Considerations for the quantitative analysis of coated reliefs.

The contrast visible in a coated relief specimen viewed by transmission electron microscopy is generated by the angular difference between the direction of contrast coat evaporation and the direction from which the relief is viewed. This contrast can often be exploited for the computer reconstruction of specimen relief from optical density data if the contrast angle is known. If an additional backing coat is present, the relief must be viewed along the direction of backing coat evaporation, and may therefore require reorientation by the use of a rotate/tilt holder. Despite attainment of correct orientation, undercutting - multiple intersections of imaging electrons with different regions of contrast coat - may yet prevent valid reconstruction. Although the danger of experiencing undercutting is high, "vertical" coating, in conjunction with minimal tilting, could be used when maximum relief information is required from a unique specimen. Bidirectional contrasting may also be useful for increasing the information capacity of relief coats.