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Thread: Convert Lux meter to PAR meter? Reply to Thread
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  Topic Review (Newest First)
12-22-2013 08:43 PM
Originally Posted by Aquaticz View Post
How do you convert Lux to par - please
Lux and PAR are measurements of the same thing - light intensity at a surface. "Lux" is the units used when measuring the light, with the spectrum weighted to match human eye sensitivity. PAR is the units used when measuring light over the complete 400 to 700 nm wave length range, with all parts of the spectrum treated equally. Because lux involves suppressing wave lengths not easily seen by humans, the two sets of units can't be directly converted from one to the other, without using the spectral distribution of the light source to adjust the lux measurement to one that treats all wave lengths equally. So, the conversion factor for going from lux to PAR is different for different types of light sources - fluorescent, the sun, LEDS, CFLs, T8 bulbs vs T5 bulbs of different color temperatures, etc.

Fortunately, for our use, we don't need to know PAR to within 1% or better. If we know it to within 10% that works perfectly well. So, it is possible to use a lux meter with the reading divided by about 60 - 80 to convert the measurement to the equivalent of PAR. Or, you can use various filters on a lux meter to tweak the frequency sensitivity of it, and the overall sensitivity of it, and make it read in PAR units, accurate to +/-10%, and possibly better, for the commonly used lighting types on planted tanks. That is what I was doing in this thread.

I have stopped making these "PAR meters", out of boredom, mostly. But, I think I can just slightly modify a lux meter like the ones I used, so it reads in PAR units almost as accurately as those I made. I'm making good progress towards doing so. That would make it an easy DIY project for anyone, or it would make selling modified units for $25 and making some profit for little work, possible. If this works out, I will start a new DIY thread about how to do it.
12-22-2013 12:35 PM
Aquaticz How do you convert Lux to par - please
08-27-2013 07:20 AM
Jalopy Hey Hoppy, does the thickness of the 1/2" acrylic disc in this version of the PAR sensor matter? Thanks.
08-25-2013 12:53 AM
Originally Posted by Jalopy View Post
Hey Hoppy, how do you calibrate your PAR meter? Thanks.
I use a Quantum PAR meter, or one of my PAR meters that I have very carefully calibrated several times. I set the two sensors side by side under a 50-50, 36 watt PC AH Supply light that is rigidly mounted above a stand I made for a 10 gallon tank. If my PAR meter reading isn't within 5% of the other meter, I adjust the spacing between the photodiode and diffuser, for the version I am making now, or for the older versions I adjusted the number of white diffuser filters I used, until I get the one being calibrated to be within about 2 digits at about 40 PAR. Since there is no way to adjust the slope of the PAR vs Quantum PAR, I only calibrate at one PAR reading.
08-23-2013 04:26 AM
Jalopy Hey Hoppy, how do you calibrate your PAR meter? Thanks.
06-21-2013 05:30 PM
Hoppy Continued from last post:
Use black electricians plastic tape to hold the top lens in place, and black out the sides of the sensor, for calibration.

With the sensor shimmed up to match the height of the Quantum meter sensor, measure the PAR from a fixed light with both meters. They should match.

They do match! So, all that's left is to cement the lens in place, put acrylic cement over the cable to housing joint to waterproof it, and cement the socket for a wand to hold the sensor at the other end of the bottom strip.

The last step is to paint the exterior of the sensor housing with black nail polish, two coats, to block all light except that that comes in from the top. You now have a usable PAR meter!
06-21-2013 05:23 PM
Hoppy Continued from last post:
Cement the bottom of the housing onto the base by putting a puddle of acrylic cement where you want the housing to be attached and pushing the assembly into the puddle, moving it a bit to be sure you have a good contact. Hold the assembly in place for about 3 minutes to let the cement set. The electric leads may slightly interfere with this, so you need to hold it until their push won't separate the joint.

Cut the sensor off the lux meter - disassemble the sensor so you can cut with a long cable lead left ahead of the coiled part of the lux meter cable. Strip the wires on that cable, and the cable to the new sensor. Leave the ground wire on the new sensor extending about 3/4 inch beyond the wires so it can be used as a strain relief for the soldered connections.

Tin all 4 bare wires. Place shrink tube segments on the cables before soldering the two connections. I use shrink tube on both connections, with two layers over the whole cable connection, with the long ground wire wrapped around the lux meter cable so the shrink tubes will grab it to help relieve the strain on the connection.

Cut a 5/16" diameter disc of Rosco #114 diffuser filter and place it on top of the upper retainer in the sensor assembly. This filter is essentially invisible, so it takes great care to be sure it is in place before attaching the top "lens".

Continued on one more post:
06-21-2013 05:09 PM
Hoppy Continued from last post:
Solder the cable wires to the diode leads.

Trim off the wires and diode leads that extend past the end of the acrylic housing. Install a 1/8" long 1/2" tube retainer (the retainers have a saw cut on one side. Use diagonal cutter to trim off more of the acrylic so it will clear the cable). Push the retainer down until it holds the photodiode against the tube stub in the fixture. This locates the photodiode in the right position. Put a drop of acrylic cement on the cable where it comes through the wall of the housing, and one other place on the retainer, to hold the retainer in place. Be sure to pull the assembly partly back off the tube stub in the fixture to make sure you don't cement it to the fixture.

Cut out two 5/16" diameter discs of Rosco #4815 pink filter, and carefully put them on top of the diode, with the assembly turned over and not on the fixture.

Install a 1/8" long retainer on top of the filters, to hold them and the diode in place. This will fit tightly inside the housing, but can be held in place with a drop of acrylic cement, if you wish.

Continued on next post:
06-21-2013 04:54 PM
Hoppy While experimenting with putting 3 photodiodes in one sensor, in order to improve the spectral sensitivity of the PAR meter, I found that I could fit a photodiode into a 1/2" diameter acrylic tube. This led me to redesign the one diode PAR meter to be 2/3 the size of what I had been making - a 1/2" diameter vs. 3/4" diameter sensor.

Assembling this is much easier if you make a simple fixture to hold it while you work on it, while also keeping the photodiode located where it should be. This is just a piece of 3/8" tube, with a longitudinal saw kerf in one side, to make it fit inside a 1/2" acrylic tube easier, with a length sticking above the base equal to the desired distance from the top of the 1/2" tube housing to the top of the photodiode (9/32 inch), cemented into a sheet of acrylic.

Fit the 1/2" tube housing on the assembly fixture, and poke the electric cable through the hole, and up out of the housing far enough to let you strip the insulation about 1/4" back. Don't do this before trying to stick the cable through the hole - it is much easier to do before stripping the wires.

After stripping the wires, gently move the cable back so the stripped wires are inside the housing.

Now, look at the face of the photodiode and locate the mark, either a "+" or just a dot, which identifies the cathode of the diode. This is the lead that connects to the red wire in the cable. Turn the housing over and carefully drop the diode in, so the leads straddle the cable, with the cathode lead on the red wire side. Put the housing back on the fixture, so the diode rests on the top of the stub of tube in the fixture. Use a wood toothpick or something similar to bend up the wires to get them in contact with the diode leads, ready to solder the connections.

Continued on next post:
06-08-2013 11:42 PM
Hoppy I have been asked if these PAR meters will work ok for reef tanks. To find out, I bought a 50-50 36 watt PC bulb, and tested a couple of them with that. One read 32 vs the Quantum meter reading of 33, and the other was only 2 units off at about the same reading. So, now I know they do work with 50-50 white/actinic light.
05-31-2013 07:29 PM
Hoppy I am still having problems with duplicating these exactly. Any minor error in the geometry make the sensitivity different, and I have to experiment to calibrate it against a Quantum meter, using different diffuser filters. So, I have spent some time trying to make it less variable. This is the design I will be using for any next batches:

Except for one part of the assembly, this is easier and faster to assemble, and the geometry is more consistent. It works exactly as well, once calibrated. The part that remains difficult to assemble is the photodiode to the tiny circuit board, cut from a much bigger one I got at Radio Shack, which will produce 28 tiny circuit boards. But, soldering the diode to the circuit board, the cable to the diode on the circuit board, and maneuvering this assembly into the 3/4" acrylic tube is still difficult. I think this will get easier as I make more of these.

I tried to photograph this as I assembled it, but my camera macro function isn't good enough to focus on the tiny parts.

I plan to make about 5-6 of these at a time, probably one batch every week or 10 days, and sell these on the For Sale forum. But, I was informed that there would also be a market for these with Reef tank keepers if they work equally well with typical Reef tank lighting. So, I ordered cheap 50-50 actinic/10000K PC bulb to verify that it is equally accurate with that mix. I haven't received the bulb yet, so I don't yet know how well it will do with that light.
04-23-2013 05:02 AM
Originally Posted by jerrytheplater View Post
Is this the meter you are offering for sale here?:

I just bought one from you and did not see this thread till now.
Yes, this is the PAR meter you bought. As I make more of these the mechanics of doing it get a bit easier so they tend to be more nearly exact duplicates. It is still difficult to get the photodiode mounted at exactly the same distance from the diffuser, so I am finding that I have to tweak the diffuser filter selections a bit at times. When all goes well I use the same 2 diffuser filters in all of them. (The meter calibration is done by choosing diffuser filters which also change the total amount of light passing through them.)
04-23-2013 01:54 AM
jerrytheplater Is this the meter you are offering for sale here?:

I just bought one from you and did not see this thread till now.
04-01-2013 04:00 AM
sowNreap Great work Hoppy!! I figured that you wouldn't quit until you had it perfect in your eyes.
03-30-2013 02:23 AM
Hoppy I think I am finally satisfied with my design, after a lot more work. I finally thought to look up the spectral response for the Apogee Quantum PAR meter, to see what they use and how good it is. I was surprised to find that they also have a far from perfect match with the 400-700 nM PAR spectral range, and, as a result their meter is only good to about +/- 4% for typical aquarium lights, and less accurate for incandescent lights. So, I tried to match their spectral response as best I could with the photodiode and Roscolux filter gels that I have. That proved to be relatively easy, once I worked out the details of what the filter response should look like. I settled on using the Roscolux # 4815 filter gel, giving this spectral response vs the Qualtum meter spectral response:

That left the problem of how to adjust the calibration of the meter without changing the spectral response. So, I noticed that in the Roscolux sample book are 19 different diffuser filters, white diffusers which would not alter the color of the light passing through them. I set up a PAR meter and checked the reduction in the reading that each of those diffusers gave me when I put one on top of the PAR meter sensor. The transmissivities ranged from 45% to 90%, so by using different combinations of those diffuser filters I could adjust the sensitivity of the meter over a very wide range.

With that problem solved, I made some modifications to the last design of the sensor, all aimed at making it easier to make the parts and to assemble them with minimal variation from assembly to assembly. My final design, which I prototyped, was:

With this design I can make 5+ of each part in about 30 minutes, with the aid of a few very simple fixtures. Assembling the parts was very easy, even with the tiny parts.

Today I received a batch of 5 more lux meters, so I was able to assemble a complete PAR meter for testing and to determine what diffuser filters I needed. It took just 2 diffuser filters, Roscolux # 102 and #114, to get a near perfect match with a Quantum meter. This PAR meter is now well within my goal of +/- 10% accuracy. I have two more just about assembled now.

If you have reasonable mechanical skills you should be able to easily duplicate this and make them to sell, if you wish to do so. The Roscolux filter sample books are free at a dealer, or you can buy one at Amazon. The acrylic tubes are available on EBay. You should have little difficulty finding usable flexible 2 conductor cable at a local electronics supply store, although Radioshack doesn't seem to have any.

I have a list of 7 people who have asked that I make them one, so I will finish off that list, and maybe make a few more, then I hope someone else picks up this relatively easy, high demand "product" for others to buy.

Here is the assembly sequence I am following:
Assemble Photodiode to item 5.
Apply acrylic cement to holes in item 5 to attach diode.
Thread cable through hole in item 6, and solder cable wires to leads.
Assemble above assembly to item 6, with acrylic cement.
Paint outside cylindrical surface of above assembly with black nail polish.
Glue item 4 to item 1 with acrylic cement.
Paint outside cylindrical surface of that assembly with black nail polish.
Load filters into item 4-1 assembly and push item 3 in to retain.
Use masking tape to assemble above assemblies for testing.
Adjust diffuser filters to get correct PAR reading.
Apply 2 drops of acrylic cement to hold item 3 in place.
Glue the two sub-assemblies together with acrylic cement.
Glue above assembly to item 7, and paint bottom of item 7 under item 6 with black
nail polish. Glue on a “socket” for holding wand made of piece of acrylic tube to
above assembly.

This modified meter works on the maximum scale of the lux meter, the 50,000lux scale. I just use a couple of drops of acrylic cement to lock the range selector slide switch in the right position.
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