Photosynthetically Active Radiation (PAR)

[Chris_150G]

This guys tests are interesting. Especially how he found that a glass top reduces PAR.
-Chris
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Photosynthetically Active Radiation (PAR) measurement

1. In order to keep lighting at constant levels optimal for coral growth it is possible to measure and monitor levels of photosynthetically active radiation (PAR) as well as LUX. Metal halide lamps alongside fluorescent lamps will loose some intensity over a period of time. By measuring PAR it is possible to adjust lighting levels accordingly:

Apogee LQM-ELEC meter and QSO-ELEC sensor reliably measure PAR levels. The sensor can be submerged in water and is supplied complete with 2m of cable.

This quantum sensor is waterproof and measures only 33x20mm. The sensor generates 0.2mV per umol/m2/sec. It is supplied with 3m of cable. It can be connected to data acquisition systems or simply connected to a multimeter capable of measuring mV. The measured voltage is then converted to the PAR/PPF range simply by multiplying the value by 5. This is a cost effective solution for your PAR monitoring and measurement.


Measuring PAR levels in an aquarium

To find a good performing MH lamp we've used an Apogee Quantum meter to measure PAR (photosynthetically active radiation). PAR levels will vary depending on the type of MH lamp and the age of the lamp.

It is difficult to tell exactly when MH lamps need replacing but a PAR meter can clearly show a decline in light intensity. It allows aquarists to keep lighting levels constant by simply adjusting the hight of the fixtures housing the lamps as this will increase PAR in the aquarium and by replacing lamps at the right time.

A standalone quantum meter can be used for monthly checks. Alternatively a low-cost quantum sensor can be connected to a data acquisition system or an aquarium controller to continually measure and record lighting levels.

If you are interested in checking your aquarium lighting levels using quantum meters please let me know as we have these meters in stock. There is also a demo unit available if you'd like to do a spot check or familiarise with these meters.

The aquarium is set up with flourescent marine tubes. The depth of the aquarium is aproximately 50cm.

1) Reading taken just under the neon tube, glass over the aquarium was removed

2) Reading taken in the middle of the aquarium, glass over the aquarium was removed

3) Reading taken at the bottom, glass over the aquarium was removed

4) Reading taken just under the neon tube with the glass cover

4) Reading taken furhter down from the top with the glass cover in place

5) Reading taken at the bottom of the aquarium with the glass cover in place

 

[Chris_150G]

In the test the PAR was compared with and without a glass top at three depths.


PAR readings
_____________No glass top___ w/ glass top ____% change in PAR w/ glass top
Upper................... 278 .................143......................... -48%
Middle.................. 116 ..................73.......................... -37%
Bottom................. 102.................. 70.......................... -31%

 

[ccCapt]

Those are some pretty poor PAR numbers, but it looks like a softie tank where PAR isn't that important.

Here's my PAR testing.
https://www.livingreefs.com/par-meter-readings-t19189.html

 

[Chris_150G]

Yes your test results are great for comparing MH bulbs. I am printing out your data right now.

This one shows how much PAR the glass top filters (unfortunately he had flourescents). I think we can assume that the glass top would have a similar affect on other types of bulbs.

 

[wontonflip]

Man, I wish I knew someone with a good meter. I might buy this one someone found:

Aquarium Lighting & Monitoring: Milwaukee Instruments Smart Lux Light Meter

I'd love to get PAR readings while testing out some LEDs. Good to know about the glass top results. Thanks for sharing that, Chris.

 

[tankedchemist]

Yes your test results are great for comparing MH bulbs. I am printing out your data right now.

This one shows how much PAR the glass top filters (unfortunately he had flourescents). I think we can assume that the glass top would have a similar affect on other types of bulbs.

Actually, I wouldn't make that assumption. Here's why:

here's a spectrum of some 10k fluorescent lights:
http://www.iquaticsonline.co.uk/ebay/large/spectrums/10k150w-spectrum.jpg

and here we have a spectrum from MH (look @ 14K spectrum, which i think most people use):
http://glassbox-design.com/wp-content/uploads/2010/01/elos-20k-14k-metal-halide.jpg

see how there's far more blue, relative to the other wavelengths, in the fluorescents? means more light at short wavelengths than MH's have.

Now for the next tidbit of info-- glass is basically opaque to very short wavelengths of light. SO, the fluorescents very likely loose a lot more of their PAR due to going through glass than MH's do.


.... but that's all theory until someone tests it. Now I want to get a PAR meter.

 

[Chris_150G]

You make a good point Tanked. Flourescents do have a different spectrum and signifigantly less PAR than MH.

However, aside from the amount of PAR, how is the PAR that these flourescents make different than the PAR of a MH?

PAR is simply a measurement of the amount of light in the 400-700nm range. I'm thinking that if the glass reduces the amount of 400-700nm light of a flourescent by say 30%, then couldn't it also reduce the PAR of a MH?

I wish the guy used MH for the test.

 

[tankedchemist]

no. As I said, the spectrum of light that each of the emits is different-- look at the graphics I linked. Glass doesn't filter all wavelengths (colors) of light in the same way. it blocks shorter (bluer) wavelengths more than it does longer wavelengths. ergo, since fluroescents have a "bluer" spectrum more of its light is lost because of the glass than the "whiter" halides. In other words I bet if someone compared the %light lost at different depths in MH and fluorescent, the % lost for fluorescent would be higher.

 

[Chris_150G]

In this situation I am not considering the entire spectrum of light created by any bulb. The full spectrum is irrelevant because it doesn't contribute to photosynthesis. I am not saying that they have similar spectrums. They are very different. But, yes the flourescent has more useless light waves being absorbed by the glass. And, the MH has much more PAR than the flourescent. But a light wave that is in the PAR range is the same regardless of what bult creates it. MH just have many more.

I am only considering the small amount of the flourescent light's spectrum that falls within PAR. That small amount was reduced by the glass. The fact that the glass reduced the PAR is signifigant because that means glass reduces PAR.

 

[tankedchemist]

okay, clearly you have a very fundamental misunderstanding of biology. Photosynthetic organisms have developed the ability to use MOST of hte spectrum of light for photosynthesis. PAR is a measurement of the wavelengths of light most closely associated with chlorophylls a and b, the two primary photoactive molecules in plants. chlorophyll a has two wavelengths it absorbs light best at: around 450nm (BLUE) and around 675 (red)... point of fact, the blue light actually results in more efficient photosynthesis.

PAR measurements themselves are a measurement of the intensity of light within the 400-700 nm range. that's visible light. Here's some wiki links:

http://upload.wikimedia.org/wikipedia/commons/7/72/Par_action_spectrum.gif

Photosynthetically active radiation - Wikipedia, the free encyclopedia

the efficiency of photosynthesis under blue light is undisputed. that's why we buy actinics. And, the fact that glass preferentially absorbs shorter (bluer) wavelengths of light is undisputed. Ergo, it's likely that bluer lights (fluorescents) will lose more of their PAR than whiter lights (MH) as a result of filtering through glass. :)

 

[ccCapt]

Dr. Sanjay Joshi is the premier lighting expert in our hobby. Here's a few of his articles that are very informative.

Facts of Light – Part I: What is Light? by Sanjay Joshi - Reefkeeping.com

Facts of Light – Part 2: Photons by Sanjay Joshi - Reefkeeping.com

Facts of Light – Part 3: Making Sense of Light Measures by Sanjay Joshi - Reefkeeping.com

Facts of Light – Part 4: Color Temperature by Sanjay Joshi - Reefkeeping.com

Facts of Light – Part 5: Everything You Need to Know About Metal Halide Lamps and Ballasts by Sanjay Joshi - Reefkeeping.com