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COMBINED OPTICAL FILTERS FOR ULTRAVIOLET
Anyone who has had occasion to deal with one or another kind of measurement in the ultraviolet is well aware how difficult it is to select optical filters for isolating a definite region in this part of the spectrum. This is due to the fact
Fig. 1. Transmittance of aqueous solutions of certain inorganic salts from 2000 to 10,000 Å.
1—CoSO₄·7H₂O, 75 g/l, \(d = 1\) cm; 2—CoSO₄·7H₂O, 300 g/l, \(d = 1\) cm; 3—CuSO₄·5H₂O, 100 g/l, \(d = 5\) cm; 4—NiSO₄·6H₂O, 500 g/l, \(d = 1\) cm; 5—KCl(SO₄)₂·12H₂O, 150 g/l, \(d = 1\) cm.
that there are almost no substances which absorb the visible and near-infrared spectrum well and transmit the ultraviolet region. Only by means of fairly complicated combinations of two, three, or even four components can optical filters be constructed that isolate one or another region of the ultraviolet spectrum.
In a short communication by M. Kasha1, “Light Filters for the Ultraviolet,” recipes are given for six “monochromatic” light filters composed of three or four components, so that the regions 2400–2800, 2600–2900, 2900–3350, 3000–3350, 3200–3600, and 3450–3900 Å are isolated, with maximum transmission from 20 to 30% (see Fig. *). All combinations are made in such a way that only some of them show negligible transmission in other regions of the spectrum. The author also pays special attention to the stability of the substances he uses.
Table 1
Components of combined light filters
| Component no. | Substance | Solution | Layer thickness | Note |
|---|---|---|---|---|
| 1 | CuSO₄·5H₂O | 100 g/l of water | 5 cm | Fig. 1, curve 3 |
| 2 | NiSO₄·6H₂O | 100 g/l of water | 5 cm | Fig. 1, curve 3 |
| 3 | NiSO₄·6H₂O | 200 g/l of water | 5 cm | Fig. 1, curve 4 (for 500 g/l) |
| 4 | NiSO₄·6H₂O | 300 g/l of water | 5 cm | Fig. 1, curve 4 (for 500 g/l) |
| 5 | NiSO₄·6H₂O*) +CoSO₄·7H₂O |
240 g/l of water 45.0 g/l of water |
5 cm | Has weak transmission maxima: 1.5% at 5700 Å and 3.9% at 9000 Å. |
| 6 | Corning filters C. S. 7-37 (5860 or 586) |
5 mm | ||
| 7 | C. S. 7 51 (5970 or 597) | 4 mm | ||
| 8 | C. S. 7-54 (9863 or 986) | 3 mm | ||
| 9 | 2,7-dimethyl-3,6-diazacycloheptane-1,6-diene iodide | 20 mg/100 cm³ of water | 1 cm | Exceptionally stable dye. Preparation: see literature 6. |
| 10 | Naphthalene | 12.8 g/l of isooctane | 1 cm | Sometimes the fluorescence of the solution must be taken into account. |
| 11 | K₂CrO₄ | 0.200 g/l of water | 1 cm | Not quite stable |
| 12 | KHC₈H₄O₄ | 5.0 g/l of water | 1 cm | Not quite stable |
| 13 | CCl₄ | Pure, with no additives | 0.5 cm | Not quite stable |
| 14 | 1,4-diphenyl-butadiene | 4.24 mg/100 cm³ of ethyl ether | 1 cm |
*) On the use of these two substances, see also 2, 4, 5.
In Table I are given the components of the selected filters, with indication of their concentrations and thicknesses, and in Table II are shown the combinations of these components and the optical properties of the filters obtained. In Figs. 1, 2, 3 are given the transmission spectra of the components; in Fig. 4, the transmission spectra of their combinations.
Table II
Combined light filters for isolating individual regions of the ultraviolet spectrum
| Combined filter | Components used, Table I | \(\lambda_{\max}\) in Å | Notes |
|---|---|---|---|
| A | \(1+6+9\) | \(\sim 3650\) | If the receiver is not sensitive to \(\lambda > 6000\) Å, this filter may be replaced by a cobalt-nickel glass filter |
| B | \(2+7+10\) | \(\sim 3300\) | Instead of component 7, cobalt-nickel glass may be used |
| C | \(3+8+11+12\) | \(\sim 3150\) | Component 12 can be replaced by 2 mm of greenish glass |
| C′ | \(3+8+11\) | \(\sim 3130\) | |
| D | \(4+8+9+13\) | \(\sim 2620\) | Gives a narrow transmission band at 3470 Å |
| E | \(5+14\) | \(2550\) | Transmits \(\sim 1\%\) at 5700 Å, which can be corrected by adding glass No. 8 from Table I |
To realize the described light filters it is necessary to have: 1) a cuvette with quartz windows; layer thickness 5 cm; 2) a glass filter; and 3) a second cuvette with quartz windows and a layer thickness of 1 cm.
The cuvettes must be placed in the light beam in the indicated order, since the first cuvette is intended for solutions of inorganic salts in water. These solutions are sufficiently stable, and 5 cm of water absorb the infrared part. Thus the rays falling on the last cuvette will be considerably weakened, and the less stable substances placed in this cuvette will be in greater safety.
Fig. 2. 1—\( \mathrm{NiSO_4 \cdot 6H_2O} \), 240 g/l + \( \mathrm{CoSO_4 \cdot 7H_2O} \), 45 g/l in aqueous solution, \(d = 5\) cm; 2—transmission filter T-54 (9863 or 986) Corning, \(d = 3\) mm; 3—the same, old filter.
Fig. 3. 1—\( \mathrm{K_2CrO_4} \), 0.200 g/l water, \(d = 1\) cm; 2—cyanine-like dye (component 9, Table I) 20 mg/100 cm³ water, \(d = 1\) cm; 3—1,4-diphenylbutadiene, 4.24 mg/100 cm³ ethyl ether, \(d = 1\) cm.
Fig. 4. Transmission bands of combined monochromatic ultraviolet filters (see Table II).
When using the indicated combined light filters, one must strive to select appropriate light sources. Thus, for example, combinations \(A\), \(B\), and \(C\) will give a good effect with superhigh-pressure lamps (SVD-Sh). The combination \(D\) and \(E\) gives nothing in this case, since in SVD lamps the resonance line is completely self-reversed and an entire region near it is also absent. Light filter \(E\) can be used in combination with a low-pressure lamp (of the “bactericidal” lamp type or the short-wave ultraviolet lamp KUF). In this case the light filter can be greatly simplified, since the low-pressure lamps themselves emit almost all their energy in the form of the line 2537 Å.
PRK-2 or PRK-4 lamps may be combined with all the indicated light filters; the use of light filters \(D\) and \(E\), however, will be rather ineffective owing to the small amount of energy supplied by PRK lamps in the region \(\lambda < 3000\) Å.
A. A. Ilyina
CITED LITERATURE
- M. Kasha, JOSA 38, No. 11, 929 (1948).
- R. W. Wood, Phil. Mag. 5, 217 (1903).
- Collection of Works on the Biological Action of Ultraviolet Rays, ed. Prof. G. M. Frank, NKZ, Medgiz, Moscow–Leningrad, 1939. Article by Komarov.
- T. Dreisch u. W. Trommer, Zeits. f. physik. Chemie B37, 37 (1937).
- H. L. Bäckström, Naturwiss. 21, 251 (1933); E. I. Bowen, J. Chem. Soc., 76, 1935.
- G. Schwarzenbach a. K. Lutz, Helv. Chim. Acta, 23, 1139 (1940).