[Poseidon]

Okay it is coming soon, the dreaded bulb decision process. In Febuary I will need to replace my bulbs as their 12 month service period will have ended. Maybe it is the head cold I have now, or the darn meds, but for the life of me I cannot find the "key" to Mr. Joshi's excellent spectral analysis charts.

Here is a chart showing my current bulb, the CoralVue 10K, and the XM 10K which seems to have its peak MUCH closer to the optimum 420nm. I am familiar with PAR and even P-I to some degree, but what the heck are PPFD and CCT? Maybe one of my cohorts will be able to digest this better then my medicated head can....



Thanks everyone!

[Ninong]

PPFD = photosynthetic photon flux density. It's a quantitative measurement of PAR (photosynthetically active radiation, 400-700 nm) in photon terms. PPFD is usually stated in microEinsteins per square meter per second. This is not the same as the measurement typically given in scientific texts -- PAR irradiance in watts per square meter. PPFD measures the number of photons with all photons considered equal, independent of their energy.

CCT = correlated color temperature. Here is Dr. Sanjay Joshi's explanation of CCT and CRI (color rendition index):

Correlated Color Temperature (CCT) and the Color Rendition Index (CRI) of the light source. Color temperature refers to the spectral distribution of light radiated from a black body as it is heated. A black body is an ideal object that absorbs all light incident upon it. This complete lack of reflectance implies that the apparent color of a black body in no way depends on the light that falls on it; a black body gets its apparent color only from the light that it radiates. This visible light radiated by a black body changes from red to blue to white as the temperature of the object increases. The color temperature of the light emitted is expressed as a temperature on the Kelvin scale, and corresponds to the temperature of the black body at which the light is emitted. This is known as the black body model.


Most lamps do not follow the black body model completely. That is, the spectra of most lamps does not correspond to the spectrum of any true color temperature. For this reason, and also because it is very hard to measure the temperature of a filament or an arc tube, a value called correlated color temperature (CCT) is commonly used rather than color temperature. CCT is a calculated value, which provides the closest approximation to the color temperature of a true black body. The temperature that is the closest match is referred to as the CCT. Color rendering index (CRI) indicates how much a lamp will cause an object to shift in perceived color compared to its color under natural light of the same CCT. CRI numbers range from 100 (no color shift) to 0 (black and white). For the purpose of analyzing the light sources available for lighting aquaria, the spectral distribution is the most important piece of information, because all radiometric and photometric quantities can be derived from it.

[Poseidon]

Okay so given that information the Coral Vue has PPFD of 66 and the XM almost twice that at 115. Does that make the XM a better bulb? My corals have done very well under my CV, but I have noticed 1 bulb has shifted more towards blue then the other. That makes me a little leary about the differences that I can not SEE, maybe the quality control consistancy was not there in the CV bulbs. I have also noticed most places no longer carry them...

[Ninong]

Originally Posted by Poseidon

Okay so given that information the Coral Vue has PPFD of 66 and the XM almost twice that at 115. Does that make the XM a better bulb?

Assuming identical conditions (meter positioned same distance from the lamp, both either with or without a UV absorbing glass shield, etc.), it simply means that the XM lamp produced 75% more PAR than the Coral Vue lamp. It tells you nothing about the spectral distribution of the PAR. It does mean that if you placed five Coral Vue lamps over a 6-ft long tank, you would still have less total PAR than if you placed three XM lamps over that same tank.

[Poseidon]

Boy, I didn't think my bulbs SUCKED that bad!!! LOL

[Ninong]

Originally Posted by Poseidon

Boy, I didn't think my bulbs SUCKED that bad!!! LOL

Don't overlook the fact that the same lamp will give different results with different ballasts. If you are changing lamps, you might want to check to see if your ballast is one of the ones that Sanjay used to test the new lamp you are considering and, if so, was it the best choice for that lamp.

[Poseidon]

Yes I checked the profiles with my ballast, as for if the IceCap electronic is THE best, I don't know, but it sure works well for me. I guess I will have to go pretty slow in acclimation when I change bulbs, or else I'll end up frying everything!

[Ninong]

I guess I should not have asked if your ballast was "the best choice for that lamp" because that is obviously a subjective judgment. I should have asked if that ballast produced the most PAR with that particular lamp.

As you already know, your particular ballast produces much, much less PAR with both of the lamps you are asking about than a cheapy M80 ballast. Since your ballast is an electronic ballast, it uses less electricity and produces less heat and weighs less, but that's about it.

According to Sanjay's tests:

250w XM 10,000K on the M80 ballast produced 182 PPFD vs. only 115 PPFD on the Ice Cap ballast. http://www.advancedaquarist.com/issu...004/review.htm

250w Coral Vue 10,000K on the M80 ballast produced 93 PPFD vs. only 66 PPFD on the Ice Cap ballast. http://www.advancedaquarist.com/issu...004/review.htm

That seems like a very significant difference.

[Poseidon]

Yup it sure is... Geez I wonder how much more growth I would have IF I had a better ballast? Budget won't allow for that anytime soon, but I don't think I would want to subject my corals to that large of a change at once anyway. Let them get used to the new bulbs, then later reacclimate with new ballasts.

I wonder if the PAR difference would show up in my camera settings? Say I get 1/60 and f5.6 now, when I go to the new bulbs I wonder if that will change to say 1/125 and f5.6? It would be interesting to see, at least to me anyway, if PAR has any influence to the camera's "eye".

By the way I have not settled on the XM, it was just a LARGE difference that I came across right away. I will do more comparisons and factor in the cost too before I make my purchase. I wish Sanjay's AWESOME charts were available a couple years ago when I made this purchase!