what do you mean by coolable hood? A hood with a hole in it for ventilation? NEW, I'm going to guess no, not for either. Try online hydroponic sites, or try ebay, I've seen quite a few light there. Search hydroponic lighting.

The coolable hoods are for rubes who don't know any better.
The glass shield filters out most of the blue light,
which is the highest energy spectrum and most important for many functions.

Not all 400 watt lights are equivalent. Generally, the best light is the 400w ceramic metal halide (cmh). Horizontal is usually the best way to get the light directly from the bulb to the leaf surface. A 400w cmh is equivalent to a 600w hps.
The cmh is full spectrum, with more red than an hps and more blue than a quartz metal halide (qmh); so it is the best all-around light bulb.

Getting the best bulb is more important than the electronic ballast feature. Ideally, go for both, but only Life Lights makes an electronic ballast for the 400w cmh; it can also run the psmh and is adjustable down to 250watts for summer time.

To lower temperature, use less watts, improve ventilation, and/or spin the lights. 76F is the best temperature when the lights are on. You can go 10-20 degrees lower when the lights are off.

Here is a good source:
www.hydro-techn.com

My advice is not to listen to people who are trying to sell you stuff but clearly don't know what they are talking about. The glass shield (sometimes called a lens although it is not a lens and it is not usually glass, but actually a type of flat shatterproof plastic) filters out none of the blue light. If it did, it would be coloured but in fact it is transparent and colourless. It does two things. Most importantly, it prevents failure of the bulb from emitting hot pieces of glass and the arc from emitting dangerous levels of UV. During normal operation, the shield does block a considerable amount of near UV and essentially all the far UV. You may or may not consider this a good thing. Many modern metal halide bulbs are O-rated, which means they have design features that make them (relatively) safe to use without a separate shield on the lamp fixture, although remember that the bulb will operate at several hundred degrees and will set fire to most things that touch it accidentally. E-rated and S-rated bulbs should not be operated in an open fixture, although S-rated bulbs can be used in a vertical position subject to certain additional requirements. It is now illegal (in most states) to sell a metal halide fixture that can operate E-rated or S-rated bulbs without a shield, although what you do in the privacy of your home is your own fire risk ;)

I refer "Shrubs n bulbs" to "Light Measurement Handbook" copyright 1998 by International Light, page 14, Fig. 3.3
You will see there an illustration showing the blue spectrum being filtered out by the glass. The shorter the wavelength, the higher the photonic energy and the more it tends to be filtered out. UV light has the shortest wavelength, and is most filtered out.

Of course, the known laws of physics prohibit a 100% transparent glass. The best pyrex borosilicate glass will filter out 6-8% of the light.

A chart at the Barloworld-scientific website shows how light transmission is associated with wavelength (spectrum).

Here is a document from Barloworld if anyone wants to look up those transmission spectrums. Here is a link which may be easier to use, it shows a borosilicate glass, you can navigate to other types of glass on the left. The transmission is flat to about 350nm, well beyond what any human can see. The 6%-8% loss you quote is almost all by reflection and does not depend on wavelength, the actual absorption within a sheet of glass is a fraction of a percent. Some glass, most notably the cheap soda glass, is slightly green because it absorbs fractionally more blue and red wavelengths. The effect is so tiny that you need to look through many inches thickness before the colour is apparent. You should not worry about a tenth of a percent loss of blue (and red) light when any glass shield is reflecting 8% of light across the whole spectrum. The question of what effects the UV itself might have on plants is an interesting one and is of great interest to any greenhouse grower, although obviously plants do grow rather well under glass without UV.

All this is of course irrelevant, since metal halide shields are made from polycarbonate or acrylic, at least all UL approved ones for high power bulbs are. You'll have to trust me that the transmission and reflection curves are quite similar. Both plastics transmit quite a lot of UV but are almost always treated with UV coatings to prevent them yellowing. Acrylic also transmits light significantly better than either glass or polycarbonate and even very thick blocks of acrylic show almost no loss or colouring.

dont need to bother with this thread picked up a t5ho 8 bulb fixture with bulbs for 220 dollars.

Shrubs has some interesting comments. I'm not sure how relevant the Pyrex borosilicate would be, since few if any glass shields for vented reflectors are of that type. I just looked at my glass shield, and it sure didn't say "Pyrex", looked like ordinary glass. The link showed other types of glass, probably better than average, but still, some had considerable drop-off just past 400nm. Even the borosilicate had considerable drop-off past 350nm.

The plant can utilize more than the human eye can see. That's why lumens isn't a good measurement for plant growth.
The UV-A and UV-B bands perform many functions relating to phenolic compounds and potency of, e.g., licorice. There is also the Violet band, which is part of the full spectrum. So the ideal spectrum would cover everything from 280 to 700 nanometers, with a slight bump in the red at 670nm.

I didn't see any reference indicating the 8% transmission loss was due to refraction. Please provide a quote with link.

The Light Measurement Handbook I cited isn't available digitally online. But here is a quote from page 14:
Transmission: Beer-Lambert or Bouger's Law
Absorption by a glass filter varies with wavelength
and filter thickness. Bouger's law states the
logarithmic relationship between internal transmission
at a given wavelength and thickness.
...
(Lermer) Perhaps the Pyrex glass was specially treated to compensate for Bouger's law, for the purpose of color rendition in photographic work.

Oh dear, where to start. You clearly didn't read my message. I did not mention refraction at any point. Or perhaps you just don't know the difference between reflection and refraction. I did write you a paragraph about the materials that are used (in UL approved fittings) for metal halide lamp shields, but you are still rattling on about glass.

And what is all this stuff about Bouguer's law (note the correct spelling)? You clearly have no clue what it means. It has nothing to do with the wavelength of the light, nor is it something you can "compensate" for. It is simply a mathematical expression of the fact that half of light is absorbed when passing through a given distance in optically homogeneous media, leading to an exponential relationship between the thickness of the media and the amount of light absorbed. However, as I already mentioned, the amount of light absorbed (as opposed to being reflected at the two surfaces) in any thin sheet of glass (or polycarbonate or acrylic) is so utterly negligible that you should be more worried about light lost passing through the air between the light and the plants.

I really do not feel like responding to someone who keeps on trying to sell something in this forum.

BTW, why is this person not banned? He is clearly ignoring the rules here.

dcarch

Contact forum support if you feel someone is flouting the rules. I used to do it all the time when Spike ran GardenWeb and they would be gone within 24 hours. I don't bother now that iVillage owns the place, I feel they are less responsive, so I just poke fun at the poor spammers :)

Shrubs said:
"I did not mention refraction at any point. Or perhaps you just don't know the difference between reflection and refraction."

(Lermer) I had assumed your were talking about refraction and not reflection, so out of politeness I thought to correct you. Definition of "refraction":

http://squ1.org/wiki/Light_Behaviour
"Refraction is an effect that occurs between transparent materials of different densities, such as air and glass. The bending evident in refraction is a physical representation of the longer time it takes light to move through the denser of the two materials, and it is dependent on the angle that the light strikes the boundary."

(Lermer) Light passing through glass would be refracted, as it's angle is changed. This is the first I've heard about glass reflecting light. Please show a reference for the alleged reflective properties of glass.

In any event, I thought we were talking about how much light (and what spectrum) makes it through the glass, not how much is reflected from the glass to the aluminum above the bulb.

I ask again for a reference showing that the light losses claimed by glass manufacturers are not true losses, because of alleged reflection or refraction.

(Shrubs) I did write you a paragraph about the materials that are used (in UL approved fittings) for metal halide lamp shields, but you are still rattling on about glass.

(Lermer) I just called Sunlight Supply, the biggest manufacturer of vented hoods w/glass. UL does not require any special type of glass, other than that it be tempered. Even with the glass types you mentioned, there was radical drop off in light transmission past 400nm with some, 350 nm with Borosilicate. As I have pointed out, the Violet, UV-A and UV-B spectrums are important for plant growth. You didn't respond to those points, except to "poke fun".

(Shrubs) And what is all this stuff about Bouguer's law (note the correct spelling)? You clearly have no clue what it means. It has nothing to do with the wavelength of the light,...

(Lermer)Perhaps we are talking about different laws, but googling "Bouger's law" I get lots of hits in the scientific literature. Including this one (ibid.):

"Absorption by a transparent or translucent filter varies with wavelength and filter thickness. Bouger's law states the logarithmic relationship between internal transmission at a given wavelength and thickness."

Please show a reference for the alleged reflective properties of glass.

You cannot be serious! Which planet did you just arrive from? I give up. You can look up your own reference for this most utterly basic physical property, or maybe just stare at your reflection in a window when it is dark outside. Or perhaps just continue to live in your own private fantasy world, your choice ;)