We present a model describing the image formation in DIC (Differential Interference Contrast) mode microscopy, by including the actual refractive indexes and reflection coefficients of objects and substrates. We calculate the contrast of flat and level objects of nanometric thickness versus the bias retardation Γ and the numerical aperture NA. We show that high contrasts, of the edge and of the inner object, can be achieved in DIC mode with special anti-reflective substrates and large NA values. The calculations agree with contrast measurements on nanometric steps of silica and explain also the extreme ability to detect single molecules (stretched DNA molecules).
© 2008 Optical Society of America
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