Discussion on PAR (photosynthetically active radiation)

[DE Lightings]

Following from this thread...

Razali,

Would like to know the comparsion between an Arcadia 30w FL of 900mm length of 7,500K compared with the Osram 39w T5 High output tube of 6,500K - which is brighter and better for plants ? Is the T5 of same length as 900mm ?
I have an existing lighting hood from Bioplast which already has Electronic ballast and waterproof endcovers for the 30W FL. Would the e-ballast be able to use for the T5 and just replace with T5 waterproof endcaps if I want to convert it ?

The difference between T5 and T8 are the diameter of the FL tubes and the waltage. The length of the tubes will be the same. If you are refering to PAR values, of cause the 6.5k are better than the 7.5k. But both tubes will be suitable for planted tanks.

If talking about brightness, the higher the waltage, the brighter it will be. Not to mention the parabolic reflectors that helps to optimum the light spread.

u cant use a 30 watts eballast to fire a 39watts t5 tubes. u need to replace them together with the waterproof endcaps.

Cheers

[ginaalan]

Not too sure on your input that a 6.5K is better than 7.5K in term of PAR (By the way, what's PAR ?). I thought a 7.5K would be having a deeper light penetration and are definitely 'brighter' (by our naked eye) when compared side by side for a same wattage FL.

As for the eballast, need to know supposingly if the original eballast used for the 30W FL light is at a bigger wattage of say 40W, would it be suitable for the 39W T5 tube or it still requires special T5 39W eballast ?

Thanks.

[solonavi]

PAR = photosynthetically active radiation
From http://en.wikipedia.org/wiki/Photosy...tive_radiation

PAR is the range of light (400-700nm) that can be photosynthesized by plants. It coincides with the range of visible light.

The acronym PAR stands for photosynthetically active radiation, but the term PAR does not include the full range of light that can drive photosynthesis in all living organisms. If it included photosynthetic green bacteria, purple bacteria and Heliobacteria, PAR would extend into the near infrared zone. These bacteria live in environments such as the bottom of stagnant ponds, sediment and ocean depths. Because of their pigments they form colorful mats of green, red and purple. These organisms must make use of the leftovers discarded by the plant kingdom, in this case, light outside the PAR range.

Cheers
JC

[joe]

Would like to know the comparsion between an Arcadia 30w FL of 900mm length of 7,500K compared with the Osram 39w T5 High output tube of 6,500K - which is brighter and better for plants ?

Both are fine. Plants won't care what you give them as long as there is light

[ginaalan]

JC, thanks for the detail explantion of PAR.

Joe, your vague input does not help at all. Plants need the optimum light to flourish and that really depends on if you are giving sufficient lighting (but not too excessive) with the correct selection of light's intensity w.r.t. the tank's size. Plant may not care (or they dont have a choice) but aquarist who wants their plants grow well cares.....

[valice]

So if we set up 5000K together with a 10000k light, we should be be able to enjoy both a high PAR for the plants (from the 5000K) and also a higher perceived brightness (from the 10000k) from our lights right?

Because from what I understand, our eyes are more receptive to the green than to the other 2 colours, so for 10000k lights, there are more green so will look brighter than the 5000k although both have the same 24W...

[solonavi]

Wattage really does not means much. Intensity of a lights is measured by Lumens. PAR is the correct measure if we are discussing on how well a plant will response.

Check out this link for a much detailed explaination.
http://www.sunmastergrowlamps.com/Su...andPlants.html

Extract:
PAR Watts for Plants

Watts is an objective measure of energy being used or emitted by a lamp each second. Energy itself is measured in joules, and 1 joule per second is called a watt. A 100 watt incandescent bulb uses up 100 joules of electrical energy every second. How much light energy is it generating? About 6 joules per second or 6 watts, but the efficiency of the lamp is only 6%, a rather dismal number. The rest of the energy is dissipated mainly as heat. Modern discharge lamps like high pressure sodium (HPS) and metal halide convert (typically) 30% to 40% of the electrical energy into light. They are significantly more efficient than incandescent bulbs.

Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts. This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.

The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).

"Illumination" for plants is measured in PAR watts per square meter. There is no specific name for this unit but it is referred to as "irradiance" and written, for example, as 25 watts/square meter or 25 w/m2.
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The main problem of PAR is that the equipment to measure it is expensive compare to Lux. Thus most hobbist will only measure the Lux of their tube. However, do note that spectrum shift (PAR) will typically occur much earlier than a drop in Lux. That is also the reason why some tubes are more expensive.

Based on my experience.
JC