A NEW METHOD FOR OBTAINING VARIABLE COLOR PHASE CONTRAST

The paper under review*) describes a simple and convenient method for obtaining variable color phase contrast. The phase plate is made in the form of a metallized mirror of black glass, in the center of which, as usual, a narrow unmetallized strip is left. The mirror is inclined at the Brewster angle. Illumination is carried out through a polarizer \(P\) (Fig. 1).

The direct rays fall on the unmetallized strip of the mirror, and the diffracted rays on the metallic mirror. If, after the phase mirror, an analyzer is installed, then by changing the angles of rotation of the polarizer and analyzer one can obtain variable—both positive and negative—phase contrast. Let us now place between the polarizer and the phase mirror a quartz plate \(Q\), cut perpendicular to the optical axis, and rotate the polarizer so that the plane of polarization of the light makes an angle of \(45^\circ\) with the plane of incidence of the rays. The corresponding diagram is shown in Fig. 2. The direction \(Ox\) is perpendicular to the plane of incidence, and the direction \(Oy\) is parallel to this plane. Direct \((OD_1)\) and diffracted \((OD_2)\) rays with vibrations in the direction \(OP\) fall on the plate \(Q\). If the thickness of the plate \(Q\) is equal to \(1.88\ \mathrm{mm}\), then the plane of polarization of yellow light \((\lambda = 0.555\ \mu)\) is rotated by \(45^\circ\), and the direct \((OJ_1)\) and diffracted \((OJ_2)\) rays with vibrations in the direction \(OJ\) will fall on the phase mirror. The ray \(OJ_1\), upon reflection from the glass (Brewster angle!), is completely extinguished, while the ray \(OJ_2\), reflected from the metal, is preserved (dark field). For rays of another wavelength, the rotation of the plane of polarization by the plate \(Q\) will occur through another angle:

The corresponding diagrams are shown in Fig. 2. Upon reflection from the glass mirror, the direct ray gives only the component in the direction of the axis \(Ox\) \((OR'_1, OB'_1, OV'_1\), respectively). The diffracted rays, being reflected from the metallic mirror, acquire elliptical polarization, the degree of ellipticity depending on the direction of polarization of the incident ray. Placing after the phase mirror an analyzer transmitting vibrations in the direction \(A_1\) (Fig. 2), we obtain for rays of different

) M. Françon, Rev. d’Optique 32*, No. 10, 557 (1953).

colors have different phase contrast. (For some colors it will be positive, for another part of the colors—negative.) Turning the analyzer to position \(A_2\) will lead to a reversal of the phase contrast. Thus, by varying the angles of rotation of the polarizer and analyzer, it is easy to change the resulting color phase contrast.

Fig. 1.

Fig. 2.

Let us note that the variables here are not only the phase shifts between the direct and diffracted rays, but also the relative intensities of these rays. This creates broad possibilities for changing the conditions of observation of the object, and thereby for selecting the optimal conditions for its visibility.

G. Rosenberg