Infrared (IR) or heat-absorbing filters are designed to block or reflect mid-infrared wavelengths but pass visible light. They are often used in devices with bright incandescent light bulbs (such as slide and overhead projectors) to prevent unwanted heating. There are also filters which are used in solid state video cameras to block IR due to the high sensitivity of many camera sensors to near-infrared light.
Invisible infrared radiation
The spectral range that is visible to the human eye ends at a wavelength of approximately 750 nm. This is where infrared radiation begins (only certain birds can see portions of infrared radiation that better penetrates haze). Most films parallel the spectral response of our eyes. But there are special infrared-sensitive films for color- and black-and-white photographs which, depending on their sensitization, react to 850 nm, 900 nm or nearly ˇ000 nm radiation. Like other films, these are also very sensitive to visible light. If we wish to image only in the infrared, filters must be used to suppress the visible, or to attenuate it strongly, so that the (weak) image produced by the infrared radiation will be sufficiently prominent.
Infrared photographs are attractive in many ways: Because of the nearly white reproduction of the chlorophyll green of vegetation, infrared black-and-white photographs render landscapes as if they were self illuminating, or immersed in an extraterrestrial light (the moonlight effect). Infrared color photographs have a fairy-tale effect because colors are reversed. The film renders highly infrared-reflecting plants in orange to purple-red tones, while filters suppress the blue and green components that are also present. In any case, the pictorial results are difficult to predict, therefore they are always good for experimentation and surprises.
Because there are no exposure meters that are sensitive exclusively to the sensitivity range of these infrared films, it is difficult to calculate exact exposures and conversions by means of filter factors. This is due to the fact that two subjects that are equally bright in normal (visible) light might reflect infrared radiation at significantly different rates. In other words, they can have very different “brightnesses” as far as the infrared film is concerned. Therefore it is always advisable to make a series of three to five different exposures. Don’t be stingy with film, because once it is out of the refrigerator, it is only good for a few months anyway, so use it up quickly!
| Infrared Filters |
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B+W Infrared Filters |
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Infrared Filter 092 (≈ 89 B) [RG 695]
The nearly opaque B+W Infrared Filter 092, which looks dark purplish red when held in front of a light source, blocks visible light up to 650 nm, and passes only 50% of the radiation just below 700 nm (thus the dark red color). From 730 nm to 2000 nm, transmission is greater than 90%. This makes photographs of pure red and infrared images possible with the best utilization of the relatively low sensitivity of infrared films. As the sensitization of infrared black-and-white films barely extends beyond 1000 nm, the red portion that is transmitted still makes a relevant contribution to the exposure. That is why this filter is the preferred filter for pictorial photography on IR black-and-white film. Its filter factor is 20 to 40.
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Infrared Filter 093 (≈ 87 C) [RG 830]
This B+W Infrared Filter blocks the entire visible spectrum, so to our eyes it looks completely opaque. Unlike the infrared filter described above, it makes pure infrared photographs possible without the visible red component. Its transmission only begins to exceed ˇ% at 800 nm, rising to 88% at 900 nm, and remains that high far beyond the upper limit of sensitization covered by infrared films. This filter is used less frequently in pictorial photography because of the dramatic loss of effective ISO. But in the scientific field, materials research and forensics, the limitation to a strictly infrared range is often important. The filter factor is very dependent on the illumination and on the characteristics of the film.
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B&H |
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Infrared Filter 099 (≈ 16) [OG 550]
The orange-colored B+W Infrared Filter 099 is the ideal
filter for photography with infrared color film, which is
also referred to as “False Color Film” because of its
charming abstract color reproduction. It blocks large
portions of short-wave radiation, up to 520 nm (blue,
blue-green) and reaches its full transmission near 600 nm,
which it retains far beyond the sensitization range of these
films. This avoids the blue cast that is caused by the
heightened sensitivity in this spectral range, and it leads
to a better differentiation of colors. The charm of these
infrared color photographs is partly due to the orange to
red rendition of green vegetation, which is due in turn to
the high infrared reflectivity of the chlorophyll in plants.
The filter factor is highly dependent on the film. Check
availability, sizes & prices at Amazon
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B&H |
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Leica Infrared Filters |
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Leica Infrared (Black). The Leica UVA/Infrared filter is targeted for the M8 series camera. Although not a traditional infrared filter in the classic sense, i.e., helping with special effects images, this filter corrects for the magenta cast occasionally found on digital sensors.
This filter does not block by means of absorption, but rather by interference of the unwanted UV and IR radiation that is repeatedly reflected between these layers, affecting wavelengths on both sides of the visible spectrum with a steep cut-off.
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sizes & prices at
Amazon
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B&H |
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Tiffen Infrared Filters |
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Tiffen #87 Infrared Filter. The Tiffen #87 Infrared Glass Filter is for black and white infrared film when you only want to transmit the infrared spectrum and none of the visible spectrum.
Tiffen, a leader in the filter industry, proudly makes its filters in the USA and backs up its products with a solid 10-year warranty.
Note! Prior testing is recommended
• Requires testing for exact exposure
• For black and white infrared film only. Check availability, sizes & current
prices at Amazon |
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The Tiffen Combination Neutral Density (ND) Infrared (IR) ColorCore Glass Filter -
Color shift from IR contamination is a critical problem in digital videography, whether you are using neutral density filters with HDSLR or HD video cameras. The CCD or CMOS sensor of a digital camera is inherently susceptible to infrared contamination, even when the manufacturer attempts to reduce this problem by adding an infrared absorbing filter on top of the image sensor. Blacks become magenta and greens a muddy brown. Infrared light focuses at a different focal length than visible light; therefore, your images may seem a bit out of focus as well. The more light you cut, the more IR contamination; therefore, whether you're reducing light in general or wish to create a shallow depth of field for a cinematic look, the more IR contamination you will encounter.
Tiffen has addressed this problem by offering their exclusive line of their Full Spectrum Water White IR Neutral Density filters in limited screw in sizes for the HDSLR. The Tiffen IR ND filters do not have a sharp cutoff at the infrared wavelengths; therefore the wide color gamut these sensors record is not compromised. The result is vibrant colors and no IR
contamination, with crisp clean images that only Water White
glass can deliver. These filters also provide a greater
latitude when editing in post production and are ideal for 8
byte compressed color. Check availability, sizes & current
prices at Amazon |
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Infrared Filters at
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B&H Photo Video
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Camera filters are transparent or translucent optical elements that alter the properties of light entering the camera lens for the purpose of improving the image being recorded. Filters can affect contrast, sharpness, highlight flare, color, and light intensity, either individually, or in various combinations. They can also create a variety of "special effects." It is important to recognize that, even though there are many possibly confusing variations and applications, all filters behave a reasonably predictable way when their properties are understood and experienced. Most of these properties related similarly to filter use in both film and video imaging. The following will explain the basic optical characteristics of certain types of camera filters, as well as their applications. It is a foundation upon which to build by experience. Textual data cannot fully inform. There is always something new out there.
In their most successful applications, filter effects blend in with the rest of the image to help get the message across. Use caution when using a filter in a way that draws attention to itself as an effect. Combined with all the other elements of image-making, filters make visual statements, manipulate emotions and thought, and make believable what otherwise would not be. They get the viewer involved.
Filter Planning
Filter effects can become a key part of the "look" of a production, if considered in the planning stages. They can also provide a crucial last-minute fix to unexpected problems, if you have them readily available. Where possible, it is best to run advance tests for pre-conceived situations when time allows.
Filter Factors
Many filter types absorb light that must be compensated for when calculating exposure. These are supplied with either a recommended "filter factor" or a "stop value." Filter factors are multiples of the unfiltered exposure. Stop values are added to the stop to be set without the filter. Multiple filters will add stop values. Since each stop added is a doubling of the exposure, a filter factor of 2 is equal to a one stop increase. Example: three filters of one stop each will need three additional stops, or a filter factor of 2x2x2= 8 times the unfiltered exposure.
When in doubt in the field about compensation needed for a filter that you have no information on, you might use your light meter with the incident bulb removed. If you have a flat diffuser, use it, otherwise just leave the sensor bare. Aim it at an unchanging light source of sufficient intensity. On the ground, face up at a blank sky can be a good field situation. Make a reading without the filter. Watch out for your own shadow. Make a reading with the filter covering the entire sensor. No light should enter from the sides. The difference in the readings is the compensation needed for that filter. You could also use a spot meter, reading the same bright patch, with similar results. There are some exceptions to this depending on the filter color, the meter sensitivity, and the target color, but this is often better than taking a guess.
Filter Grades
Many filter types are available in a range of "grades" of differing strengths. This allows the extent of the effect to be tailored to suit various situations. The grade numbering range can vary with the effect type, and generally, the higher the number, the stronger the effect. Unless otherwise stated, there is no mathematical relationship between the numbers and the strengths. A grade 4 is not twice the strength of a grade 2. A grade 1 plus a grade 4 doesn't add up to a grade 5.
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