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Feb., 1950
ROSS : HEATING OF FILMS AND SLIDES
41
It is estimated that such a cell and lens absorbs only 10% more of the visible light, compared with an ordinary lens. This is, in effect, recovered by avoiding the use of a separate spatter glass.
It may be added that the measured absorption of visible light from an arc by a simple cell comprising two glass windows and two inches of water is about 15%. The addition of 2 mm. thickness of ON20 glass to such a cell absorbs only a further 6 % of visible light from an arc, due to low reflection losses. An ON20 glass filter in air absorbs about 12% of the visible light from a tungsten bulb.6
By combining the curves of Fig. 2 for filters with those of the light sources, Fig. 1, we obtain the distribution of the energy of the filtered light, Fig. 4. In the case of the arc, the dotted line shows the effect of filtering out the ultraviolet radiation with a filter similar to the Wratten 2A filter. When using a water-cell, this effect may be obtained by adding ortho-nitro-benzoic acid and a little sodium carbonate.
High Intensity Arc Filtered Through 2m/m Water 2, 2% ON20 Glass
Tungsten Bulb (3ioo°K
Filtered Through 2% ON20 Glass
&
1-0 \z
16
■8
20 22
2 4
4 -6
Visible Wavelength./-*
Fig. 4. Spectral distribution of energy from arc and tungsten sources, after unwanted radiation has been removed by practical filters.
Luminous Efficiency of Radiation
When a beam of radiation is absorbed in any way, the energy of the radiation is converted into heat. Unless there is sufficient cooling, the temperature of the object absorbing the radiation will therefore rise. Contrary to what is often implied, it makes no difference to the heating effect whether this radiation is in the infra-red, the visible or the ultra-violet regions of the spectrum. One way of looking at this is to realise that just because the human eye can see radiation of certain wavelengths, this is no reason for these wavelengths losing their heating effect. Consequently, in any projector, wherever light is absorbed, heat is generated there. All that can be done by filtering the light is to remove the unwanted radiation — those wavelengths which contribute to the heating effect without adding to the visible light — but we can never get rid of all the heat.
In the curves of Figs. 1 and 4 the heating effect may be determined by measuring the area between each curve and the base line. Comparison between the areas beneath the curves for unfiltered and filtered light shows immediately the great reduction of heating obtainable by filtering.
Another way of specifying this effect is to compare the amount of visible light (which we want) obtained from a light source, with the total amount of