VARIABLE COLOR-AMPLITUDE PHASE MICROSCOPY 165 



producing a phase contrast image (Frangon and Nomarski, 1950a). 

 (See also Section 8 for a description of Frangon's phase contrast system.) 



9.6. Variable color-amplitude phase microscopy 



Barer (1950) has suggested a modification of a system of variable 

 phase microscopy in order to introduce color differences between the 

 deviated and the undeviated light. The proposal is to replace the 

 polarizer or analyzer with a type of dichroic filter or combination of such 

 filters. A propert}^ of such a dichroic filter is that the spectral trans- 

 mission varies with the plane of polarization of the light passing through 

 it. For example, a combination of filters may be constructed such that 

 if a polarizing element is also in the path of light, a band of wavelengths 

 that appears green is transmitted with one orientation of the polarizer; 

 but, if the polarizer is rotated 90°, the band of wavelengths appears red. 

 For a given orientation of the polarizer with respect to the filter, the 

 color of the transmitted light is the same whether the light is incident 

 first on the polarizer or first on the filter. Therefore, if the diffraction 

 plate contains zonal polarizers made of polarizing film oriented at 90° 

 and if the filter described in the example is placed in front of the light 

 source, the filter can be adjusted by rotation with respect to the zonal 

 polarizers so that the light emerging from the conjugate area of the 

 diffraction plate is red and the light emerging from the complementary 

 area is green. A rotation of the filter through 90° causes a reversal in 

 color between the two bands of wavelengths transmitted through the 

 conjugate and complementary areas. The formation of a phase con- 

 trast image depends on the overlapping of wavelengths within the two 

 bands of color. If light of the same wavelength is present in the bands 

 emerging from the conjugate and complementary areas, then the deviated 

 and the undeviated light of the wavelength interferes, as in the standard 

 phase microscope, to form an image that is colored (bright contrast) 

 or black (dark contrast). The remainder of the light from the comple- 

 mentary area, composed of wavelengths which cannot interfere with 

 light from the conjugate area, superimposes an image that is colored and 

 is analogous to the image formed with darkfield illumination. The 

 remainder of the colored light from the conjugate area is spread over the 

 field of the eyepiece. Similarly, if such a combination of dichroic 

 filters replaces the analyzer instead of the polarizer, the polarized light 

 emerging from the zonal polarizers is incident on the filter, and again 

 rotation of the filter changes the composition of the band of wavelengths 

 transmitted to the eyepiece by the filter from the conjugate and comple- 

 mentary areas. 



