The subject of light control and the various methods of controlling it have been discussed a number of times on museum-L, so you can find more detailed information in the archives.  Following are the basic concepts.

1) UV light is the highest energy, creates the fastest damage, and can not be seen by the human eye.  Therefore, all of it should be filtered and removed by whatever light control system is selected.  Two different general types of UV filtration exist.  The first uses an absorber that intercepts the UV light, absorbs its energy, then is broken down by that energy and is destroyed.  As time goes on, it loses its effectiveness.  The second interacts with the UV light, absorbs the energy, then give it off, usually as heat, without being broken down.  This is the type of absorber you want.  You will have to ask the manufacturer of the product for this information, and request a UV absorption curve (or spectral curve including UV) after aging to verify it is stable with time.

2) Once the UV is removed, visible and IR (infra-red) light are left.  IR is heat and also can not be seen by the human eye.  The choice of the removing heat depends upon the location of the museum and the specific needs.  During the heating season, which can be much of the year in northern climates, the heat gain reduces the amount of fuel that must be consumed to heat the building.  But it also raises the temperature during the summer.  

3) The visible light is what allows us to see objects and colors.  It also causes light damage to collections.  The only way to lower the amount of visible light is to reduce its intensity.  Reduction of intensity means less light, of course.  There is no way around this.  Sunglasses don't work unless they are dark - otherwise they are just glasses.  In most head-to-head tests that I have been involved with, museums choose bronze tint over gray tint at the same level of transmission.  It appears brighter.

4)  A couple of other considerations.  Rigid panels such as plexiglass also help to stabilize the temperature and slow down environmental changes.  They function as storm windows.  Film does not.  Rigid panels can not be raised on a very dark day, while film can if it is mounted on a pull-down roller.  However, to work effectively in this manner, some other type of UV absorber must be on the window, or full UV exposure will occur when the film is raised.  An excellent system would be to install clear, 100% UV filtering film or plexiglass at the window, then mount visible light filtering (tinted, 10% transmission) film on a roller to the interior.  The roller film should stay down as much of the time as possible, but can be raised on unusually dark days, making sure to pull it back down when the museum closes for the day.

5) There are many considerations and alternatives for light control.  The best strategy is to have a conservator conduct a light control study, so that sample control methods can be custom-selected and tried in a real world environment prior to making a final decision.  Too many rash decisions have been made in the past, with the museum coming to regret their choice later.

I hope this helps!

Marc

President, American Conservation Consortium, Ltd.
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