Image contrast of biological materials in high-voltage electron microscopy

A new and simple expression has been formulated for the estimation of contrast of amorphous specimens in the tilted-beam mode of dark-field (DF) imaging in a high-voltage electron microscope. DF contrasts have been calculated for AIDS1.DNA, Salmonella typhimurium DNA, human haemoglobin and myoglobin of sperm whale in the range 0.1-3.0 MV and compared with the corresponding bright-field (BF) contrasts. DF contrast is seen to be significantly greater than BF contrast for a wide range of microscope apertures (alpha) and voltages (phi). The ratio of DF to BF contrast was observed to increase with increasing phi and alpha for all the aforesaid specimens.