Lighting - PAR - Dana Riddle

When I visited The Aquatic Wildlife Company's facilities in Cleveland TN, and met with John Walch and Dana Riddle, I also had the pleasure to meet with the major funders of AWC, Mr. and Mrs M. Clark. A summary of the talk given by Dana Riddle on the subject of PAR can be accessed from our web site. This article supplements that summary. It is not the text of the lecture given at the recent Vegas Conference.

To conduct the experiments I ran, I used a Dawson Light meter (cost with electrodes well over 4000 dollars)that allowed me to measure the output of the lighting sources I tested in the photosynthetically active wavelength ranges. I was also able to measure UVa and UVb and could have measured UVc but did not.

The main purpose of this testing was to determine how much light (or energy or quanta which is how the corals perceive light) was actually emitted by a number of bulbs in use in the hobby on one hand, and on the other to determine whether light wavelengths outside of the PAR range were useful at all in terms of coral growth.

The PAR range is in the 400 to 700 nanometer wavelength spread I should clarify. That is not only the green range but also the blue and red ranges. We know that plants and zooxanthellae, or symbiotic algae, use the blue and red ranges for photosynthesis and not just the green range (if you wish more info on this you should read up on the two photosynthetic ranges PS-I and PS-II).

We can measure Lux of course, a measure of the intensity reaching a specific spot in the aquarium. Submersible Lux meters are available and are not nearly as expensive as the Dawson quantum meter.

The difference between the two measurements is that one measures intensity in all wavelengths whereas the other one measures only that intensity which can meaningfully be used in photosynthesis, pigmentation and growth.

PAR is measured in micro-moles or micro-Einstein. The sun typically emits a PAR value of 2000 mE.

PAR strength is not uniform across the emitted visible light area. Different light sources emit PAR values that change depending on where within the lighted area they are measured. This is of importance in placement of corals as you would not want to place one in an area where the PAR happens to be low (the area underneath the end of VHO tubes for instance).

4 VHO's emit about 1000 mE and 4 of the PC 96 watt just a fraction more than that. That is actually not bad at all. Match that with the 2000 mE from the sun and you can see that VHO and PC lights actually emit quite a bit of PAR desirable light.

Of course these measurements fall off as we go deeper in the tank. That needs to be kept in mind when placing corals since at lower levels the PAR that started off at 1000 mE's, may only be 100 or 200. This is not considered enough for coral growth. 300 to 400 mE is about the lowest I would recommend as a safe level (combined with all other tank parameters being at the right levels of course - in this respect read the other articles in this series on http://www.athiel.com).

The manner in which a bulb is mounted affects PAR readings rather dramatically. What I have found is that different lighting sources emit different PAR values depending on their geometry (shape) in different areas of the tank. As long as the PAR that can be measured in all areas is high enough for coral growth this is not of much importance, but because of the shadows certain bulbs create, where PAR is very low due to the lamp's shape, this is a factor to be taken into account.

As I publish more data, says Dana, details on high and low PAR values emitted by a large number of bulbs that I have studied will become available. In the interim, you can ask questions and I will try to answer them based on my notes.

You can direct your questions to Albert because he passes them on to me in bulk and I can then answer all of them at once. I prefer this method as I have a very busy day here at The Aquatic Wildlife Company and cannot spend hours checking up on individual messages I am afraid. I wish I could but I have 24 000 gallons of systems to supervise, testing to do, checking on corals, and so on.

In my experience a 400 watt MH placed horizontally or in a pendant not a reflector, is what will give you PAR readings that are high enough and evenly distributed throughout the aquarium.

Additionally when comparing 400 watt bulbs, the one that emitted the best PAR values overall is the 6500 K bulb (brands are not important at this point).

What one needs to keep in mind though is that as the bulb ages PAR values will diminish. The advantage of measuring PAR is therefore obvious: you will know when time has arrived to replace your bulb or bulbs.

Note that pendant mounted 400 watt bulbs emit PAR levels higher in most cases than the sun. Of course the area they cover is restricted by the shape of the pendant and the degree to which it spreads the light. Most pendants are rather narrow and focus the light straight downwards, hence the high PAR readings.

Based on these findings you will need one 400 watt light per 2 feet of aquarium to obtain sufficient PAR in all areas of the tank. Note though that even though these lights give off a lot of PAR, once you measure right underneath the water, the level falls very rapidly. At AWC just a few inches under the water PAR was only at 400 mE from a whole series of 400 watt MH's over a 1200 gallon vat with the lights shielded for UVa and UVb and only about 9 to 9.5 inches away from the water (all are pendant lights).

So if you think you are overlighting you may be surprised at what you find when you actually measure the PAR value. Intensity may be very high but, as I explained in this and other articles, that is not what corals need. Intensity is what our eyes perceive. PAR is what the corals use for growth and that is what we need to pay more attention to in my opinion.

Note that I mentioned Iwasaki bulbs. I need to add that these bulbs give off a fair amount of UV light and need to be shielded. We have found at AWC that UV is not necessary for coral growth and pigmentation and, in fact, many studies done on the effect of UV has demonstated that it can be, and often is, detrimental to them and may leave sequellae as a result of overexposure to UVa and especially UVb.

End of part 1 of this article.

Check this document again.

Thank you.

Go from here to Other AWC Related Articles and read the transcripts of my interviews with Dana Riddle and John Walch.

Albert J. Thiel, Copyright 1997