Perforated carbon film, 3.05mm grid

This specimen is one of the quickest and easiest to use for performance checks since it gives information about nearly all causes of loss of resolution.
The Agar Scientific perforated carbon films yield holes small enough to be imaged at an instrumental magnification of x200,000.

If these holes are not present in a grid square they will be found in an adjacent square.
The holes are round, with smooth unthickened edges. Perforated carbon films are thin films of carbon which have been treated to obtain a large number of small holes.

The examination of the Fresnel fringe around a hole when the objective lens is slightly defocused enables the astigmatism to be corrected.
The clarity of the fringe also gives information about the mechanical and electrical stabilities and available resolution of the instrument.
When higher resolution equipment is used, the slightly underfocused image of the carbon film itself is used for astigmatism correction.