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Feb., 1950
ROSS : HEATING OF FILMS AND SLIDES
39
Heat-Absorbing Filters
In order to reduce the heating effect it is clearly desirable to remove the infrared radiation, and perhaps also the ultra-violet radiation. Fig. 2 shows the absorption curves of some typical infra-red filters. It will be seen that water,4 even in a very thin layer, is virtually opaque to radiation above 2-35 microns (1 micron = 1 |Ji = one-thousandth of a millimetre). This is of particular value in keeping cool the lenses and other parts of the optical systems, for crown-glass begins to absorb quite heavily at longer wavelengths than this.
Probably the best infra-red filter is provided by ferrous sulphate or ferrous ammonium sulphate dissolved in water.5 The addition of a few drops of sulphuric acid makes the solution more stable. A convenient way of preventing the formation of air-bubbles on the windows of the cell is to add a few drops of detergent or wetting-agent such as " Teepol," which prevents the released air from adhering to the windows. A solution of the strength shown, when in a cell 5 cms. thick, has a very pale blue-green colour, due to slight absorption of the deep-red part of the visible spectrum, but for most applications this is hardly visible.
Perhaps the most convenient infra-red filter is provided by type ON20 heat-absorbing glass made by Chance Bros.6 This is almost colourless (the curves of Fig. 2 have been drawn for ferrous ammonium sulphate and ON20
Relative Transmission
i'OO
eo
60
20
0-2 cm Water
5 cms Ferrous Amm. Sulphate
(5 2Gms Per Litre)
Fig. 2. Transmission curves for various infra-red absorbing filters.
•4 -s ^
ViSI&LE
12
18
20 22 ZA
Wavelength ->*
filters appearing visually to have the same colour) while it absorbs well in the near infra-red. Its transmission increases slightly as the wave-length increases and only becomes negligible above 3.4 microns. For many applications this type of filter is suitable, and the heat which it absorbs may be dissipated by natural convection to the air, or the glass may be cooled by blowing air on it.
Combined Heat-Absorbing Cell and Lens
It is probable, however, that the most suitable infra-red filter for practical use in a high-power projector is a combination of a thin layer of water with a sheet of ON20 glass. This absorbs well in the near infra-red, provides full protection to the lenses and is reliable and stable in use. Fig. 3 shows diagramatically such a filter combined with the first condenser lens of a process projector.7 The arc runs at 300 amps, with a 16 mm. positive carbon, and 2f ins. away from this is the front window of the cell 5 J ins. diameter. This window is made of quartz in order that it may readily withstand the heat of the arc and its flame. On the other side of the quartz there is a narrow water channel, the