Does glass block light?

thelastpirate said:

Clear glass in and of itself is "neutral density". It blocks light equally across the spectrum. Depending on the quality of glass, this light blockage can be as much as 30%. Auto glass (your car windows) tests at 70% visible light transmission, meaning it blocks 30% of the light passing thru it. I would assume that a "protective" piece of glass for an HID fixture will be tempered, and right at that 70% visible light transmission. The benefit of being able to get the lamp closer to the plants will MORE than offset any loss of light.

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This may be true of visible light, but it is not true of UV light.

I agree about it being compensated for by heat, but you have to realize that the heat is not just from the heat of the bulb and air. The plants are heated by light and IR that goes through the glass.

Wind through a cool tube does not interrupt electromagnetic radiation, which is what heat from a bulb's glass is. It may cool the glass and therefore reduce its IR emission, and remove the air heated by conduction, but that is all.

thelastpirate said:

Clear glass in and of itself is "neutral density". It blocks light equally across the spectrum. Depending on the quality of glass, this light blockage can be as much as 30%. Auto glass (your car windows) tests at 70% visible light transmission, meaning it blocks 30% of the light passing thru it. I would assume that a "protective" piece of glass for an HID fixture will be tempered, and right at that 70% visible light transmission. The benefit of being able to get the lamp closer to the plants will MORE than offset any loss of light.

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I ain't buying a 30% loss. I used a spectrophotometer daily for a year and even with a *HEAVY* coating of ITO (a thin film to make the surface electrically conductive) it rarely dropped below 82% transmission.
Plain glass was at 95% transmission.

We also used other chems for coating but the ITO was the 'darkest'

HydroChron said:

I am using a air cooled hood that has safety glass. I have read that the glass diminishes the light and takes away UVA. I am wondering if anyone thinks that it is a really bad idea to use glass and if i would be better off taking the glass out or if it is not that big of a difference. I have heard both sides but one thing i am noticing now is that the nicer grows that i see never have glass between there lights and plants. I would love to settle this once and for all so PLEASE put in some input and let me know what the true pros out there think!! thanks for your time!

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Glass does not block light noy unless if it is smoked or timted. Glass does have a bad refractive index so mirrors next to plants can cause damage because of how it bends the light and reflects it back.

kirby said:

A lot of people will take the glass off their lights when they receive it.. the main reason for that glass is to block harmful UV rays

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@#$%@#$^@#^Y WHAT!?>!!> THATS NOT TRUE

Ok first, the main reason for the glass is to seal the lamp and isolate its air from the grow room air. This allows for a separate circuit exhaust system which in addition to cooling all your lights without heating your space also keeps that same air odor free.

Does glass block a LITTLE light? Yes, but so does humidity, were talking little percentages here and the amount of light and UVB blocked by tempered glass is insignificant.

Even more convincing is the fact that I can keep an air cooled 1000 within 12" of the plant tops, if it was uncooled I would have to raise it at least another 20in and it would heat my room. This is not only a temperature issue, but also a lumen intensity issue, light is many times brighter at 12in then 36in and I would bet that this extra intensity would be better during flowering then the uvb rays that are blocked by the clear glass.

There is no question that there are advantages to air cooling a light, that does not mean you cannot grow open lamp but its awfully 20th century.

yea im pretty sure it is all about space and ventalation. If you have the space and the ventalation to move enough air to keep those bulbs cool then do it that way. But if your having heat issues then the glass is the way to go. there is no way that the amount taken away by the glass can be more the amount gained by getting the bulb much closer

good post speed

Does anyone know whether or not Plastic will block the same UV light that glass will? And also does anyone know which brand or aircooled hoods is the best one. HTG or Hydrofarm etc......etc.....?

HydroChron said:

Does anyone know whether or not Plastic will block the same UV light that glass will? And also does anyone know which brand or aircooled hoods is the best one. HTG or Hydrofarm etc......etc.....?

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Plastic will most likely block IR too (can cut t with an IR laser, so it absorbs That spectrum) But I'd most concerned with it melting, or outgasing as it gets warm. Remember those tents that poisoned a bunch of plants?

As for visible, eyeglasses are made from CR39, polycarb normally.

UV? I dunno. toss a pair of self tinting glasses under the bulb and plastic in question. If it tints, then its not blocking.

what is with the tents?

HydroChron said:

what is with the tents?

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There was a series of grow tents that out gassed and poisoned plants.

did they find out why? im assuming the plastics that were used

There is a plastic on the market that blocks LESS light than the purest glass. It's expensive though, and I can't recall the name of it. It's available in tube form and somewhat heat resistant and someone on here or ICMAG made cooltubes with it successfully.

As others have said yes glass blocks a little but not in any way significant enough to offset the gains given when you can put your lights closer. The gains in getting your lights closer with a vented hood are massive.

Glass on industrial fixtures is there for safety AND they generally have a frosted finish which IS specifially intended to block harmful UV light. Don't make so much fun of that one guy, he was partly right. Glass on agricultural lights isn't there for that purpose though, so that's why I said partly.

Bulbs that are coated in phosphor are also done to block UV, as well as to change the spectrum.

HydroChron said:

did they find out why? im assuming the plastics that were used

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yes the plastic from china used in hydro huts and habitats. some sunhut were affected as well!