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LEDs and PAR/Quantum Meters Part 3: Quantum Sensors and LEDs

Posted 05/02/2012 at 10:17 AM by GeorgeMonnatJr
Updated 06/18/2012 at 07:30 AM by GeorgeMonnatJr

I've finally reached the meat of my blog where I post the stuff that I find myself repeating in multiple threads, continuing from [URL=""]my general thoughts on technologies (Part 1)[/URL] and [URL=""]the definition of PAR (Part 2)[/URL].

As before, this blog is as much a place for me to keep my thoughts straight and engender discussion and something to refer to instead of retyping the same material a lot as it is a place to share information.

I'm not even close to being an expert on reef tanks, yet (maybe in a couple of decades), and I'm not a biologist or botanist. If you are an expert and you see something factually wrong please let me know, and I'll correct it. (Comments on my grammar and citation style will probably be ignored )

I do know LEDs and silicon-based photometers/photo-sensors (my title at work is Optoelectronic Engineer). I find myself re-posting the same graphs and explanations, so I figured I'd put it all in a blog that I can refer people to as needed.

[B][SIZE="6"][COLOR="Navy"]Quantum Sensors and LEDs[/COLOR][/SIZE][/B]

[I][B][SIZE="5"][COLOR="Navy"]The Apogee Quantum Sensor[/COLOR][/SIZE][/B][/I]

As discussed in [URL=""]Part 2[/URL], Photosynthetically Useful Radiation (PAR) is defined as all photons, or light, in the range of the visible spectrum, 400nm-700nm, because that is the light energy needed for photosynthesis of all types to occur. Silicon-based light or photon sensors, like the photo-diode, have a sensitivity or response to light in that range, but the response is not uniform. Photodiodes are much more sensitive to amber or orange/red (600nm-650nm) than to blue (400nm-480nm). Apogee's SQ line of quantum sensors is calibrated to make the photodiode curve more uniform, but it still isn't perfect (and can't be close to perfect for less than $1,000's) [URL=""][10][/URL].


Here again are the spectra for my TMC AquaRay LEDs (based on CREE LEDs) [URL=""][4][/URL].


If you overlay the blue-driven white LED spectrum over the curves for chlorophyll, it looks pretty much like this chart from MARINELAND [URL=""][11][/URL]. (I call them blue-driven white LEDs instead of just white LEDs as a reminder from [URL=""]Part 1[/URL] - it's very important.)


The two major things that I notice from that graph are:

1) the high AquaRay blue and MARINELAND white LED peaks line up very nicely with the ~460nm chlorophyll b blue peak and the high AquaRay white LEDs line up nicely with the ~440nm chlorophyll a blue peak (without either creeping below 400nm into the dangerous UV-A) and

2) the Apogee SQ-110 quantum sensor [URL=""][10][/URL] that I own is only about 80% "sensitive" to blue light around 450nm.

Here's the crux of my discussion: [B][COLOR="DarkGreen"]If the lights over the aquarium are blue and blue-driven white LEDs with the majority of light energy around 450nm and the quantum sensor only reads 80% of photons with wavelengths around 450nm, then the PAR meter reading for those LEDs must be multiplied by a 1.2 correction factor.[/COLOR][/B] Some will say it's technically 1.25 (1/0.8), but without accounting for the phosphor generated non-blue contribution I don't think 1.25 is perfectly accurately and maybe a little worse.

To back that up, I'll quote a second-hand email that a friend posted in my local forums, Austin Reef Club (ARC) [URL=""][12][/URL].

[quote="victoly in ARC"]
Email from Apogee Instruments on Measuring LED with Quantum Sensor:

"In regards to measuring LEDs with our quantum sensor, there are some caveats to doing so. The following link shows the spectral response of our quantum sensor ([url]http://www.apogeeins...alresponse.html[/url]). As the graph shows, Apogee quantum sensors underweight blue light, and as a result, photon flux measurements for blue LEDs will be too low. They also overweight red light up to a wavelength of approximately 650 nm, above which they do not measure, and as a result, photon flux measurement for red LEDs will either be too high (if the LED output is all below 650 nm) or too low (if a non-negligible fraction of the LED output is above 650 nm). Additionally, LEDs often have a very narrow spectral output, with a sharp peak of only a few nanometers. So, unless the quantum sensor has a perfectly flat spectral response, meaning it weights all wavelengths of light exactly the same, there will be errors. Electrically calibrated Apogee quantum sensors will likely provide a reasonable measurement for white LEDs because they are broadband, and because electrically calibrated quantum sensors are calibrated under CWF lamps. However, for narrowband LEDs, like red and blue, Apogee quantum sensors will not provide an accurate measurement.

As a less accurate method you can use the same spectral response graph as mentioned above to get a relative idea of the error. For example, a 450nm blue LED will have a relative response of approximately 0.8. Therefore, you can figure that the photon flux reading from the sensor is reading approximately 20% low. Just remember, this approach is only relative so give yourself a wide margin of potential error. A blue/white configuration should give you reasonable accuracy, particularly from the broadband spectrum of the white.

It would be more accurate to figure out the non-blue component of the LEDs and multiply that by 1.0, then figure out the blue component of the LEDs and multiply that by 1.2. That is a lot of work I'm not currently equipped to handle. I've chosen to multiply the total by 1.2 for the main reason that it's conservative and helps prevent bleaching of corals and others due to light shock or too much light.

One thing to remember is that pretty much any quantum sensor you buy for < $500 will be based on the same technology as the Apogee SQ-110 and will have a similar response curve, meaning that the rough factor of 1.2 still applies.

I've noticed a lot of posts here on Reef Central and elsewhere where people complain about corals and/or others bleaching with the root cause of too much light discovered later. A perfect example is
[URL=""]is it just me or it seems many LED users get coral bleaching/ lightening?[/URL]

[QUOTE=kevensquint;20162939]I hang out in 3 forums in different parts of the world and everyday there is a new thread or two with coral bleaching under LEDs. I myself have been frustrated with the same situation under my Radion. Some may blame poor or lack of acclimatizing the corals to the light, maybe the case sometimes. But I'm going to propose an idea based on what I have experienced...[/QUOTE]

[I][B][SIZE="5"][COLOR="Navy"]How I Put Together My Equipment[/COLOR][/SIZE][/B][/I]

Instead of paying hundreds of dollars on a PAR meter, I bought an [URL=""]Apogee SQ-110 sensor (click on "Order Sensors" on left)[/URL] and borrowed the digital multimeter (DMM) from work. As long as your DMM reads in millivolts (mV), or thousandths of a volt, then you can use it with the SQ-110 quantum sensor to get PAR readings. There are a lot of cheap DMMs available that will work just fine. You simply multiply the mV reading (or thousandths of a volt) by 5 to get the PAR value. [I]As a bonus, you have a DMM that can be used to test for stray voltages, grounded circuits, live wire and many more things, too, instead of just a more expensive meter that only reads PAR.[/I]

The gain/output on the Apogee SQ-110 is 5.00 µmol m-2 s-1 (or written as µmol*m2*sec) per mV, so if you get a mV reading of 20mV (0.020 V) that's 100 PAR. Since I have LEDs, I then multiply it by 1.2 for 120 PAR.

(20 mV) * (5 µmol*m2*sec/mV) * 1.2 = 120 PAR


(0.020 V) * (5000 µmol*m2*sec/V) * 1.2 = 120 PAR

Note: Don't make the same mistake I did of paying for a Fluke 113. I spent the extra money ($95), because I wanted accuracy and needed to use the DMM for other, non-aquarium things. Unfortunately, [B]the Fluke 113 doesn't work with the SQ-110[/B]. It's a low impedance DMM for better accuracy and doesn't have the option to change its impedance. The sensor was built to work with a standard voltmeter.

[URL=""]Part 1: Technologies[/URL]

[URL=""]Part 2: PAR[/URL]


[4] Quality Marine: AquaRay USA, [URL=""]Spectrum of Marine White and Reef Blue LEDs [pdf][/URL], 02MAY2012

[10] Apogee Instruments, Inc., [URL=""]Quantum Sensors and Meters: Spectral Response[/URL], 02MAY2012

[11] MARINELAND, [URL=""]Light sustains life[/URL], 02MAY2012

[12] victoly, Austin Reef Club (ARC), [URL=""]My First Coral and Clam post #9 on 29APR2012[/URL], 02MAY2012
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  1. Old Comment
    As I fear that coral bleaching with LEDs might concern me as well I would like to be able to at least evaluate the output they are giving. So your suggestion to get only a sensor is very appealing. Could you please clarify the key differences between SQ-110 and SQ120? I understand that the calibration procedure for these is different - 110 being calibrated against sunlight whereas 120 using an artificial lighting. Which one should people get ?

    Thank you!
    Posted 05/05/2012 at 05:52 AM by mpc mpc is offline
  2. Old Comment
    GeorgeMonnatJr's Avatar
    Originally Posted by mpc View Comment
    As I fear that coral bleaching with LEDs might concern me as well I would like to be able to at least evaluate the output they are giving. So your suggestion to get only a sensor is very appealing. Could you please clarify the key differences between SQ-110 and SQ120? I understand that the calibration procedure for these is different - 110 being calibrated against sunlight whereas 120 using an artificial lighting. Which one should people get ?

    Thank you!
    Both calibrations are done at the factory, so the user doesn't need to do it. It's really what you prefer as your reference, the sun or electric light. From what I can tell, the electric light calibration of the SQ-120 (page 7) is to sodium arc, MH, and/or CFL - not LEDs. I can understand not calibrating it to LEDs, because the variations are too numerous to list.

    Since I'm still a newbie concerning coral, when I was trying to decide which sensor to purchase, I looked through the forums and saw most people were using the SQ-110. Here are some examples.

    LED lighting and PAR measurements -- Advice?

    PAR meter around?

    LED PAR Readings

    That made sense to me, because we are concerned with replicating the sunlight that our livestock needs to grow. So I chose the SQ-110. If I had gone with MH instead of LEDs, it might have been a more difficult choice, but with LEDs I chose the sun as my reference.

    Thanks for asking. Because of your question I found more information on the Apogee site.
    Posted 05/05/2012 at 10:48 AM by GeorgeMonnatJr GeorgeMonnatJr is offline
  3. Old Comment
    Timfish's Avatar
    George, I like this series about lighting! While you are focusing on the issue of getting PAR readings for LEDs I feel it is important to p[oint out the issue with Apogee PAR sensors underweights blue light and overweights red applies to every light source not just LEDs. If a PAR reading is taken of T5s it will underweight blue to the same degree it underweights blue light from an LED source. I have long complained what we need is an easy way for the average aquarist to get a spectral chart of the light at a specific location in a tank.
    Posted 05/24/2012 at 07:18 PM by Timfish Timfish is offline

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