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Differential interference contrast (DIC) microscopy, also known as Nomarski Interference Contrast (NIC) or Nomarski microscopy, is an optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples. DIC works on the principle of interferometry to gain information about the optical path length of the sample, to see otherwise invisible features. A relatively complex lighting scheme produces an image with the object appearing black to white on a grey background. This image is similar to that obtained by phase contrast microscopy but without the bright diffraction halo. DIC works by separating a polarized light source into two orthogonally polarized mutually coherent parts which are spatially displaced (sheared) at the sample plane, and recombined before observation. The interference of the two parts at recombination is sensitive to their optical path difference (i.e. the product of refractive index and geometric path length). Adding an adjustable offset phase determining the interference at zero optical path difference in the sample, the contrast is proportional to the path length gradient along the shear direction, giving the appearance of a three-dimensional physical relief corresponding to the variation of optical density of the sample, emphasising lines and edges though not providing a topographically accurate image. ==The light path== ''1. Unpolarised light enters the microscope and is polarised at 45°.'' :Polarised light is required for the technique to work. ''2. The polarised light enters the first Nomarski-modified Wollaston prism and is separated into two rays polarised at 90° to each other, the sampling and reference rays.'' :(詳細はNomarski prism causes the two rays to come to a focal point ''outside'' the body of the prism, and so allows greater flexibility when setting up the microscope, as the prism can be actively focused. ''3. The two rays are focused by the condenser for passage through the sample. These two rays are focused so they will pass through two adjacent points in the sample, around 0.2 μm apart.'' : The sample is effectively illuminated by two coherent light sources, one with 0° polarisation and the other with 90° polarisation. These two illuminations are, however, not quite aligned, with one lying slightly offset with respect to the other. ''4. The rays travel through adjacent areas of the sample, separated by the shear. The separation is normally similar to the resolution of the microscope. They will experience different optical path lengths where the areas differ in refractive index or thickness. This causes a change in phase of one ray relative to the other due to the delay experienced by the wave in the more optically dense material.'' :The passage of many pairs of rays through pairs of adjacent points in the sample (and their absorbance, refraction and scattering by the sample) means an image of the sample will now be carried by both the 0° and 90° polarised light. These, if looked at individually, would be bright field images of the sample, slightly offset from each other. The light also carries information about the image invisible to the human eye, the phase of the light. This is vital later. The different polarisations prevent interference between these two images at this point. ''5. The rays travel through the objective lens and are focused for the second Nomarski-modified Wollaston prism.'' ''6. The second prism recombines the two rays into one polarised at 135°. The combination of the rays leads to interference, brightening or darkening the image at that point according to the optical path difference.'' : This prism overlays the two bright field images and aligns their polarisations so they can interfere. However, the images do not quite line up because of the offset in illumination - this means that instead of interference occurring between 2 rays of light that passed through the same point in the specimen, interference occurs between rays of light that went through ''adjacent'' points which therefore have a slightly different phase. Because the difference in phase is due to the difference in optical path length, this recombination of light causes "optical differentiation" of the optical path length, generating the image seen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「differential interference contrast microscopy」の詳細全文を読む スポンサード リンク
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