MH colour temp vs growth

[tempest115]

Hey All

I have tried many searches on this and read many different websites, and am probably more confused than i was when i started. i have a 6x2x28"" tank with SPS and clams. i have 3x 250w halides (lumen bright) and 4 x 24w T5 blue's. i currently run 2x AB16K bulbs and one generic chinese 20K bulb. i have borrowed an apogee quantum meter and tested the PAR output from both bulbs. i have found that the 20K bulb is within 10% of the aqua-medic 16K globe, with the 20K having slightly more PAR at the bottom of the tank (expected). I like the look of the 20K better but after reading it seems that the majority of people don't seem to get good growth under 20K globes. My question is, with the PAR being more and less equal, why should growth be significantly different under the different lights? below is the approx PAR i am getting at different depths straight under the globe.

depth
0 1100
10 900
20 700
30 550
40 400
50 350
60 310

 

[Wes]

With PAR and all other variables equal. Color temp should not effect growth rate.

Usually bluer bulbs have lower PAR than the "warmer" bulbs. That is where the generalization comes from.

 

[prattreef]

+1 There are very few instances of bluer lamps having higher PAR than warmer (whiter) lamps, so the trend to slower growth under Bluer lamps is generally true. There are exceptions though, and you seem to have found one-- likely because the 16K lamp is not really 16K and is just a poorly made blue lamp.

 

[tempest115]

thanks guys. i thought this was the case. the 16k lamp is a aquamedic, so i would have thought should have been very good with regard to output. the 20k bulb i have though i suspect isn't a true 20k, probably why the output is close to that of the AB globe. it still appears a lot bluer than the 16k globe though. i shall see how my frags go under the 20k light and hope they grow!