Filters to Pass Visible Light and block IR
and ones to Pass IR and Block Visible Light


Blocking Visible Light and Passing Infrared

This is easier than the other way around!

Here are several ways to do this.

1) Edmund Scientific in Barrington, NJ, USA (1-609-573-6250) sells a variety of filters.

Some of these are longpass filters made of Schott glass. These are high quality filters that are better than necessary for most purposes. It is advisable to choose a very long cutoff wavelength, preferably near or over 800 nanometers, since wavelengths in the 700's of nanometers are slightly visible. Wavelengths in the low-mid 800's are even very slightly visible.

Edmund also sells more general purpose visible-blocking IR filters.
So do some surplus dealers that advertise in magazines, including Popular Electronics and Electronics Now, and probably also some photography magazines.

2. Use a combination of deep red, deep green, and deep blue stage lighting filter gels. Adding deep blue-green to this and/or using two layers of each color makes the filter darker, but less efficient. Stage lighting filter gels of nearly all brands, colors, etc. pass IR.

Workable Stage Lighting Filter Gel Combinations

                                                     Approx. Wavelength nM
Combination                                          for 50 %    for 90 %
                                                     blocking    blocking
---------------------------------------------------------------------------
Edmund Scientific:
#823 (med.red) +#874 (med. grn) +#877 (med. blu. grn.)   740        765

Roscolux:
Two #19 ("fire") +#83 (med. blue) +90 (dk. yel. grn.)    720        740

GamColor:
Two #250 (med. red) +#690 (bluegrass) +#850 (pri. blue)  710        730
----------------------------------------------------------------------------
Combining deep red, deep green, and deep blue Plexiglass also worked the last time I tried this.

3. Wratten filters of numbers 87, 87C, and 88A block visible light and pass IR. The 87C is the most visible-blocking of these, with almost complete blocking of wavelengths below 800 nM, and some significant absorption even into the mid 800's.

Wratten filters are available at some photographic supply stores and some scientific supply stores.

4. Someone out there reminded me of the color film trick, and gave me some specifics. Developing unexposed color slide film produces a filter that passes infrared and blocks visible. He mentions Ectachrome E6 film and claims optical density figures of 3 for visible (99.9 percent absorption) and .3 for 780 nM (50 percent absorption).

More IR and IR filter info at other sites:

IR and red filter info at Willem-Jan Markerink's photo site.

The Infra-Red FAQ v1.10 (96/10/19)

Reflecting Visible, Passing IR

To do this, use "cold mirrors". Edmund Scientific sells these.
Please note that these do not block visible as well as visible-absorbing filters.

Reflecting IR, Passing Visible

This is usually done with "hot mirrors". These are also available at Edmund Scientific.

The glass used for the outer bulbs of low pressure sodium lamps is also IR-reflecting. It is coated with a special grade and thickness of either tin oxide or indium oxide in order to do this.
If you decide to break or disassemble a low pressure sodium lamp, please beware that the bulb contains a vacuum and could violently implode if broken. The inner arc tube contains a near-vacuum and can also implode if broken. The inner arc tube may also contain substantial amounts of sodium, which is hazardous. Sodium quickly forms sodium hydroxide upon exposure to moisture or humidity. If exposed to very dry air, sodium oxide forms instead, which becomes sodium hydroxide when exposed to moisture or humidity. Sodium hydroxide is corrosive and hazardous to most living tissue.
Sodium reacts very vigorously with water, forming both sodium hydroxide and hydrogen gas. Violent spontaneous combustion has been known to occur from this.
Break sodium lamps with appropriate precautions and only at your own risk.

Absorbing IR, Passing Visible

This one is more difficult, since sharp short-pass filter dyes seem to be hard to find in any way.

If some significant absorbtion at visible wavelengths is tolerable, and degree of absorption of different visible wavelengths is tolerable, then there may be a solution.

One option is just that, solutions of copper compounds in water largely absorb most infrared. Copper compounds used must be of the more common "cupric" or "copper II" valence, not "cuprous" or "copper I". These have a very gradual cutoff, and absorb visible red light greatly, and also yellow and orange significantly, and these absorb yellow-green a bit also.

The filters used in acetylene welding goggles absorb IR more than visible, especially if they are glass rather than plastic. Green glass sunglasses are a milder version of this, and offer some infrared absorbtion.

Someone posting to Usenet in May 1996 suggested Schott KG-1 glass and Hoya HA-30 glass filters. He believed these might be in use in some video cameras to attenuate IR and/or make the spectral response of silicon photodetectors more like that of the human eye. Please beware that such glass filters probably do not completely block IR and probably also partially block red visible light.

To pass a specific band of wavelengths and greatly block (mostly reflect) just about everything else, use dielectric interference bandpass filters. They are very effective.
However, they are usually quite expensive, and the passband shifts to shorter wavelengths and often gets narrower and often gets less transparant for light rays hitting the filter in a non-perpendicular manner.
These filters are available from Edmund Scientific.
A few of these are sometimes available from a few surplus dealers. Check the ads in Popular Electronics, Electronics Now, and other magazines related to fields in which such filters might be useful.

Written by Don Klipstein.

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