Over the rainbow - a talk by David P
The world of infra-red, ultra-violet reflectance, and ultra-violet fluorescence photography
Club Member David P provided us with an introduction to the worlds of infra-red [IR] and ultra-violet [UV] photography during this week’s Zoom meeting, taking us into places in this world you cannot normally see with the naked eye. He started the talk by illustrating how a raindrop splits the spectrum of sunlight into the colours visible in a rainbow, but also separates out the infra-red and ultra-violet components of the rainbow which are invisible to the naked eye. He also provided a diagram of the electromagnetic spectrum to show how small a proportion of this complete bandwidth is actually visible to human eyesight. This part of the talk was accompanied by photographs of a rainbow taken in infra-red (recording radiation normally invisible and located above the arc of colour), visible light, and ultra-violet (recording radiation normally invisible, this time located below the arc of colour).
David went on to explain that modern cameras are designed to prevent these IR and UV wavelengths reaching the sensor or film in a camera. Modern sensors are actually sensitive to IV and UV wavelengths, but this is not desirable in normal operation. This IR attenuation is achieved by use of a 'hot mirror' in the lightpath blocking much of the IR spectrum, and by additives in the lens material being incorporated to attenuate much of the UV spectrum.
Infra-red photography – over the rainbow
Both black and white and colour film is available for IR photography, although it should be noted that this can give an extremely grainy negative, and that in the case of the colour film the result appears as a false colour image. Such film must be loaded into the camera in total darkness.
Some ‘hot mirrors’ will allow a little IR light to pass through to the sensor. You could test your own camera by taking shots of a TV remote control lens being operated in a very dark room. If such a test shot is successful, then when taking IR pictures a deep red or IR filter is required in front to the lens to cut out the visible light spectrum, resulting in very long exposures and the need for a tripod. It should be noted that the IR light will focus differently to visible light, so the image focus point needs to be adjusted to compensate. Due to the light blocking nature of the IR filter, composition, focus, and exposure all need to be established before the filter is mounted. Some, but not all, cameras my even work in ‘auto’ mode. A lens mounted IR filter can be found for about £20, dependent on lens filter size of course.
David has had one of his cameras modified to remove the 'hot mirror' effect, allowing him to take photographs in the IR band, some of which we have seen in club competitions. When this change is implemented a new IR pass filter is inserted into the camera in lieu of the IR blocking ‘hot mirror’. This pass filter can be selected / tuned to specific frequencies that will affect the recorded image. With this modification the camera does not need the dark red or IR filter on the front of the lens so the image exposure time is reduced, although these new pass filters do cut out a stop or so of the incoming radiation. When such a camera modification is implemented the choice of filter frequency needs to be selected, the autofocus system recalibrated for IR, a new white balance setting established, and a general camera service carried out. Following such a camera conversion, the camera is only useful for IR photography. This is not a cheap option.
Infra-red pictures have a very limited colour range compared with normal photographs, and when converted to monochrome present an unusual tonal balance. Greens appear very pale or even white, blue sky and water both appear very dark. Eyes in a portrait are also dark, and even more interestingly skin is uncannily smooth as blemishes disappear due to the IR light being reflected off of an underlying layer in the skin rather than from the surface. Photoshop can be used to generate a range of false colour images; interesting but not lifelike. The degree of colour information recorded will depend on the tuning of the pass filter selected. For instance the 830nm filter will only generate a black & white image.
Ultra-violet fluorescence [UVF] photography – under the rainbow
When UV light falls onto some materials it can be reflected as a lower frequency of light that is within the visible spectrum – they fluoresce. This appears to brighten those materials, and is commonly incorporated into washing powders and white paper to make them more appealing to buyers. The black lights used in locations such as nightclubs, or in banknote testing machines, also use this property.
Some materials in nature also have this reaction. Scorpions, fungi, resinous trees and plants were given as examples showing such effects, with pictures of the surprising differences provided. An ethereal hawthorn flower spray was particularly impressive, as was a myrtle plant with a spread of colours apparent.
UVF images can be taken with a standard camera, used in conjunction with a UF-A light source. The requirements are a darkened room and a suitable torch incorporating a UV-A pass filter. The image subject is then ‘light painted’ via a system of trial and error. A tripod and an extended exposure are again required. UV-B and UV-C light sources should not be used, and care must be taken to ensure that the strong UV light is never allowed to impinge on anyone’s eyes.
David has found the Convoy S2+ torch with a Banggood 2 filter disc is an effective light source, available for about £10 from Gearbest.com. A figure 186650 rechargeable battery and suitable charger is also required.
Ultra-violet [UVR] reflectance photography
UVR photography requires a camera converted to full spectrum operation. This can then capture IR, UV, and visible light, controlled by use of suitable filters located in front of the lens to only allow through that part of the spectrum required for each image. The conversion is achieved by incorporating a quartz filter in place of the standard ‘hot mirror’. Additionally a particular type of lens is required, one not designed to block UV light. Such lenses are often found as enlarger lenses from older equipment.
For UVR images a Baader U Venus filter was recommended. David showed us a range of images taken with this set up, including flower blooms that appear totally different under UV light, and portraits where the skin shows many surface features not apparent in normal visible light.
David concluded his talk by bringing to our attention a book written by Adrian Davis called “Photographing the unseen world” published by Crowood, who also sell direct and often at competitive rates. Adrian Davis gave a talk at the club a few years ago.
David also discussed the many differences in vision provided within nature.
Cameras ‘see’ in R G B , mimicking the vision of primates, including humans.
Most other mammals see in B Y only , so are R + G colour blind.
Insects see in Y B UV, giving them a different vision spectrum to us
Day flying birds see in R G B UV, yet another vision spectrum
Mantis shrimp see with 12 to 18 different types of receptor. Unimaginable!
A very interesting talk, opening up many further fields of photography and helping us to explore that unseen world, should that challenge appeal to us.