Tunable Sharp Cutoff Liquid Optical Filter K. A. Ingersoll

Bell Laboratories, Murray Hill, New Jersey 07974. Received 2 April 1973.

Virtually every application involving optical radiation requires the use of filters to remove unwanted bands of ra­diation. A common requirement is the elimination of ra­diation below a particular cutoff wavelength λc and the transmission of radiation beyond λc. Recently a series of sharp cutoff liquid optical filters were described in which the cutoff wavelength varies from the uv to the NIR.1 ,2

For these liquids, the cutoff wavelength may be altered by changing the concentration of the dye or the thickness of the optical path. This paper describes an optical cell with a variable pathlength designed for use in conjunction with these liquid filters. A pathlength change from 1 mm to 14 mm changes the cutoff wavelength by typically 300 Å.

Fig. 1. Sectional view of liquid filter cell. This construction al­lows the cell to be mounted in any position.

Figure 1 shows a sectional view of the optical cell. A hollow piston, with a quartz window attached to one end, travels in or out turning the adjusting head (a threaded barrel) that is anchored to one end of the cell by means of a locking flange. As the piston is moved in, shortening the optical path of the liquid in the inner chamber, liquid is forced through small apertures into the outer chamber. Liquid pressure drives a movable metal ring in the outer chamber, providing increased space in the outer chamber for the incoming liquid. O-rings provide confinement of the liquid. For water-based filters,1 rubber O-rings were found to be satisfactory. For liquids employing carbon tetrachloride as the solvent,2 O-rings made of an inert material such as Teflon would be required. Access to the cavity for changing liquids is provided by a detachable quartz window on one end of the cell.

Figure 2 shows transmission curves for three representa­tive liquids. The bounds of the shaded areas correspond

Fig. 2. Representative transmission curves in the sharp cutoff region. Shaded areas represent the wavelength range covered by varying the optical pathlength of the cell from minimum to maximum. The three filters are composed of yellow, red, and blue food colors in water.1

July 1973 / Vol. 12, No. 7 / APPLIED OPTICS 1393

to minimum and maximum travel of the optical cell. The subsidiary transmission band, peaking at approxi­mately 4500 Å, is shown for the minimum pathlengthof the deep red filter; it does not appear in the maximum pathlength condition. This band may or may not be de­sirable, depending on the application.

In addition to its usefulness in optical experiments, a cell of this type, using the three basic subtractive colors, could possibly find application in color photography.

The author thanks L. F. Johnson for his many helpful comments.

References 1. K. A. Ingersoll, Appl. Opt. 10, 2781 (1971). 2. K. A. Ingersoll, Appl. Opt. 11, 2473 (1972).

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