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Old 09-09-2012, 01:49 AM   #106
Hoppy
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I have been trying find a simple way to make the lux meter readout be the actual PAR number. It seems like it should just take a voltage divider to do that, but after asking mistergreen some questions I started having doubts. I tried to use a 10K trimmer pot as a voltage divider, but that didn't work at all. So, I bought a 1 megohm potentiometer to play with. I first found that the readout portion of the lux meter does not supply any voltage to the sensor, so it has to be reading just millivolts from the photodiode. Then I shorted the connected sensor with the potentiometer to see how much resistance I can use in a voltage divider without skewing the reading. One megohm did nothing to the reading, but at around 200-300Kohms it did drastically change the reading. So, a one megohm pot would work as a voltage divider, as far as the sensor part is concerned. In order to get enough sensitivity to adjustments I decided that a 1 megohm resistor in series with a 10Kohm resistor and 10Kohm pot would work best. But, having the 10K ohm resistance across the readout (milivoltmeter) might be a problem. I'm still thinking about that part. (The photodiode resistance is around 10 Kohms, if that matters, and I think it does, since it seems to show that 10 Kohms in the circuit with the milivolt generating photodiode doesn't affect the readout - maybe. And, the milivolts from the sensor, divided by 61, across a 10 Kohm resistor is essentially the same as having the photodiode connected to the readout) More to follow!
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Old 09-11-2012, 09:14 PM   #107
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Into each life some rain must fall! I find that you can't change the readout of these cheap lux meters so they read about 1/60th of the values they now read. At least you can do it with a voltage divider. After spending about $30-$40 on resistors, pots, etc. I find that the readout is the same whatever I put in the circuit between the sensor and readout. As I was trying to sleep last night I think I figured out why that is: They advertise these meters as having automatic zero compensation, which has to mean that it readjusts its readout as needed to maintain the original accuracy. So, when I add a voltage divider, it just adjusts the readout to ignore the voltage divider. Just how that is done, I don't know, but I do know I am not going to succeed with this approach.

Fortunately, as I was still trying to sleep I figured out an easy way to accomplish the same thing with Rosco gel filters - it's obvious, isn't it? If I use multiple neutral density filters, plus a spectrum adjusting filter, with combined transmissivity of .0166, I am dropping the readout by a factor of 1 to 60, which is what it takes to make the reading a good approximation of PAR instead of lux.

After a few hours of work I find that I can do this with 4 layers of filters:



It won't be perfect, but it will greatly reduce the emphasis the lux meter puts on 550 nm, yellow green, light, and drop the sensitivity down to .0166 of what it is now. The sensor is so simply made it will be a cinch to put the stack of filters in place. So, now I just need to find those specific filters at a reasonable price.
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Old 09-13-2012, 10:30 PM   #108
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Roscolux filters are sold at a theatrical lighting store in Sacramento. They have the free sample books, with what appears to be every color filter that Rosco sells in it. The samples are about 1.5" x 3", enough to make filters for at least 8 "PAR meters". When I get time to concentrate on this I will modify mine with a stack of filters. Even if a full sheet of these filters costs $10, it would add a trivial amount to the cost of modifying lux meters in quantity.
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Old 09-13-2012, 10:55 PM   #109
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Keep up the good work hoppy!
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Old 09-14-2012, 06:53 PM   #110
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The first chart is the frequency response of the basic Mastech luxmeter sensor, from the tiny instruction sheet that comes with it. The second chart is what it would be by placing a stack of Roscolux filters - 3 layers#3308, #398, #397, 3 layers #4315 - over the photodiode assembly in the sensor. This theoretically will reduce the readings by 1/57.4, to convert the reading to an approximation of PAR, plus change the frequency response to better match the needed response for a true PAR meter.

I calculated this using the frequency response numbers from http://www.rosco.com/filters/roscolux.cfm and a spreadsheet calculator. This was about the 6th attempt, and I think it is the best I can do, other than adjusting the attenuation, if desirable after calibrating this against a PAR meter. I would like to get less attenuation of the green part of the spectrum, but so far I can't find a combination of filters that will do that. It will probably be next week before I get around to actually trying this.

EDIT: I can't stop doing this!



With a couple of small changes in the filter stack, changing to 2 layers#3308, #97, #398, #397, 3 layers #4315 gives less attenuation of the green area and possibly a better response overall.
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Old 09-15-2012, 02:35 AM   #111
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Oh no .. Hoppy's addicted to Lux2par ... no it's not a new designer drug but something much better!

I have faith that you'll get it almost exactly the way it needs to be and then a lot of people will benefit from all your hard work.
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Old 09-15-2012, 04:25 PM   #112
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Last night I disassembled the sensor to see how I could stack filters in it:


The electrical tape is due to my experiments with a voltage divider.


You can see how simple the assembly is, and how small the filters can be.



This thing is designed with someone like me in mind! I can cut squares of filters, about 3/8 inch square, and just drop them in the square recess. Maybe a tiny drop of silicone sealant to hold the stack in place.

I think the next step will be to measure the actual attenuation of each filter, by before and after lux readings for each filter type. Possibly today.
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Old 09-15-2012, 10:03 PM   #113
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Now I'm making progress! I tried measuring the % transmittance for each filter I want to use, by laying them on top of the little round case for the photodiode, then carefully putting the cosine diffuser half of the housing back on top, without installing the screws. I measured reductions in readings that were similar to the specified transmittance for each filter, but higher, in each case. I decided that was due to light leakage around the filters, so I stuffed all but one of the filters in the square box that now holds the factory installed filter. This time when I measured the % transmittance for the stack, I got 2.1% instead of the spec 2.0% - very close! So, now I have a PAR meter that probably reads a little high, but I can add one more filter on top of the little round lid for the diode, to finish adjusting this to read correctly, when I get our club's PAR meter to use. I expect to be off about 15%, which I can correct with one filter, if I select it right. And the spectral response for the meter will be: (If the final filter is the neutral density filter I think I can use.)



This will have a nice broad spectral response, not terribly different from the Quantum meter response:
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Old 09-16-2012, 12:33 AM   #114
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Default thanks for the hard work

i understand a little OCD is good for the soul i do this all the time else nothing would get done...i will follow with great interest your findings.
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Old 09-16-2012, 07:24 PM   #115
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I tested my PAR Volksmeter against a Quantum PAR meter and found I was reading about 40% high, so I added the last filter that I had calculated would be needed. Then rechecked it against the Quantum meter:


Now, I would call that a success! Since I have the Quantum meter for awhile I think I may set up a LED light (the above check is with a 36 watt 6700K /AH Supply PC light) and check at various PAR levels, to be sure.

The final filter stack is: (All Roscolux filter numbers, all of which are in the free sample booklet)
2 - Roscolux #4915: CalColor 15 Lavender
3 - Roscolux #398: Neutral Grey
1 - Roscolux #397: Pale Grey

I measured the PAR of full sunlight at noon PST with both meters:


That is incredibly close!
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Old 09-16-2012, 10:22 PM   #116
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Default cool so cool

so how much do you think it would take to put one of these together say for a project all together?. i would like and i know others would a list of all the hardware you used to get her this far i am satisfied it is close enough for horse shoes or my needs. i have read the whole thread but i know you made some changes up to this point.
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Old 09-16-2012, 11:15 PM   #117
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The cost would be about $15 plus shipping for the lux meter. The filters are from the free sample book you can pick up at all dealers in Roscolux filters. All of the other modifications I made didn't work out at all well, other than the pair of connectors that allow me to make another sensor and use it in place of the original sensor. That was about $5 as I recall, if you want that feature. Adding the filters is very easy to do, so no real skill is needed, just reasonably steady hands. All of the filters will fit in the square "box" over the photodiode. And, there are just 4 screws to remove to get to that "box". At this point the PAR meter isn't water proof, but I'm planning another effort at making a sensor that will be.
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Old 09-16-2012, 11:43 PM   #118
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Way to go Hoppy! I knew you'd get it. I think even I do the filter mod.
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Old 09-20-2012, 10:09 PM   #119
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I finally made a waterproof sensor that reads in PAR units, but reads about 10% low. I ended up using this design:


It is made from a piece of 3/4 inch diameter acrylic tube, an acrylic half dome, a strip of 1/8 inch thick acrylic, for a handle. First I cut a piece of tube about .02-.03 longer than the photodiode with its leads - Excelitas VTB8441BH - from Newark Electronics, about 3/8 inches long. Then I drilled a hole in one side at about the height that I expected the electric cable to come in, just big enough to get the cable through it. I had a 1" by 5" piece of acrylic !/8 inch thick sheet, so I glued that tube to that with Weldon #16. When that dried I used black nail polish to paint the inside of the tube a solid black.

To solder the cable to the photodiode I drilled a shallow 5/16 inch diameter hole in a scrap of plywood. Then I jammed the photodiode in that holes with the leads up. This acted as a third hand to hold it when I wrapped the two cable wires around the leads and soldered them on.

To get the diode inside the tube I poked the other end from inside the tube, through the hole in the side, and pulled it until the diode could fit into the tube. I centered the diode, with the ends of the leads against the bottom of the tube enclosure, and used masking tape to hold the cable steady against the bottom piece.

I used Loctite instant mix, 5 minute setting epoxy to form a pool of epoxy about 1/8" deep, deep enough to cover the cable hole in the tube, to anchor the photodiode in place.

Then the hard part began: selecting Roscolux filters to convert the reading to PAR. First I wanted to change the wave length sensitivity from a peak at about 600 nm to something more nearly flat over the 400-700 nm range. At the same time I needed to greatly reduce the sensitivity of the photo diode. This required a spreadsheet calculator, with all of the transmissivity data for lots of filters, by wave length. First I found a filter to knock off the 600 nm peak, without too much damage to the rest of the sensitivity. Then a filter to correct, as much as I could, the damage that was done to the rest of the sensitivity. (This means first reading the graph of sensitivity for the diode at every 20 nm and putting that in the spreadsheet, then multiplying all of the filter sensitiviies by the diode sensitivity, and dividing by the resulting maximum peak sensitivity to get a graph with the peak at 1.0.) To act as a diffuser to emulate the cosine diffuser, I used a Roscolux diffusing filter, and multiplied those sensitivities in with the others. When I had the best looking spectral distribution I could get, which took several tries, ending up with 5 filters, I cut half inch diameter discs of each and stacked them on top of the photodiode, and sat the hemishpere on top, and measured the output with my "standard" light. I got 1112 vs the 25 I wanted. So, I had to find a stack of neutral filters that would drop that 1112 down to 25. That took another 6 filters.

After a final trim of the stack of 11 filter discs I restacked them on top of the photodiode and laid the hemisphere on top and repeated the reading - I got 22 this time, which was close enough, since no other combination was nearly that close.

The filters I used are: #163, #4915, #4230, 2 #R09, #97, 2#397, #98,#398, and #3318. Unfortunately there is no Rosco filter in the sample book that cuts out the infrared and near infrared.

By now the acrylic cement and the epoxy had set up very well, so I glued the hemisphere on top with Weldon #16, and put a filet of that cement around the electric cable on the outside to better waterproof it.

After it set up good, I put a bead of Weldon #16 around the joint at the hemisphere to tube interface to better seal that.

One more calibration, and it still read 22, where 25 would be the desired reading. A final calculation of the sensitivity vs wavelength gave me:



It looks pretty ratty, but it should still work ok. Eventually I will try it at noon in full sun, to see if the 10% error is the same at high light.

Much of the cost of this was in Epoxy and nail polish! About $9 for them, plus $4.26 for the photodiode and about $7 for shipping.
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Old 09-21-2012, 06:55 AM   #120
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Your way over my capabilities now! LOL

But I do wish I had a waterproof one. I moved and altered my lighting setup a bit and now need to know what it's reading in the tank itself.

If you decide you want to make a couple let us know. I'd like to buy one. I think I could do the adapter part to be able to switch between this sensor and lux sensor. But like I said this mod is beyond my ability.
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