[Coral-List] PAR intensity of different kelvin globes

delbeek at waquarium.org delbeek at waquarium.org
Fri Aug 11 16:10:28 EDT 2006

Steve Dalton <sdalton at nmsc.edu.au> said:

> I recently purchased two 400watt metal halide lights and two what I assumed 
> were 14000 kelvin globes to conduct some coral bleaching experiments on 
> subtropical hard corals.  After setting up the experiment I was not able to 
> get consistent PAR intensity across all temperature treatment so I had to 
> purchase another 400 watt light.  The problem was that the globe that were 
> provided with the additional fitting appeared more blue than the original 
> globes and when I measured the PAR intensity output of the three individual 
> lights the two globe which were first purchased were nearly twice the value 
> of the more blue light.  The supplier indicated that the third globe was a 
> different brand and so sent another globe that was the same brand as the 
> first two.  This did not solve the problem because the new globe was still 
> noticeably bluer than the original ones and the PAR output was still 
> considerably less.  Could it be that the original globes were a lower 
> kelvin value say about 100000K which would explain why they were less blue 
> than other globes?  Or the more blue globes are 200000K and marked 
> incorrectly.  Does anyone know how I can tell what kelvin rating these 
> globes are and whether PAR intensity decreases with kelvin rating when the 
> globes have the same watt rating?
> If anyone can help me regarding these question I would really appreciate 
> your comments

Steven: Kelvin ratings on commercial lamps sold in the Aquarium industry are 
more often than not more abuot marketing hype than fact. The lamps that are 
bluer can either be made so by reducing the chemicals that produce longer 
wavelengths and thus letting the blue that is there to be expressed more, this 
would then result in a lower PAR value, or the lamps can be made bluer by 
adjusting the chemical balance to produce more blue while the other 
wavelengths remain about the same ... your results seem to suggest the former 
situation. Measurement with a spectraphotometer is really the only way to know 
for sure what is going on. Measurements done to date have shown significant 
differences in some cases between the provided Kelvin rating and what the 
results from the spectrophotometer show for some lamp brands.

Be aware also that there can be significant differences between lamps of the 
same Kelvin rating, and as the lamps age there will be further changes, usually 
the blue drops significantly compared to the rest of the spectrum since the 
chemicals that produce blue are less stable. Metal halide lamps are, by their 
nature, unstable so it is difficult to get ones that maintain spectrum and are 
identical when it comes to spectrum.

If you are not concerned about the amount of blue, then the Iwasaki 6500 K 
lamps are good and produce the most PAR for your dollar/pound. I don't have 
access to my home computer at the moment, but if you do a search for Sanjay 
Joshi, you will find his website where he has examined a LARGE number of 
metal halide lamps for spectrum and PAR output and you should spend some 
time there to learn more about PAR vs. Kelvin as how these relate to aquarium 
metal halides. 

J. Charles Delbeek
Aquarium Biologist III
Waikiki Aquarium
2777 Kalakaua Ave.
Honolulu, HI, 96815


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