Liquid Filters for the Visible and Near Infrared K. A. Ingersoll Bell Telephone Laboratories, Inc., Murray Hill, New Jersey 07974. Received 5 August 1971. Many experiments in spectroscopy require the use of optical filters. The type most commonly used are glass filters such as those produced by Corning Glass Company. In most applica- tions, glass filters are convenient to use for they come in a standard 5 cm × 5 cm size and are readily available. However, for some experiments this type of filter cannot be used for a variety of reasons. Excessive heating from a source of high intensity radiation may fracture the filter, and effective cooling is difficult. A geometry other than the standard 5 cm × 5 cm square is sometimes required, but fabrication of a number of glass filters of Table I. Cell Thickness and Liquid Composition Data December 1971 / Vol. 10, No. 12 / APPLIED OPTICS 2781
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Liquid Filters for the Visible and Near Infrared K. A. Ingersoll
Bell Telephone Laboratories, Inc., Murray Hill, New Jersey 07974. Received 5 August 1971.
Many experiments in spectroscopy require the use of optical filters. The type most commonly used are glass filters such as those produced by Corning Glass Company. In most applications, glass filters are convenient to use for they come in a standard 5 cm × 5 cm size and are readily available. However, for some experiments this type of filter cannot be used for a variety of reasons. Excessive heating from a source of high intensity radiation may fracture the filter, and effective cooling is difficult. A geometry other than the standard 5 cm × 5 cm square is sometimes required, but fabrication of a number of glass filters of
Table I. Cell Thickness and Liquid Composition Data
Fig. 1. Transmission curves of sharp cutoff liquid filters.
Fig. 2. Transmission curves of sharp cutoff Corning glass filters.
unconventional geometry is very costly, and here too there remains the possibility of fracture from excessive heat. There is also the inconvenience of disassembling equipment to change filters.
A suitable cell containing a liquid with appropriate optical properties would solve these problems. The liquid can be pumped through a heat exchanger for cooling, and the high cost of a number of specially designed filters would be eliminated. To change filter characteristics, one simply changes the solution. The need for such a filter prompted an investigation of the optical properties of different dye and solvent combinations. This paper describes some liquids that have been found to be useful as sharp cutoff filters in the visible and near ir. They are compared with Corning glass filters used for the same purpose.
The solvents used included carbon tetrachloride, methanol, trichloroethylene, and water. Carbon tetrachloride has good transmission (essentially transparent from 2650 Å to 12 μ), and it will dissolve many dye-containing substances such as wax crayons, grease pencils, and oil paints. However, care must be taken to contain its toxic vapors. Water, or course, is a useful filter in itself, absorbing radiation beyond 1.4 μ.
The liquids selected for the filters consist of NaNO2, K2Cr2O7,
and pure food colorings, with water as the solvent, and oil paint in carbon tetrachloride. Data were taken on a Cary 14 spectrophotometer, and Fig. 1 shows the transmission curves of the liquids. The cell thickness and composition of each liquid are listed in Table I.
It is seen from Fig. 1 that these liquids possess sharp cutoff characteristics and bandpass regions of high transparency. Furthermore, in the case of the food coloring dyes, the cutoff can be varied by changing the concentration. For example, by varying the concentration of the red dye, one can shift the cutoff of the filter from 5800 Å to 6400 Å (at 2% transmission) with very little reduction in transmission at longer wavelengths [see curves (13) and (8) in Fig. 1]. Similarly, the cutoff of the yellow dye can be varied from 4900 Å to 5340 Å, the green dye from 6700 Å to 7200 Å, and the blue dye from 6600 Å to 7200 Å. The red and green dyes have proven useful in the operation of an ir-pumped visible laser.1 Sharp cutoff filters consisting of NaNO2 and K2Cr2O7 dissolved in water and Grumbacher's oil paint dissolved in carbon tetrachloride are also shown in Fig. 1. The ratio of paint to carbon tetrachloride is not known, but the color of the solution is a pale yellow-green. The NaNO2 and K2Cr2O7 have played important roles in the attainment of continuously operating solid state lasers.2,3
Figure 2 shows transmission curves of sharp cutoff Corning glass filters. Comparison of Figs. 1 and 2 shows that the optical cutoffs of the liquid filters are comparable in sharpness with those of the glass filters. Furthermore, the liquid filters show little if any visible fluorescence under uv radiation, a serious drawback of some of the Corning glass filters. No fluorescence is observed from the liquid filters in the ir (to approximately 3 μ) when irradiated by high intensity visible light.
The investigation is continuing to find other liquids with desirable optical characteristics.
The author wishes to thank L. F. Johnson for his many helpful comments and M. D. Sturge for the use of the Gary 14 spectrophotometer.
References 1. L. F. Johnson and J. H. Guggenheim (to be published). 2. L. F. Johnson, G. D. Boyd, K. Nassau, and R. R. Soden,
Phys. Rev. 126,1406 (1962). 3. G. D. Boyd, R. J. Collins, S. P. S. Porto, A. Yariv, and W. A.
