My New Gadget

Huzzah! I can finally measure my light properly!

I got my new Apogee SQ-500 quantum sensor in the mail yesterday and I found some time to play with it tonight. This thing is pretty slick. This sensor comes in a few different styles – you can get it attached to a handheld meter that gives you a digital readout (MQ-500), you can get it as just the sensor with a USB termination on the end (SQ-520), or you can get it with bare wires (SQ-500), which is what I opted for. In order to read the data from this sensor, you just hook up the wire leads to your multimeter and read the DC voltage output.

Quantifying PAR Light

These sensors measure PPFD, which stands for Photosynethic Photon Flux Density and is measured in micromoles per meter squared, per second (µMol/m2/S). Put simply, PPFD is a measurement of the number of photons in the PAR range (Photosynthetically Active Radiation = light usable by photosynthesis) landing on a square meter per second. Measuring PPFD is the best way to determine how much usable light a plant is getting. Trying to use inexpensive lux meters for this task will produce wildly inaccurate readings due to the fact that they are designed to measure lumens, which are weighted according to how the human eye perceives light.

In my brief testing so far, the main issue I have is that the signal is so low (I’m talking microvolts), that a single measly millivolt is equivalent to 100 PPFD. As most multimeters only read down to the millivolt level, I am only able to get readings of hundreds of PPFD at a time (1mV = 100 µMol/m2/S, 2mV = 200 µMol/m2/S, etc.). The solution to this is to find a way to cleanly amplify the signal produced by the sensor by 100x, so 1mV = 1 µMol/m2/S. Once I figure this out, I’ll be able to get an accurate reading like 124 µMol/m2/S instead of just seeing 100 µMol/m2/S. It looks like there is a part available to do this, but doing this kind of stuff yourself is the fun part, so I’m going to try and figure it out. If any of you electronics nerds know an easy way to DIY it, please let me know!

First Reading – Space Bucket PPFD

Now, onto my first test results. I was most curious to see what sort of light my plants were getting in my space buckets, so I grabbed one of them and threw the sensor in at a few different distances from the light. I use 2 spacer rings in my space bucket, so the bottom of the bucket to the surface of the COB is about 24″, and this is where I started. I set current to my Vero 18 to 1,400mA and got to testing.

The sensor sitting on the bottom of the bucket, 24″ from the COB.

At the very bottom of the bucket, the sensor was getting ~500 µMol/m2/S

Next, I propped the sensor up at 18″ away from the COB. This upped my reading to about 600 µMol/m2/S.

I raised the sensor so it was 12″ from the COB.

At 12″ away, I was getting 1,100 µMol/m2/S! That’s awesome!

Finally, I placed the sensor 6″ away from the COB.

I was getting 1,900 µMol/m2/S at this height. Crazy!

The readings I got from my bucket were waaaaay higher than I expected from a single Vero 18, which is neat. In comparison, the floor level of my grow tent beneath 4x Cree COBs was only about 300 µMol/m2/S, though the lights are quite a bit further away and the tent was not sealed up, which let some light out.

I’m very excited to do some more testing with this device and will definitely share my results as I gather more data.




  1. Thank you so much for this! Could you please do the measurements in an unpainted bucket just to quantify how well the reflective white works for you?

    I’ve heard that light is spread in an inverse square basis, so your readings should have been lower theoretically, but who cares right? at the end I just want to see what the truth is

    • LEDGardener

      March 17, 2017 at 1:26 pm

      Glad it was of use to you! I’ll track down some unpainted pails soon and post my results – I expect there won’t be much difference.

      Regarding the inverse square law, here’s my very uneducated guess: I think that the inverse square law applies to light in OPEN space, as it’s about light spreading out and covering more area. In the bucket, a lot of it is reflected off the walls, kind of like a giant fiber optic cable, though not nearly as efficient.

  2. Congrats on the buy!
    PS Buy a dc milivoltmeter?

    • LEDGardener

      March 17, 2017 at 7:48 pm

      I didn’t know millivolt meters existed… you learn something new every day. I’ll look into one of those if i can find one comparable in cost to my other idea, thanks!

  3. A high bin cxb3070 vs clu048-1212 of the same spectrums comparison would be great to show how much more light the tripple the price cree gets you over the citizen.

  4. The SQ500MM is built on this base schematic, with a few tweaks to improve low noise performance.

    Hope that helps 🙂

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