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Light Filters — Jones
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To determine quantitatively the absorption of a light filter for radiation of different wave-lengths a spectrophotometer is used. An essential element of this instrument is a device, such as a prism or diffraction grating, for dispersing or separating into its component parts the radiation from some suitable source (such as the electric arc or incandescent lamp) which emits many different wave-lengths. In this way a spectrum is formed and by means of a narrow sht suitably placed, radiation of any desired wave-length may be isolated. One-half of this monochromatic radiation is then allowed to fall upon
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WAVELENGTH (m/i)
Fig. 2. Spectrophotometric transmission curve of green filter.
the filter being examined and the intensity of the radiation transmitted by the filter is measured by comparing it in a suitable photometer with the other half of the monochromatic beam which has not been subjected to the absorbing action of the filter. In this way values of transmission, T, where
r = ^ (see Fig. 1),
for a series of different wave-lengths are obtained. These values plotted as a function of wave-length give a curve which shows the absorption characteristics of the filter in graphic form. This is called a spectrophotometric curve. It is customary in plotting such a curve to multiply Ix/h by 100, thus expressing transmission in percentage. Such a curve is shown in Fig. 2, applying to a gelatine filter made by the use of toluidine blue.
For many purposes the expression of absorption in terms of optical density, equation (3), is more convenient than in terms