Negative phase contrast

The situation is somewhat comparable to positive and negative phase contrast in optical microscopy. The lens imperfections have been used to Introduce a phase shift of jt/2 in the same way as the quarter wavelength ring in the optical microscope. Only beams passing through the "window in which the condition sin X =- I is met, interfere with the required phase relationship that causes the image to represent maxima in projected potential as dark areas. 

In addition, there are various technical corrections that must be made to the image data to allow an unbiased model of the structure to be obtained. These include correction for the phase-contrast transfer function (CTF) and, at high resolution, for the effects of beam tilt. For crystals, it is also possible to combine electron diffraction amplitudes with image phases to produce a more accurate structure (7), and in general to correct for loss of high resolution contrast for any reason by "sharpening" the data by application of a negative temperature factor (22). 

Matrix embedding material can sometimes be a negative The analyst may not be able to easily extract the from the matrix in order to use the method. Where possible, the matrix before the analysis, taking careful note of the loss of weight. Some common matrix materials are vinyl, rubber, tar, paint, plant fiber, cement, and epoxy. A further negative interference is that the asbestos fibers themselves may be either too small to be seen in Phase contrast Microscopy (PCM) or of a very low fibrous quality, having the appearance of plant fibers. The analyst s ability to deal with these materials increases with experience. 

Dispersed phase particle size measurements of acrylonitrile blends were made directly from phase contrast optical micrographs and from TEM negatives [172] with a similar system. 

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