The wavelengths of electromagnetic radiation known as light are measured in nanometers or billionths of a meter. The range of light visible to the human eye ranges from about 400nm to 700nm. The lower-frequency wavelengths beyond 700nm (red) are known as infrared, the term infra referring to the fact that they are closer to the starting point of the electromagnetic spectrum.The higher frequency wavelengths beyond 400nm are referred to as ultraviolet, the term ultra meaning that they are higher in frequency.
Infrared light is absorbed and reflected differently than visible light, and if we filter out the visible component, the effects can be recorded by film and digital sensors which are sensitive to wavelengths beyond 700nm. In film photography, special film is required, but in digital photography, all cameras are inherently sensitive to infrared light. However, since infrared light can cause blurriness and color shifts, digital cameras employ filters known as “hot mirrors” to filter it out. Fortunately, these filters can be removed, and older cameras employ weaker hot mirror filters.
Film vs. Digital
Film and digital infrared images are very different in appearance. Infrared film requires special handling and processing, and is therefore less accessible than digital. Processing for color infrared film is hard to come by, but black and white can be processed in the home darkroom using conventional chemistry. Some film cameras which employ infrared film frame counters are unsuitable for IR photography as they will fog the film.
On the other hand, any digital camera sensitive enough to IR radiation can be used simply by purchasing a suitable infrared-pass filter. All digital cameras are inherently sensitive to IR, but employ IR-blocking filters (which can usually be removed) to improve color rendition and sharpness.
Film vs. Digital
Film and digital infrared images are very different in appearance. Infrared film requires special handling and processing, and is therefore less accessible than digital. Processing for color infrared film is hard to come by, but black and white can be processed in the home darkroom using conventional chemistry. Some film cameras which employ infrared film frame counters are unsuitable for IR photography as they will fog the film.
On the other hand, any digital camera sensitive enough to IR radiation can be used simply by purchasing a suitable infrared-pass filter. All digital cameras are inherently sensitive to IR, but employ IR-blocking filters (which can usually be removed) to improve color rendition and sharpness.
NIR vs. FIR
There are two types of infrared imaging. Pictorial photography falls under the category of near-infrared (NIR) imaging. Far-infrared (FIR) refers to infrared imaging used to record temperature variance. The brightly saturated colors used in FIR photography are actually a color coding used to differentiate between temperature ranges, and are not related to the color of the light in any way. It’s a common misconception to refer to this form of infrared imaging as “color infrared photography”.
Cameras
Many digital cameras can be converted to infrared by replacing their hot mirror filters with a clear filter. This generally renders them unsuitable for normal photography, although an external UVIR or hot mirror filter can then be used restore normal color balance. The results may not be as accurate as with the factory filter, but this can often be corrected by using a custom white balance, or through post-processing. If the original filter is retained, the conversion is fully reversible. Suitable UVIR and hot mirror filters are available from B+W, Formatt and Tiffen.
Lenses
Unlike UV photography, any lens can be used for IR photography. Given the insensitivity of most digital cameras to IR wavelengths due to the hot mirror filter employed, faster lenses tend to work best. Lenses which feature an infrared focus index are helpful, but not essential
Filters
The range of light visible to the human eye ranges from about 390nm to 750nm. Infrared pass filters, also known as “cold mirrors” suppress most or all of the visible spectrum, but allow infrared light to pass. Their are four classes of these filters each having a different point at which they begin to pass infrared light. Filters which are higher up in the IR band (for example the #87 series) record less color separation information, and so are suitable mainly for monochromatic IR photography. Listed in order of sensitivity or “start point” are the following filters used in near-infrared pictorial (NIR) photography:
- #25, #29 (Red) are very popular filters used in black and white photography and are an excellent foray into infrared photography. When used with black and white film, effects similar to IR photography such as deep skies and high contrast in foliage can be achieved. When used with infrared films, true IR effects can be achieved. #25 suppresses wavelengths below 580nm and #29 below 600nm. B+W, Cokin, Formatt, Heliopan, Hoya and Tiffen all manufacture #25 filters, while #29 is available from B+W, Formatt and Tiffen.
- #70, #89B (Dark Red) is the second most usable filter in this class, as it suppresses most visible light, but allows a small portion of the upper red bandwidth to pass, beginning at about 650nm. This can be helpful for cameras which are less sensitive to IR, or to achieve different effects by allowing visible light to interact with infrared light. The B+W 092M is an excellent choice.
- #88A is the most usable filter in this class, as it suppresses all visible light, yet passes infrared radiation as low as 720nm, just beyond the visible range. This allows it to work with most digital cameras, even if they have not been converted. Newer cameras might require longer exposure times. The Hoya R72 and RM72 are coated glass versions, while the Singh-Ray I-Ray is a resin version.
- #87, #87C, #87B are ideal for use with cameras that are very sensitive to IR or that have been converted by removing their hot mirror filters. It begins to pass IR at 800nm, well into the infrared range, and peaks at around 900nm. Look for the Tiffen #87, B+W 093 and the Hoya RM90.
Other types of filters to consider are the UV/IR and hot mirror. These filters do the opposite of those listed above, and are used to restore natural color photography capability to cameras which have been semi-permanently converted for UVIR use.
- UV/IR filters suppress both ultraviolet and infrared radiation, and are also suitable for general photography where these wavelengths can produce color shifts and blurry images.
- Hot Mirror filters suppress only infrared and are especially helpful when using Neutral Density filters which are invisible to infrared light.
Processing
All infrared images require special processing, although some cameras naturally produce more aesthetically pleasing images than others. Color infrared film is generally processed by a lab, although you can process black and white infrared film yourself using chemistry but with longer processing times.
Digital infrared images can be processed a variety of ways using both the RGB and the L*a*b* color spaces. The latter can be used in color infrared photography to overcome the extremely narrow range of color separation which occurs from 650nm to about 800nm. Beyond about 800nm, color separation is minimal at best, and images more suited to monochrome (black and white) IR.
Often the red and blue channels are swapped using the channel mixer in RGB, or by cutting and pasting pixel information between these two channels. This is useful for color work, but more importantly improves contrast in black and white conversions.
In L*a*b, the endpoints of the a and b curves are adjusted to match the color information in the histogram, and the b curve inverted to restore blue to the skies.
As in conventional black and white conversions, it’s advantageous to capture images in color and use this information to optimize contrast in the black and white image.
All infrared images require special processing, although some cameras naturally produce more aesthetically pleasing images than others. Color infrared film is generally processed by a lab, although you can process black and white infrared film yourself using chemistry but with longer processing times.
Digital infrared images can be processed a variety of ways using both the RGB and the L*a*b* color spaces. The latter can be used in color infrared photography to overcome the extremely narrow range of color separation which occurs from 650nm to about 800nm. Beyond about 800nm, color separation is minimal at best, and images more suited to monochrome (black and white) IR.
Often the red and blue channels are swapped using the channel mixer in RGB, or by cutting and pasting pixel information between these two channels. This is useful for color work, but more importantly improves contrast in black and white conversions.
In L*a*b, the endpoints of the a and b curves are adjusted to match the color information in the histogram, and the b curve inverted to restore blue to the skies.
As in conventional black and white conversions, it’s advantageous to capture images in color and use this information to optimize contrast in the black and white image.
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