or 50% @650.. Lower end is a bit more problematic though... Of course this was already known..


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