MH Lamp Spectra

[kaltika]

I have been encouraged to take some lamp spectra for the benefit of reefer-kind. To that end, what follows is an analysis of a few lamp spectra, to be compared with the spectrum of the sun.

Anyone knows that hot things emit light. A physicist calls this blackbody radiation and the formula is easily found on the web. There is also a saying among chemists, that a chemist knows that nothing is as simple as the physicists think. I am a physical chemist, therefore I have a slightly more complicated view of things that is probably necessary.

Attached is a spectrum of the sun, overlaid with a blackbody radiation spectrum of the appropriate temperature (5800 Kelvins). The blackbody curve stops at 1000nm because our favorite critters are all living underwater, and water absorbs almost all of the light at 1000nm or more. The curves match pretty nicely.

[kaltika]

Attached here is a collection of the spectra of three lamps I had handy. The MH lamps are 250 watts, fired by electronic ballasts, each is 3 months old. The NO fluorescent is just the fluorescent bulb over my desk at work, not sure what the color temp is supposed to be, but I'd guess its a 5500K lamp.

[kaltika]

Here is a view of the NO lamp compared to the sun and two blackbody radiation spectra. The peak is a little to the red side of the 5500K blackbody, in the yellow region of the spectrum. (these are typical fluorescents like are used in schools.)

[kaltika]

Here we have the Corallife 12000K MH bulb spectrum compared to the sun and the 12000K blackbody spectrum. You can see that the shape of the Corallife bulb matches the 12kK spectrum quite well at longer wavelengths, but reaches a maximum just red of the sun's spectrum (520nm or so), and cuts off at much longer wavelengths than the true 12kK blackbody.

[kaltika]

Next is the Radium 20kK bulb. Of course it looks nothing like a real 20kK blackbody, as such a bulb would kill your fish. It does, however, match the curve of the 12kK blackbody, much like the 12kK bulb, but it peaks a little to the blue of the sun's spectrum(around 490nm), and extends a little farther into the UV. The 20kK bulb also has much less intensity in the red region than the 12kK bulb. The blue color we see when looking at these bulbs is due to the strong feature at 456nm, which is much stronger in the 20kK bulb than in the 12kK bulb, but is present in both.

[kaltika]

If everyone would like to see more spectra, I'll see what I can do. One of these days I plan on getting some spectra of some more NO fluorescents of different types. If you would like the raw data for these plots to play around with on you own, let me know.

[stereomandan]

Thanks for taking the time to do this Mat

I wish I could send you a PC bulb and a VHO bulb for comrparison.

Dan

[kaltika]

One of these days I'll manange to con someone into letting me borrow one. when I do, I'll post it.

In the meanwhile, you said you were unclear about the physics of the MH lamp in that other thread, yet your description was quite accurate. A MH is a mercury vapor lamp with transition metal iodides included (they need to be heated into the gas phase, hence your 10 minute warmup time) to expand the spectrum and increase efficiency. (one gas exciting another as you described) In fluorescent bulbs, the mercury vapor in the lamp excites the coating on tube, giving you lower efficiency, but usually broader spectra, as you are exciting a solid film on the tube surface. Solids emit broader lines.

just a littl emore FYI,

-mat