Electronic recording of the infrared image:
Digital 'still' cameras

The resolution and functionality of digital cameras continues to rise whilst prices continue to fall. As mentioned above all these cameras are fitted with charged coupled devices (CCDs) which have an inherent sensitivity to both infrared and ultraviolet radiation as well as the visible spectrum. This has caused a significant resurgence of interest in infrared imaging (incidently not just electronic - as the results obtained electronically often spur on the keen enthusiasts to re-engage with the film based methods). First generation digital cameras and the cheaper end of the range of second generation cameras had poor or no infrared filters built over the CCD. This led to them being too red in tungsten lighting and to being good little infrared cameras! (The web is awash with reports from enthusiastic amateurs demonstrating their results).

Progressively digital camera manufacturers have included effective infrared absorbing filters over the CCDs which make them effectively a lot less sensitive to infrared - but not impossible to use. The Nikon Coolpix 950 for example was widely reported as excellent for infrared imaging but its successors the 990 and 880 have been shown to be much less effective because of the inclusion of more effective filtration. With the certainty of loosing one's guarantee and at the risk of accidentally damaging one's camera it is possible to remove the infrared filter from in front of the CCD and replace it with a glass block of the same thickness in order to retain the same effective image distance. Wooten in a web published article describes the process for the Nikon cameras. Kodak, at least, has realised the commercial potential of supplying a camera version, ex-factory, with no infrared absorbing filter - the DCSIR (Figure 56). A US company - MaxMax - also supplies a range of cameras especially modified for infrared - see web resources. Even with an infrared absorbing filter over the CCD most digital cameras are capable of producing an infrared image simply by exposing for longer - as most of these filters are not very efficient.

Figure 56 (left). The DCS IR camera from Kodak with no integral infrared absorbing filter.

A simple experiment will confirm whether a digital camera can be used for infrared. Set the camera up in a darkened room aimed at a television/video infrared remote control, making sure that the LCD screen is activated. The infrared emission of the remote control caused by pressing the control buttons should be visible on the camera's LCD if it can be used for infrared imaging.

The use of such cameras has some compelling advantages:

• they allow you to see the image before, during and after capture - the LCD display converting the infrared image into a visible one
• their autofocus capability often works, eliminating the frustrating problems of chromatic focus shift
• the images produced can be separated, in register, into the respective spectral channels in Photoshop, thus allowing a whole range of image enhancement techniques to be used
• those cameras capable of being used with external flash synchronisation(all the professional models) can be used with electronic flash (rich in infrared) to photograph dynamic events and living subjects

The technique of infrared photography with these cameras follows the same principles as with film based systems. An infrared transmission filter must be placed over the recording lens. As with film the choice of filter is a critical issue with CCDs. Many digital cameras (especially those with integral 'hot' - or infrared absorption - filters) will not record far into the infrared spectrum. For these cameras the selection of a pure infrared filter like theWratten 87C will almost certainly produce no result. A broader spectral transmission is required: the Wratten 89B is widely reporting as being excellent.

The infrared image records in the red channel of a composite R-G-B image, but a much better result can be achieved by switching the camera to monochrome or black-and-white image and selecting the highest resolution/least compression option available on the camera. It is always advantageous to record a colour 'control' photograph immediately before or after the monochrome infrared - without moving either camera or subject. This is useful for comparison purposes and aids interpretation of the infrared record enormously. It also enables the construction of a false colour infrared image if that is desirable, see Building a false colour image in Photoshop.

Digital cameras which use film-plane algorithms for focussing will autofocus an infrared image; others which use infrared sensors outside of the image forming path may not (especially if a large filter holder is obscuring the filed of view of the 'rangefinder' window. The authors tested a Nikon Coolpix 990 with infrared and ultraviolet radiation to check the autofocus at reasonable magnifications (Figure 57 ) and demonstrated that the autofocus capability worked well. One reason why out-of-focus results can be obtained is that the camera is 'asked' to focus both infrared and ultraviolet at the same time - all ultraviolet filters 'leak' infrared and many infrared filters leak ultraviolet - and the CCD is sensitive to visible, infrared and ultraviolet. It is imperative therefore that 'pure' infrared (or ultraviolet) is presented to the CCD by selecting appropriate filter combinations. For infrared, the infrared transmission filter, eg. Wratten 89B, needs to be supplemented by an ultraviolet absorbing filter, eg. A Wratten 2E. For ultraviolet imaging the reverse applies; the ultraviolet transmission filter, eg. Wratten 18A needs to be supplemented by an infrared absorbing filter or "hot mirror."

Figure 57 (above). The autofocus mode of the Nikon 990 digital camera evaluated. The images of a rendered wall demonstrate that the Nikon copes with each wavelength selected. The ultraviolet record was imaged by combining an infrared absorbing filter - 'hot filter' - with the Wratten 18A; whilst the infrared record was obtained by combining a Wratten 2E - ultraviolet absorbing filter - with the Wratten 89B infrared transmission filter.

The sensitivity of different CCD/over-filters varies enormously and it is impossible therefore to give any useful recommendations for exposure etc., except to say that exposures are likely to be very long - often several seconds with most continuous sources of radiation. Even with manually 'forcing' the camera to its maximum ISO setting and widest aperture one can expect 1/2 second exposures. A tripod and cable release are, therefore, de rigueur for this technique. One must, of course, disable any autoflash capability. With external electronic flash it is possible to record moving subjects, although again the 'slower' cameras will need to be manually controlled so as to override the autoexposure capability and ensure a wide enough aperture and high enough amplification from the CCD (highest equivalent film speed).

One consequence of forcing digital cameras to record such weak images is that the signal to noise ratio of the charged couple device is seriously reduced and noise becomes a problem in the final image. This 'dark field noise' has the effect of lots of pixel sized highlights in dark areas of the image. The noise is caused by an aberration of the charged couple device. The electronics themselves generate heat and this energy causes some pixel electrons to be activated and then trapped as 'false positives' ie. not generated by light energy falling on the pixel. This results in a blotchy image in dark shadow areas of the image: it becomes worse in hot conditions and with very long exposures. CCDs built into astronomical imaging devices (which have very long exposure times) are often super cooled in order to avoid this problem of noise. It is possible to 'filter' out this noise at a post imaging stage in an image manipulation software program such as Adobe Photoshop by a technique known widely in astronomy as dark field subtraction. Two images are required - one of the infrared record (with noise) and one with no ambient illumination (the noise). The image of the noise is then effectively subtracted from the real image using Photoshop's layer subtraction feature. A quick alternative is the use of a plug-in program like Quantum Mechanic which does the filtering for you. Quantum mechanic makes the reliable assumption that the noise artefacts are mostly in the chrominance channels of a colour image and leaves the luminance channel untouched. It is not however as effective as full manual dark field subtraction.

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