Reflected infrared photography
The practical operating "window"

From the previous discussion it is possible to overlay all the spectral transmission or sensitivity curves on a common axis. The result is a harmonization diagram (Figure 49) that demonstrates that the practical window for reflected infrared photography, using conventional films and optics, is from 720nm to 910nm with a peak sensitivity at 820nm. It can be seen that the film is sensitive in this region, the filter and lens both transmit in this region, and the xenon flash emits between these wavelengths. The following tables contain simple and practical recommendations for biomedical applications. They are a guide and a starting point for experimentation.

Figure 49 (above). A harmonization diagram demonstrates that the practical window for reflected infrared photography, using conventional films and optics, is from 720nm to 910nm with a peak sensitivity at 820nm. It can be seen that the film is sensitive in this region, the filter and lens both transmit in this region, and the xenon flash emits between these wavelengths.

It is critically important that a standard 'control' image - in colour or panchromatic black-and-white - that represents the visual appearance of the subject, be included as a reference with all scientific invisible radiation records. Obtaining exactly the same viewpoint, with the same lighting can be a very challenging task; this is exacerbated still further when undertaking multi-spectral analysis, where at least three cameras are involved. The authors designed a simple system (Figure 50) where the electronic flash is actually fixed to a tripod and quick release plates fitted to tripod and each camera allow the rapid change over of cameras.

Figure 50 (left). A practical working arrangement for multispectral analysis. The flash is mounted to the tripod and remains in a constant position whilst cameras are changed easily by utilizing quick release plates.

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