Prof Robert Williams Wood (1868 - 1955)

It is not generally appreciated that Professor Robert Wood (Figure 9) of Johns Hopkins University in Baltimore, described by his biographer (Seabrook, 1941) as 'The modern wizard of the laboratory', was undoubtedly the father of both infrared and ultraviolet photography (Figure 10). The discovery of the radiations beyond the visible spectrum and the sensitization of emulsions capable of recording them pre-date Wood, but he holds the distinction of being the first to intentionally produce photographs with both infrared and ultraviolet radiation and also for being the first to photograph the ultraviolet fluorescence phenomenon.

Figure 9 (left). Professor Robert Williams Wood.

Figure 10 (left). Wood's biography.

Robert Wood was an American physicist who became a professor and fulfilled various roles at Johns Hopkins University in America from 1901 until his death in 1955. He was internationally known for his work in optics and spectroscopy, in which fields he undertook fundamental research in resonance radiation and in the use of absorption screens in astronomical photography.

In February 1903, Wood described the invention of a filter for ultraviolet transmission, which would exclude all visible light. The first version was made with nitroso-dimethyl-aniline but this still let through too much blue light for effective ultraviolet photography. The addition of a small amount of the dye uranine to the formula made a filter for photography, which transmitted, exclusively ultraviolet.

The basic silver halide emulsion is only sensitive to blue/ultraviolet end of the spectrum and it was some years before the spectral range of film was extended by dye sensitization. 1904 saw the possibility of extending the sensitivity of photographic film to the infrared region with the discovery of kryptocyanine (Kornfeld, 1938), and although infrared emulsions were not available commercially until the 1930's, Wood was the first to publish infrared photographs - landscapes - taken in 1910 on experimental film (Figure 11).

Figure 11 (above). One of Wood's infrared landscape photographs.

In a report of the Traill-Taylor lecture to the Royal Photographic Society in 1910, many photographs illustrated Wood's early application of both infrared and ultraviolet photography. He noted how chlorophyll reflected infrared strongly and how blue sky recorded almost black on the infrared record. Reflected ultraviolet photographs were presented of both landscapes and lunar surfaces, whilst a magazine cover with various inks and dyes demonstrated the different absorbencies in ultraviolet and natural light. In the vote of thanks Wood was credited with 'opening up two new worlds; the worlds at each end of the spectrum, beyond the point of limit of vision'. Wood's best infrared landscapes were exhibited at the annual exhibition of the Royal Photographic Society in 1911 and were published in the Illustrated London News at the time. According to Corrigan (1927) Wood had utilized the extended red sensitivity of the 'new' emulsions to record his infrared results but the poor sensitivity led to exposures of the order of ten minutes, and even then required the negatives to be intensified before they could be printed!

In a paper presented to the French Society of Physics in April 1919, Wood described how his filter made from a special barium-sodium-silicate glass incorporating about 9% oxidized nickel (later to become universally known as the Wood's filter) had been used for secret communications during the War. His 'invisible radiation' technique worked either in infrared, which he suggested was useful during the daytime, or in ultraviolet, which he used for night-time applications. Towards the end of his address, he suggested that this might be useful not only for communication between aircraft and airfields but for many other applications and then went on to describe the remarkable effects of this kind of radiation on the human skin. He presented three photographs in his 1919 publication. This made him the first to present ultraviolet photographs of the human body. The first photograph with the reflected ultraviolet technique showed a face with teeth fluorescing brightly and he noted how the skin recorded grey (Figure 12). The other two photographs of hands demonstrated the beginnings of a fluorescence technique. He used the oxide of nickel filter as the excitation filter and measured the radiation to be at 366nm. He then used a barrier filter made from a concentrated solution of potassium dichromate which he noted was deep yellow. He applied this technique to the back of a hand and recorded both the reflected ultraviolet photograph (Figure 13) and then his 'phosphorescence' photograph, as he called it (Figure 14). He noted several patches on the skin that had a marked phosphorescent appearance.

Figure 12 (left). Wood's first photograph using the reflected ultraviolet technique.

Figure 13 (left). Wood's reflected ultraviolet photograph of the back of a hand.

Figure 14 (left). Wood's 'phosphorescence' photograph of the back of a hand.

Thus by 1920 Wood had clearly established the techniques of ultraviolet photography, infrared photography and ultraviolet fluorescence photography making him therefore truly the founder of all invisible radiation photography. It was to be many years however before these techniques were to be taken up in medicine. With the ultraviolet technique in particular, the inability to visualize ultraviolet with the naked eye meant that for decades the emphasis was on the fluorescence induced by the high-energy ultraviolet radiation. The recognition that the photographic emulsion was inherently sensitive to ultraviolet and that the Wood's filter was capable of stopping the fluorescence from affecting the film if it was placed over the camera lens, eluded researchers for many years.

References:

• Corrigan, J., 1927, "Infrared photography with commercial panchromatic emulsions" Brit J. Photogr. Aug 5 460 -461.
• Kornfeld, G., 1938, "The limits of infrared sensitizing", J. Chem. Phys. 6:201 - 202.
• Seabrook, W. 1941. "Doctor Wood, Modern Wizard of the Laboratory." New York.
• Wood, R., 1903, "On screens transparent only to ultraviolet light and their use in spectrum photography," Phil. Mag. 5 S6:(26):257-263.
• Wood, R., 1910, "Photography by invisible rays," Photogr. J. 50 (Oct.):329-338.
• Wood, R., 1919, "Communications secretes au moyen de rayons lumineux," J. Phys. Theor. Appl. 9:77-90.

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