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May 2012

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There is no reason to be intimidated by dichroic filters.

by Michael S. Eddy

Dichroic Color Filters
If you have a favorite front light color or a wash on a wall or cyc that you use all the time, dichroic color filters may be the way to go. They will last a lot longer than plastic color filters, thus reducing maintenance issues. In addition, dichroic filters are more efficient than plastic color filters, which means that you get more light output from your light. By using optical thin films, dichroic filters pass the wanted amount of color—up to 95%—and reflect the unwanted color spectrum and heat energy. A typical plastic color filter transmits significantly less, with some of the darker filters transmitting only about 5% of the light. The rest of the spectral energy is absorbed into the filter. This is what causes plastic filters to fade, shrink or burn through.

There is no reason to be intimidated by dichroic filters, and with proper handling, they are essentially no maintenance, unless you drop them, because then you will need to clean up the shards. One important tip is to always install dichroic filters with the coated side of the filter towards the lamp. To determine the coated side of the filter, touch the point of a pen or pencil to the glass. If you are touching the coated side, the tip will appear to touch its reflection in the glass. On the un-coated side, the reflection appears with a small gap between the points.

The cost is often considered high, and dichroics are certainly more expensive than plastic filters, but if it is a fairly permanent color, figure your cost of new plastic filters, the time for the changes and the labor to access some of those hard-to-reach locations when looking at affordability. Go ahead and speak with the manufacturers or your theatrical dealer about getting a sample kit to see if this is an option for you.

I asked Barco/High End Systems’ Keith Cannon, optical lab manager, and Brandon Harding, dichroic glass sales manager, to answer some commonly asked questions about dichroics.

How do you clean them?
“Typically you can clean them with alcohol or even a mild detergent with water and then dry it,” says Cannon. “Use alcohol on a cloth and wipe it in one direction slowly so it dries as you go, this won’t leave trails on the glass. You don’t want to clean it in a swirling motion, just straight across from one edge to the other.”

Do the coatings degrade over time?
“No,” answers Cannon. “We have filters that are more than 20 years old, and they look the same as ones that we coated yesterday.”

Is there a limit to the colors that you can get?
“There are no limits to the colors you can produce with dichroics, even custom colors are possible,” says Harding. “It is just a matter of designing the color with the thin films,” adds Cannon. “They don’t always respond like you would want to; it isn’t like mixing paint. It is a little trickier than that to get just the right mix for that color. I do gel matching all the time, and while you might not get the exact match, but you can get very close. It is due to the different performance of the two mediums.”

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Michael S. Eddy writes about design and technology. He can be reached at mseddy2900@hotmail.com.   .(JavaScript must be enabled to view this email address)