Infrared photography
The infrared photography (scientific "Ultrarotfotografie") is engaged in the production of images by taking advantage of light wavelengths longer than those of visible light ( infrared radiation ) are. A distinction is made between two areas of application:
- Near infrared (700 - 1000 nm), medium infrared see thermography and
Infrared false color photography (900 nm) aims at the following effects:
- Infrared wavelengths are scattered to a lesser extent than visible light by haze and air pollution. The recognizability of objects in spite of such visual impairments is therefore better in the infrared range than in that of visible light.
- The photographed objects have different reflection properties than visible light in these areas. This allows specific material properties to be identified (e.g. exploration of damaged forests).
- The infrared radiation is invisible to the human eye, hidden recordings with imperceptible artificial lighting are possible (surveillance, security services, military, nature observation).
This article only deals with photography with infrared film or with digital cameras at wavelengths between 700 and 900 nanometers, i.e. just beyond visible light.
The history
Infrared photography was not possible until the early 20th century because silver halide emulsions were not sensitive to infrared radiation without the addition of coloring as a color sensitization. The first infrared photography was published in 1910 by Robert W. Wood , who discovered the unusual effects that now bear his name (see Wood Effect ). Wood's photographs were taken on experimental film, which required very long exposures; so most of his work focused on landscapes.
Infrared sensitive photographic plates were developed in the United States during World War I for improved aerial photography. False color infrared photography was only introduced with the introduction of the Kodak Ektachrome Aero infrared film , item no. 8443, practiced in the 1960s and used by many fine art photographers, mostly for its unusual results: Jimi Hendrix , Donovan , Frank Zappa and Grateful Dead released albums with infrared photos on the cover . The unexpected colors and effects that infrared film can produce went well with the psychedelic aesthetic that became popular in the late 1960s. Infrared photography can easily seem like a gimmick, but photographers like Elio Ciol have made very subtle use of infrared-sensitive black and white film.
The Ministry for State Security of the GDR used infrared photography for covert observation in the dark.
For several years (as of 2015), infrared photography has been used in traffic speed monitoring under the catchphrase black light flash .
How it works
Infrared radiation (IR radiation) - also known as thermal radiation - is part of optical radiation and therefore part of the electromagnetic spectrum. The human eye can perceive light between 380 and 780 nanometers, the commonly used panchromatic film material is usually sensitized to a similar spectral range (approx. 400–650 nm). Special filters suppress the visible light during the recording. Everything below the spectral range to which the filter is transparent appears dark, everything above appears light. Infrared material offers the possibility of recording these wavelengths due to the extended sensitization (> 700 nm). Digital sensors can usually also see infrared light, but this property is often suppressed by camera manufacturers in favor of image quality.
The figure above shows three vegetation spectra. These each have similar courses, but differ in their albedo. In the visible range of the spectrum (0.4 to 0.7 µm) the so-called green peak can be seen, which is generated by the strong light absorption of chlorophyll in the blue and red spectral range and the weaker absorption in the green range (around 0.55 µm) becomes. The green peak makes vegetation appear green to the human eye. The strong increase in reflection between 0.7 µm and 1.3 µm ensures that the film is exposed to a high degree of exposure. It is caused by multiple scattering on the leaf structure because there are no absorbing substances in this spectral range.
The recording
Cameras
The sensors of modern digital cameras are very sensitive to infrared light. However, since this interferes with the imaging performance in the visible range and leads to strong blurring, blocking filters that cannot be removed by the user are usually built into the cameras to block these wavelengths. In most cases, the residual sensitivity in the infrared range with complete filtering of the visible light is sufficient for infrared recordings with digital cameras. Some digital bridge cameras from Sony also offer a " NightShot mode" in which the camera's internal infrared blocking filter is swiveled out and therefore not blocked. Unfortunately, the manufacturer apparently does not intend to use this function for infrared photography: In this mode, the camera does not allow exposure times shorter than 1/30 of a second and only with the aperture fully open, which leads to excessive overexposure in daylight. This can be remedied by adding a neutral density filter in front of the lens or, on some models, by swiveling the blocking filter out unnoticed by means of a magnet for the camera control.
With most digital SLR cameras , the internal IR blocking filter can be removed in a special workshop and replaced with another filter in front of the sensor. This allows the usual short exposure time , so that you can work with such a camera without a tripod . There are now different variants of this conversion to choose from, including one that makes the camera very universally applicable by placing the filter required for the respective use in front of the lens . Canon offers (2016) a variant of a commercial housing that is supplied without a built-in IR cut filter and was specially designed for astrophotographers.
In principle, all conventional cameras can be equipped with infrared film; an exception are models that use a picture counter based on infrared light; These models include, for example, the Minolta Dynax 4 , but also various newer cameras from other manufacturers. It should be checked in each individual case how severe the impairments of the negative really are.
filter
The same subject photographed using an infrared filter . The dark leaves appear light because the chlorophyll reflects more infrared light.
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In order to display the invisible radiation in infrared photography, the visible light is suppressed by a suitable filter up to a certain spectral range . Any longer-wave radiation above the threshold value can be recorded according to the sensitivity of the recording material. For this purpose, filters from various manufacturers in different wavelengths are offered in specialist shops:
- Heliopan offers filters made of Schott glass, which are transparent from the spectral range
RG 695 (89B), RG 715 (88A), RG 780 (87), RG 830 (87C), RG 850, RG 1000. - Good results can also be achieved with the Hoya R72 (720 nm), which is permeable to a residual visible light and thus offers more design freedom with regard to false colors.
- An IR filter 007 is available for the Cokin filter system P.
- By Kodak , the filter 87, 87C, 88A, 89B are
- B + W supplies the filters IR 092, IR 093 and IR 099
For initial tests, however, you can also make do with provisional solutions: 1-2 layers of unexposed and normally developed slide film, attached light-tight in front of the lens, achieves very similar results. The two superimposed filter foils of 3D anaglyph glasses (red / cyan glasses folded on the nosepiece) also largely filter the visible light and allow infrared light to pass through.
material
panchromatic film on the left, infrared sensitive film on the right
The recording can be made using both conventional film material and digital sensors. Infrared films are offered as:
- Black and white films that are more or less sensitized in the infrared range . The visible light is completely or largely (red filter) switched off by camera filters. A typical effect is an extremely dark sky and a white coloration of the leaves (see picture comparison).
- Color films , the color rendering of which shows "wrong colors", that is, the colors shown do not correspond to the perception of the human eye, but the infrared areas are "translated" into those of visible light (so-called false color film ). Such materials are used in addition to the artistic field in aerial photographs, for example for forest damage mapping .
The processing of film material must take place in absolute darkness. Since the film backing acts as a light guide, light can fall through the film tongue onto the unexposed film. If possible, the film should be placed in the camera in the darkroom .
Procedure
Exposure metering and distance setting
The determination of exact exposure times for infrared films is very difficult because the commercially available exposure meters are less sensitive to infrared light than to visible light. When exposing infrared images, one relies on empirical values and the data sheets of the film manufacturer. Exposure series are also essential, so that after the film has been developed, there is a good chance of finding usable material for further processing.
The distance setting is also not trivial, as you have to focus manually and use the infrared index, which is identified by a special marking on the lens. However, not all interchangeable lenses have such a marking anymore . Mirror lenses do not require this adjustment because mirrors do not have chromatic aberrations.
Zoom lenses can disperse more light through their complex optical systems than is the case with fixed focal lengths. This means that an infrared photo taken with a 50 mm lens can be richer in contrast and sharper than the same image taken with a 28-80 mm zoom at 50 mm focal length .
With digital cameras, the recording can usually be checked on the display and corrected if necessary.
Digital cameras require a white balance
The same functionality with automatic Levels reworked
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The standard settings for the white balance of digital cameras for general lighting situations (sunlight, incandescent lamp, neon light, etc.) cannot be used optimally in infrared photography. Manual white balance is required using one of the following methods:
- with an attached filter against an evenly bright green surface, for example a lawn / meadow in sunshine,
- against a blue cloudless sky with an attached filter,
- alternatively a red monitor image without an attached filter.
After a manual white balance, green should already appear almost white in the unprocessed image and the sky usually turns yellow / orange. How to do a manual white balance depends on the camera model and can be found in the operating instructions. If this white balance can be saved, the setting can be used for each further IR exposure. When performing white balance with the filter attached, care should be taken to ensure that the exposure is at least one second.
post processing
With an infrared film, identical chemical and optical processes are available that can also be used with normal material. Apart from special development processes for various films, the resulting originals can be used for normal enlargements or digitizations.
There is a larger selection of tools and modifications available for digital post-processing, but this depends on the software used. In general, a so-called channel shift is used for colored IR recordings in order to achieve a more natural coloring of the sky. This is a technique in which the recording is first separated into its three channels and then reassembled after swapping the red and blue channels. If the result is to be a black and white image, we recommend editing the color template and finally converting it to a monochrome image for publication.
In order to create a corresponding white balance within a RAW file, most RAW converters lack the option of specifying a color temperature below 2000 Kelvin. This limitation can be remedied using a separate camera profile. To do this, it is loaded into the RAW converter beforehand.
attachment
literature
- Gerhard Isert: Photography with infrared . Isert, Halle 1941
- Albert Nürnberg: Infrared photography . Knapp, Halle 1957
- Alfred Ullmann: Photo tricks . Fotokinoverlag, Halle 1959
- Rudolf Hillebrandt: Infrared photography on a different wavelength . Publishing house photography, Schaffhausen
- Klaus Mangold: Digital infrared photography . Edition ProfiFoto, 2010. ISBN 978-3-8266-9053-2
Physical principles and light generation in the infrared range:
- Rudolf Borchert and Werner Jubitz: Infrared technology . 2nd ext. Edition, Verlag Technik, Berlin 1954
Web links
- Infrared photography 680nm vs 720nm
- Tutorials, design tips and image examples for digital IR photography in color and b / w
See also
- False color photography
- Scientific photography
- Dark flash
- Wood effect
- Ultraviolet photography
- Infrared astronomy
Individual evidence
- ↑ Infrared briefcase. In: German Spy Museum. Accessed March 5, 2020 (German).
- ↑ Christine Bilger: Stationary speed cameras in Stuttgart - tunnel speeders should be caught with black light , Stuttgarter Zeitung
- ↑ Black light speed cameras: Is infrared the latest trend in speed cameras in Germany? , Association for Citizen-Friendly Transport Policy eV
- ↑ Housing without Canon IR cut filter , accessed January 1, 2016.
- ↑ Article on infrared photography , accessed July 19, 2016.