LEDs and PAR/Quantum Meters Part 1: Technologies

This three-part blog article is a summary (after some introductory information) of my experiences reading PAR values of LED aquarium fixtures and how to take and adjust them. 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. You can skip to [URL="http://www.reefcentral.com/forums/blog.php?b=519"]Part 3: Quantum Sensors and LEDs[/URL] if the background info is too boring.

[B][SIZE="6"][COLOR="Navy"]Part 1: Aquarium Light Technologies (General)[/COLOR][/SIZE][/B]

Most of what I've learned recently concerning how corals and other photosynthetic reef life uses light from metal halide (MH), T5 fluorescent, light-emitting diodes (LEDs), or any other light fixture technology comes from Carl Strohmeyer's "[URL="http://www.americanaquariumproducts.com/Aquarium_Lighting.html"]AQUARIUM LIGHTING[/URL]" article [1]. He runs [URL="http://www.americanaquariumproducts.com"]American Aquarium Products[/URL] and is trying to sell fixtures, but I continued reading numerous articles and have not found any errors with his logic and findings. I'll quote him from time-to-time, especially for PAR and PUR, but want him to get all of the credit he is due.

Working with LEDs I already knew they are horrible for color rendering, because each LED is a color spike meaning you don't get a broad range, or spectrum, of colors illuminated. LEDs can make clothing or other items look like a completely different shade or color to the human eye. I knew T5s and MH lamps are vastly superior to LEDs for color rendering, but florescent bulbs don't have the focused power to penetrate into water and MH lamps waste a ton of power as heat (a huge concern in Texas).

[I][B][SIZE="5"][COLOR="Navy"]Metal Halide (MH)[/COLOR][/SIZE][/B][/I]

Here is a typical MH output spectrum as seen on Wikipedia's Metal Halide page [URL="http://en.wikipedia.org/wiki/Metal-halide_lamp"][2][/URL].

[IMG]http://i209.photobucket.com/albums/bb16/GeorgeMon/Pets/800px-Metal_Halide_Lamp_Spectrum.jpg[/IMG]

Notice MH is heavy in the green, yellow and orange and not so much in the blue and red. That becomes obviously important later in this discussion. The spectrum "curve" changes with different phosphor coatings to get different "color temperatures", and MH lamps for reef aquariums are better optimized than the chart above. Here's another that I found that is specifically for reef lighting from a great series of articles on MH for reef aquariums [URL="http://www.personal.psu.edu/sbj4/aquarium/articles/MetalHalideLamps3.htm"][3][/URL].

[IMG]http://i209.photobucket.com/albums/bb16/GeorgeMon/Pets/MetalHalide250WcomparisonChart.gif[/IMG]

You can see how the higher color temperatures shift the peaks into the blue, but there are still significant peaks in the green, yellow and orange. One troubling aspect of MH is on the left side of the graph where there is a significant amount of Ultraviolet A (UV-A) energy (315nm–400nm) which can damage coral, fish, people and equipment.

[I][B][SIZE="5"][COLOR="Navy"]Light-Emitting Diodes (LEDs)[/COLOR][/SIZE][/B][/I]

Here are the spectra for the TMC AquaRay LEDs that I'm currently using. They are based on high-quality/power CREE LEDs and are typical for blue and white LEDs from all manufacturers [URL="http://www.aquarayusa.com/led-spectra-aquabeam.pdf"][4][/URL].

[IMG]http://i209.photobucket.com/albums/bb16/GeorgeMon/Pets/AquaRayLEDProfileCombo.jpg[/IMG]

You can see the high blue peak on the graph for the "white" LED. That's because white LEDs are typically blue LEDs with a phosphor coating to make "white" light. Different phosphors give the different color temperatures. These are called blue-driven white LEDs which so far are the only kind of white LEDs I've seen for reef aquariums (the other type is an array of multiple, single-color LEDs like RGB or RGBA which can lead to really bad color rendering).

A few notes on the LED spectra of which importance becomes apparent later. Note the high blue peak around 440nm-460nm, the lack of UV-A energy (315nm–400nm) and the lower levels of green, yellow and red. LEDs are also much more efficient than MH in that more of the electricity gets converted into useful light and not useless light (as discussed in the next section), or worse, heat. The MH lamps still do a better job of rendering colors for human eyes (broader spectrum), and people have been successfully using them for reef aquariums for decades.


[URL="http://www.reefcentral.com/forums/blog.php?b=518"]Part 2: PAR[/URL]

[URL="http://www.reefcentral.com/forums/blog.php?b=519"]Part 3: Quantum Sensors and LEDs [/URL]


[B][SIZE="6"][COLOR="Navy"]References[/COLOR][/SIZE][/B]

[1] Carl Strohmeyer, [URL="http://www.americanaquariumproducts.com/Aquarium_Lighting.html"]AQUARIUM LIGHTING (Updated 4/03/12)[/URL], 01MAY2012

[2] L. Michael Roberts, Wikipedia, [URL="http://en.wikipedia.org/wiki/Metal-halide_lamp"]Metal-halide lamp[/URL], 02MAY2012

[3] Sanjay Joshi, Ph.D. and Timothy Marks, [URL="http://www.personal.psu.edu/sbj4/aquarium/articles/MetalHalideLamps3.htm"]Spectral Analysis of Metal Halide Lamps Used in the Reef Aquarium Hobby Part 3 — 250-Watt Metal Halide Lamps[/URL], 02MAY2012

[4] Quality Marine: AquaRay USA, [URL="http://www.aquarayusa.com/led-spectra-aquabeam.pdf"]Spectrum of Marine White and Reef Blue LEDs [pdf][/URL], 02MAY2012