creg said:
i dont really know anything about par but i had a feeling that was the case.
Understanding PAR is worthwhile because it allows you to understand the fundamental behavior of plants and algae. This then allows you to avoid critical errors in judgement. People are always worried about whether their light fixture has "enough" light to grow plants, never realizing that invariably, they actually have too much light, and this then sets up a never ending destructive chain of events that many never recover from.
PAR is an acronym for Photosynthecically Active Radiaton. Plants harvast the energy of the sun by using photons of light. These photons are like missiles projected through space. There are trillions and trillions of them raining down every second. When photons strike an object, one of three things may happen:
1. They are reflected (this is how we are able to see thing).
2. They pass through unhindered or only slightly deflected.
3. They are absorbed and destroyed.
In a plant leaf, all three things occur, but the most important is item 3, where there is a violent collision with the pigment cells. These special cells absorb the energy of the photon and redistrubute the energy to other cells that produce sugar.
A PAR meter measures this radiation in terms of the number of photons striking the leaf per second over a unit area (1 cm by 1 cm). The units are "micromoles per second per square centimeter." The unit "mole" is 6.0221415 X 10**23. Written out it looks like this:
602,214,150,000,000,000,000,000 So that's about six hundred billion billion.
So that's a really big number. A micromole is 1 million times smaller, so it looks like this:
602,214,150,000,000,000 so that only about six hundred million million.
Again, some of these million miilion photons pass through a leaf, some of them are reflected away from the leaf and the rest are absorbed and destroyed by the leaf. The ones absorbed by the special pigment cells are the ones that generate the power necessary to produce sugar. But the leaf can only handle some of these in a productive and organized way. The pigment cells are arranged in a special way to transfer and use the energy, sort of like roads and motorways. The traffic has to be controlled. When the traffic system is overloaded with photons the energy produced becomes disorganized and uncontrolled. The energy then causes chaos and distruction.
Plants only need a total photon bombarddment of between 6 thousand million million and about 15 thousand million million photons per second per square centimeter - so that's about between 10-25 micromoles at the minimum in order to survive. At 30-40 micromoles this is considered low light. 50-70 is considered medium, and when you get up tp 100 to 150 this is considered high light.
I've actually never seen a case where someone's lighting fixture did not produce enough micromoles for their plants to survive. It's really not that difficult to produce 25 micromoles. If you have your tank in a room with an open south facing window, with the lights turned off, you can get 5 or 6 micromoles in that room without even breaking a sweat.
That's why I seriously doubt that this lighting fixture of yours will have any trouble generatong more than the minimum PAR. It's a much more likely case that it will produce too much PAR, so the thing to consider is whether or not the unit comes with a controller that will allow you to decrease the power or to control the number of LEDs that are illuminated.
So now, you know more about PAR than 99% of the world's population, and for the rest of your life, you'll never again be able to say that you don't know anything about PAR.
Cheers,
you are a very smart man ceg, thankyou for teaching me about par 
i can now say i know a thing ot 2 about par