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I am looking for a material that can be used in a non commercial environment that will allow all (or nearly all) light to penetrate but will be heat resistant (By heat resistant I mean it will not allow the heat to penetrate to the other side, not that the materiel itself won't get hot - although they may be dependent on each other).

To better understand my requirements, I am trying to illuminate a room with 40,000 lux of light using a few metal halide (MH) bulbs, for a scientific experiment. The bulbs will consume 1800 - 2400 watt which means it will produce a lot of heat. Since real people will partake in the experiment (sitting about 6-12 inches from the light source in order to increase the amount of lux reaching the subject) I am concerned the heat produced by the bulbs will be too difficult to bear. Therefore, I want to place some material (such as a glass or plastic) in between the subject and the light source so maximum light penetrates with minimum heat.

What is the best way to do this?

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    $\begingroup$ P.S. It's going to be a trade-off between weight, light transmission, cost, thermal conductivity. Infrared will be the problem. Your best bet will be looking for so called filter "gels" as termed in the lighting industry, particularly stage lighting. They tend to be developed for precisely your requirements; to absorb infra-red and dissipate it as heat, while leaving the colour spectrum intact. They are basically plastic sheets with tuned optic/thermal properties. IR heat shields I believe are what you might want. LEE filters are the renowned manufacturer. Talk to them. $\endgroup$
    – Jodes
    Apr 15, 2015 at 7:35
  • $\begingroup$ A two-layer glass construction with air in the middle would minimise convective/conductive heat transfer. For minimising infra-red heat transmission, see Jodes' comment above. $\endgroup$
    – Li-aung Yip
    Apr 15, 2015 at 8:10
  • $\begingroup$ even better if there is no air, but vacuum $\endgroup$
    – Pedro Quadros
    Apr 15, 2015 at 10:25
  • $\begingroup$ Why not start with LED or flourescent lights? Those will generate a lot less IR for the amount of visible light. $\endgroup$
    – Olin Lathrop
    Apr 15, 2015 at 13:56

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Illuminate the room indirectly via dichroic reflectors at 45 degrees to each transparent wall. These will reflect light at 90 degrees (i.e. into the room) but allow the heat to pass straight through.

(NB you may want watercooled heatsinks behind the reflectors, or some other adequate arrangement to dispose of the heat.)

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  • $\begingroup$ Brilliant solution :) $\endgroup$
    – MeEngineerTrustMe
    Aug 8, 2020 at 4:51
  • $\begingroup$ @MeEngineerTrustMe ... Not an original idea though, I believe some photographic enlargers did this to avoid heating the film. $\endgroup$
    – user_1818839
    Aug 8, 2020 at 14:16
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Infra-red will be the problem.

It's going to be a trade-off between weight, light transmission, cost, thermal conductivity.

Your best bet will be looking for so called filter "gels" as termed in the lighting industry, particularly stage lighting. They tend to be developed for precisely your requirements; to absorb infra-red and dissipate it as heat, while leaving the colour spectrum intact. They are basically plastic sheets with tuned optic/thermal properties. IR heat shields I believe are what you might want. LEE filters are the renowned manufacturer. Talk to them

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To start, ordinary window glass will probably do just fine in this situation, but there is a small chance that local heating will cause it to crack. I'd probably test that first before moving on to anything more exotic, which will be harder to find/replace and cost more.

In my lab we primarily use Pyrex (Borosilicate glass) glass as window material to let in large amounts of light while blocking air flow. It handles heat quite well and is transparent from the visible spectrum and into the short IR. I've also had success recently with Gorilla glass, which is more expensive, but comes in very thin sheets.

Fuzed quartz will also handle the heat easily, but is will cost more than the Pyrex.

I'd caution against plastics, many of them absorb strongly in the IR (which your lamps are producing in addition to visible light). In our work we use strong lamps to radiantly heat plastics to the point of melting and ignition. One or two 500 W lamps a few inches away will get the plastic melting and bubbling within minutes. The Pyrex in between, which is nearly touching the lamps, barely notices the heat even once the plastic catches fire.

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  • $\begingroup$ Thanks. You said it is transparent into the IR spectrum. Does this mean it allows heat through? As others said the main source of heat is IR light.. Also, do the house dishes produced by Pyrex have the same properties? $\endgroup$
    – Michael M
    Apr 16, 2015 at 2:13
  • $\begingroup$ Sorry, I wasn't very clear there. Borosilicate only absorbs some of the IR spectrum, not all, but it lets basically everything visible through. A window will shield against air heated by the lamps and cut out some of the IR, depending on material. Also, be aware that all of that visible light will carry energy with it will transfer heat to the subject, there's no avoiding that. $\endgroup$
    – Dan
    Apr 16, 2015 at 3:06
  • $\begingroup$ I forgot to mention: home cookware is soda lime glass. A bit less sturdy than "true" pyrex, but has nearly identical IR properties. $\endgroup$
    – Dan
    Apr 16, 2015 at 3:14
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I would suggest either polycarbonate or perspex/plexiglas/acryclic. They both have good thermal properties, with a thermal conductivity of the order of 0.2 W/m/K, and are good substitutes for glass in terms of optical properties. The thicker the sheet, the better the thermal insulation, but depending on the size, you may need to provide some reinforcement so that the plastic sheet doesn't bend under its own weight.

Depending on where you are, there are various people who sell this type of engineering materials. I tend to use Engineering Design Plastics, because they're local (for me), but there are others.

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You might want to use (if you can afford it) a material known as aerogel. It is known to be one of the best insulators known . The material has some drawbacks though, it is very brittle so you would have to build a support matrix to use it for your application and the material has a slight bluish tinge to it, don't know if that would be a problem for you.

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