PRINCIPLES OF COLOR PHASE CONTRAST 101 



provided that no optical path difference exists between the conjugate 

 and complementary areas for some wavelength Xq and the dispersions 

 are such that the optical path difference is — Xi /4 for some wavelength 

 Xi and is +X2 '4 for some other wavelength Xo. Xo, Xi, and X2 are 

 related either by the inequality Xi < Xq < X2 or by the inequality 

 Xi > Xo > X2. The dispersion curves cross at Xq. It is assumed that 

 the conjugate and complementary areas are of equal thickness t, and 

 it is convenient to consider the reversal color difTraction plate in terms 

 of an example. 



Suppose that Xi < Xo < X2 and that, if a particle has an optical path 

 greater than that of the surround, X2 and neighboring wavelengths 

 contribute a bright-contrast image of the particle whereas Xi and 

 neighboring wavelengths form a dark-contrast image of the particle. 

 The ideal color phase contrast image would be obtained if the require- 

 ments for achromatic dark contrast were satisfied for all wavelengths 

 Xi ^ Xo — e, the requirements for achromatic bright contrast were 

 satisfied for all wavelengths Xy ^ Xo + 7- and the transition from dark 

 to bright contrast took place within a very narrow wavelength band 

 e + 7. This would mean that the difference between the refractive 

 indices of the substances constituting the conjugate and complementary 

 areas is proportional to the wavelength in the regions X^ ^ Xo — e and 

 Xy ^ Xo + 7 and that the phase-accelerating or phase-retarding function 

 of either area of the diffraction plate is reversed for each of these wave- 

 length regions; i.e., the two dispersion curves are effectively inter- 

 changed on either side of the band of wavelengths included between 

 Xo — e and Xo + 7. Therefore, in order for the conditions for ideal 

 reversal color phase contrast to be fulfilled, the dispersion curve for the 

 material with the lower index of refraction in the region of shorter 

 wavelengths must describe a refractive index that decreases with 

 increasing wavelength in the region X^- ^ Xo — e, then increases rapidly 

 by an amount approximately equal to Xq '4/ with increasing wave- 

 length from Xq — e to Xo + 7 and again decreases with increasing 

 wavelength (or remains constant) in the region X/ ^ Xo + 7. At 

 present, no material with such dispersion characteristics is known, 

 and it is impossible to realize the ideal reversal color phase plate. 



If two substances with ordinary dispersion properties are selected for 

 the reversal color diffraction plate, the material with the greater disper- 

 sion in the range X2 — Xi is deposited on the complementary area if the 

 optical path difference between the conjugate and complementary 

 areas is to change from — Xi '4 for Xi to X2/4 for X2. Suppose that the 

 band of wavelengths around Xi corresponds to a blue-green color and 

 that the band of wavelengths around X2 corresponds to the color red. 



