Filters

Like films and radiation sources, much has changed with the availability of filters for invisible radiation photography, and many have been withdrawn from the market. For reflected ultraviolet photography, for instance, Kodak now only manufactures the 18A. The same is true of all Pilkington filters - the familiar Chance OX1, OX5 and OX7 much referenced in the literature are no longer manufactured.

The reflected ultraviolet technique requires a filter to be placed over the lens that will transmit only ultraviolet to the film called an ultraviolet transmission filter. All UV transmitting filters are made of glass, as gelatin absorbs ultraviolet. (Note - the skylight or UV filter commonly used over camera lenses as physical protection from damage and optical protection against UV haze effects - is an UV absorbing filter).

The Kodak Wratten 18A is still the standard filter with a transmission window from 300 to 400nm. (The generic term "Wood's filter" - named after Professor Robert Wood the 'Godfather' of invisible radiation study - applies to any filter with this range of transmission). It should be noted from Figure 25 that not only is there a window of transmission in the ultraviolet region of the 18A spectral curve, but that there is also a window in the 700 to 900nm region, which also makes it very effective as an infrared filter - provided of course that the recording medium is also sensitive to infrared. Normal photographic film is insensitive to infrared radiation (see Infrared photography) but Charged Couple Devices (CCD) found in digital cameras are sensitive to both (see electronic recording). Figure 26 shows the broader transmission curve for the Kodak Wratten 18B, which was very popular and is found widely in circulation in laboratories and studios. Curiously the Wratten 18A filter is now only available from Kodak as a very expensive 'spare part.'

Figure 25 (above). The spectral transmission curve for the Kodak Wratten 18A filter demonstrates not only the window of transmission in the ultraviolet region but also a window in the infrared region between 700nm and 800nm.

Figure 26 (above). The spectral transmission curve for the Kodak Wratten 18B filter no longer sold commercially shows a wider transmission window in both the ultraviolet and infrared region making it a less 'pure' filter but often easier to get a result with.

B+W a German company (6550 Bad Kreuznech, Postfach 2463, Germany) - a division of the Schneider Optics empire, supplies an ultraviolet transmission filter (code number 403) with a very similar transmission curve to the Wratten 18A, at a much lower price (Figure 27). Precision Optical Ltd in England (425 Stratford Road, Shirley, Solihull, West Midlands, B90 4AE, UK), supply Schott, UGI, UG5 and UG11 filters (which are the equivalent of the old Chance Pilkington OX1, OX5 and OX7). The curves for these are shown in Figures 28, 29, and 30). Oriel Scientific in USA (250 Long Beach Blvd, PO Box 872, Stratford, CT 06497, USA), supply interference filters for ultraviolet in 10, 20, 50 and 100nm wavebands throughout the ultraviolet spectrum. (Dye absorption filters are only applicable down to 310nm; interference type filters made of quartz or silica must be used below this wavelength). Tiffen - the major supplier of photographic filters in the USA supply an equivalent to the Wratten 18A in a wide range of sizes to fit common lens filter threads. The Rolyn Optics Company (706 Arrowgrand Circle, Covina, California 91722, USA) supply UG1, UG5 and UG11 filters in various sized glass squares.

Figure 27 (above). The spectral transmission curve for the B+W 403 UV transmission filter - a less expensive alternative than the Wratten 18A.

Figure 28 (above). The spectral transmission curve for the Schott UG1 ultraviolet transmission curve.

Figure 29 (above). The spectral transmission curve for the Schott UG5 ultraviolet transmission curve.

Figure 30 (above). The spectral transmission curve for the Schott UG11 ultraviolet transmission curve.

As all ultraviolet transmission filters are visually opaque, a filter holder mechanism must be used which will allow the filter to be placed quickly over the lens after the visual focus point has been established. It is important that the holder provides a light-tight mount. Examples of such holders are Kodak Pathe's Porte-filtre Professionel No.2 for large format work (Figure 31), and the Nikon AF1 holder for 35mm, designed with a light-tight mount that can be flipped up and locked when ready to take the ultraviolet exposure (Figure 32).

Figure 31 (left). Kodak Pathe's Porte-filtre Professionel No.2 for large format work allows the visually opaque UV transmission filter to be dropped in after establishing the visual focus.

Figure 32. The Nikon AF1 filter holder swings up over the lens once the visual focus has been achieved.

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