Cees your the local lighting expert can we get your opinion on CHM.
First, I'm going to be honest and say that I haven't done extensive research on CMH, and point number one below is why. Since your post, I've looked a bit more.
1. Lumen Output
The
Philips Site lists the lumen output of a 400W CMH at 39,200 lumens
I have seen various quotes for 400W HPS, but your garden variety is rated at about 50,000 lumens.
Therefore, not even considering spectral differences, you're talking about 98 lumens/W vs. 138 for HPS. That's roughly 25% better for HPS, and you can get better HPS bulbs.
2. Spectral Quality
It is undebatable that bulbs putting out primarily red-spectrum light produce great bud.
I don't think that anybody would mind adding blue spectrum on top of that, and I don't think you could argue that it would hurt - unless you were toasting the plants. This is not likely to result from the blue, however, as it is the infrared at the tail end of the red spectrum that is the culprit for the "hotness" of HPS bulbs.
The problem is that it's not as if you're simply "adding" blue spectrum. If you have one bulb that's putting out 55k lumen of red/yellow, as HPS does, and another putting out 55k of combined blue/red, the bulb with the more complete spectrum has less red spectrum.
Lumen rating is based on the energy output (number of photons), taking into account the
luminosity function . If you go back to the
CMH thread and look at the two spectra, the lumen rating is more or less proportional to the area under the spectrum, but weighted for the sensitivity of our eyes. (If it were not weighted for sensitivity, the output - called radiant flux - would be directly proportional to this area).
If you take a look at that luminosity function plot, you'll also see that a larger portion of the CMH spectrum falls where our eyes are most sensitive. The luminosity function therefore has a higher value, and these wavelengths "count more" toward the lumen rating.
What this means is that by adding blue and staying at the same lumen output, you are sacrificing red, effectively putting yourself somewhere between a MH and a HPS. This is no surprise, as this is essentially what a CMH is. In addition, you're losing overall output due to the overlap of the CMH spectrum with our vision's sensitivity. Make sense?
3. Heat Output
I have not dug through the thread to determine if there are any actual data on heat output, as the two points above make them uninteresting to me.
As I have mentioned before, it's better to just get the right bulb and ventilate properly. If you are in a tiny space and they really provide that much heat savings, then perhaps they could fill a niche there. If someone can point me toward some of this data, perhaps I could offer an opinion on that.
4. Summary
Why get one bulb that does half the job of the correct ones? If you've put a couple hundred bucks into a decent 400W HPS, cough up the $50 for a conversion bulb and do 'er right!