Lighting: T5 vs Metal Halide???

[StirCrazy]

Originally Posted by Myka

You make some very good points. When you take away all the variables, and actually compare them on fair grounds T5s just won't keep up to MH - especially when you get a bit of depth to the water.

Why do actinics give a false high reading on PAR meters?

PAR meters are calabrated for either sun light or electric light, and at a base spectrum. the longer wave lenght of the blue light in an actinic bulb is way out of perportion for either calabration as they are done based on 5500K(if I remember corectly). so when it recives a higher level of actinic than the calabration set had it thinks it is more powerfull and reads acordingly. the same holds true for say 3000k, it will report less PAR than there actualy is.

they pick the middle for the calabration and then it is only out a bit on the ends so you get a happy medium. I think at 400nm mine is 2.7% higher than actual ouput. so it isn't a lot, some meters are scewed by as much as 10%.

this is why I don't like to compare different colors of lights with out a disclamer, for an example, my AB 10K 250watt SE driven by a M80 ballast and my special home made reflector, would put about 550 to 600 units of PAR on the bottom of my 24" deep tank, so that is through 22" of water and 7" of air (I don't run the water 2" below the top, only 1 but my sensor is 1" tall also) for a total distance of 29" A tek unit with a 10K bulb load (4 bulbs) placed directly on the top of the tank for a distance of 24" from the sensor (22"water 2" air) got a max reading of 295 units of PAR. Still a respectable amount of light. now as for the disclamer, a 10K ab is actualy 12600ishK so a bit more actinic than the 10K T5s. so lets use the 10% which is the worst I have heard of and take that off my ABs it would still come to 495 to 540 Units, but seeing as there is only a 2000K difference in the bulbs I would guess it would be a lot close to the actual reading, as it takes some good actinic, as in 20K bulbs or pure actinic bulbs to get the meter to over estamate.

Steve