Film-Tech Forum ARCHIVE: ORC Heat Filter

This is topic ORC Heat Filter in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Barry Floyd (Member # 385) on 01-15-2001, 10:07 AM:

I've got a question...

What exactly is a "heat filter" in the lamphouse, and what function does it serve?

The manual for my 4000 watt ORC lamphouse says it should have one in the lamphouse, but I can't find it. I've looked and explored inside the lamphouse and all I've found is the bulb, the reflector, the douser, and some dead bugs.

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Barry Floyd
Floyd Entertainment Group
Nashville, Tennessee
(Drive-In Theatre - Start-Up)

Posted by Mark Gulbrandsen (Member # 72) on 01-15-2001, 10:48 AM:

Barry,
The heat filter is actually a piece of optical glass that has a special narrow band interference coating on it. This coating reflects infrared and Ultraviolet light back, usually onto a piece of asbestos or other heat absorbing material. This helps the projector to run allot cooler and only visible spectrum light makes it to the film.
These filters are expensive and do fade over time(years) and have to be ocassionally replaced. When you install one, and you should, be sure to put the coated surface towards the lamp. If you install it backwards then infrared and UV light makes it through the glass, hits the coating and bounces back through it again. This could result in excessive heating and the filter cracking and you loosing a 200.00+ part
Mark @ GTS

Posted by John Pytlak (Member # 331) on 01-15-2001, 11:24 AM:

A "heat filter" removes the infrared energy from the light beam illuminating the film in the projector aperture. Xenon and carbon arc lamps produce significant radiant energy, ranging from ultraviolet (UV below 400 nanometres wavelength), to visible (between 400 and 700 nanometres) to infrared (IR above 700 nanometres). Since only visible energy can be seen on the screen, good projection design eliminates the UV and IR portions of the spectra to minimize any film heating and risk of film damage/fading.

Smaller projectors (e.g., slide projectors and 16mm projectors) often used a "heat glass" like a Pittsburgh 2043 filter to absorb unwanted infrared. But all the excess energy is absorbed by the filter, so high power levels could crack or shatter the filter. The spectral "cut" of these filters was not precise, so they often removed some visible red light, making the light on the screen appear more cyan.

Theatre projector lamphouses usually use dichroic heat filters that are made by depositing very thin layers of metal (often nickel) and dielectric on a substrate, such that light passing through the coatings is separated into various wavelengths by constructive and destructive interference. For example, a metal reflector can have a dichroic coating that reflects visible light, and absorbs infrared energy. Or a piece of quartz or heat resistant glass can have a dichroic coating that transmits visible light, while reflecting IR to a heat sink.

Sometimes, multiple "heat filters" are used --- e.g., a dichroic coating on the main reflector that reflects visible light, along with a separate filter in the light beam that transmits visible, and reflects any remaining IR to a heat sink. Whatever design is used, much heat energy is removed from the light beam, so it needs to be properly managed with "heat sinks" and ventilation.

Dichroic coatings are fragile, and can be damaged by improper cleaning, reducing their efficiency. Excessive heating (i.e., poor ventilation) can also damage the coatings.
http://automatedhd.com/dichroic/coatings.htm
http://www.dichroic.com.tw/02.htm
http://www.dichroic.com.tw/03.htm

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by John Eickhof (Member # 323) on 01-15-2001, 01:04 PM:

Hi Barry, You should have an angled bracket located in front of the hand dowser (between the dowser and the front bulkhead of the lamp) It is there where a two piece IR filter is located, if the brackets are there and the glass filters are not, you need to order
2 each, ORC #1130730 'IR filter'
Otherwise, misaligment of lamp or even high current can or may be damaging the film. If your regular supply house can not furnish this part, please contact me for a quote, these are still available from Strong / ORC.

Posted by Harry Robinson (Member # 265) on 01-15-2001, 01:47 PM:

John,
My H-10 Christie came without a heat filter. Opinions seem to vary whether I need one in this unit because of it's realatively low wattage. Frankly, the cost is pretty high on these things (somewhere around 350 bucks?), so I wonder if I can get by without one?

Posted by John Pytlak (Member # 331) on 01-15-2001, 02:48 PM:

Certainly anything larger than 2000 watts should use efficient heat filtration. But even a smaller lamp can cause heat damage if improperly aligned and focused.

It's always best to run a test. Using a loop of a dark-scene trailer, you should be able to run several hundred passes without visible heat damage or film deformation. Of course, a black-and-white (silver) image is most sensitive to heat damage, as it absorbs both visible and infrared energy.

I highly recommend reading the Kodak paper "Projection Performance of Theatrical Motion-Picture Films Using Xenon Short-Arc Lamps" by Paul Preo, published in the September 1983 SMPTE Journal.

Posted by John Eickhof (Member # 323) on 01-15-2001, 09:58 PM:

Hi Harry! The H-10 has an inherent low light output anyway (as did the H-20s) I don't think using a 1kw bulb that the lack of a heat filter will be any problem. However, the
H-10 does require a 'negative lens' or 'beam spreader' located in the snood to eleiminate the hot spot of light in the center of the aperture due to the design of the early Christie reflector! (this is the same case for the H-20) Thus Christie made two negative lenses, one that was uncoated (for the H-10)
and one that was dichroic coated (optional for the H-10 but stock on the H-20) I have several of these lenses in stock as used pull outs, (from reflector upgrades) Email me! I'll make you a 'deal you can't refuse' It is easy to install, it just fits into a built in recess in the snood in front of the Hand dowser and is either held in by a spring clip, or two small flat pieces and screws.
You will find that you will get e more evenly distributed light through both scope & flat with the lens installed.

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John Eickhof President, Chief Slave
Northwest Theatre Equipment Co., Inc.
P.O.Box 258
Wendell, ID. 83355-0258
208-536-5489
email: jeickhof@nteequip.com

Posted by Gordon McLeod (Member # 33) on 01-15-2001, 10:41 PM:

The negative lens in the lamphouse is to enlarge the spot of light as the reflector was originally designed for 16mm