The calibration of beam measurement devices in various electron microscopes, using an efficient Faraday cup

The exposure meter accessories for Siemens Elmiskop I and IA and Phillips EM 200 microscopes use the final screen as an electron collector. A robust and efficient Faraday cup is described, and this was used to calibrate the collection efficiency of these flat screens. The results in each case could be expressed as a linear function of beam voltage. A negligible fraction of the beam is scattered below the specimen level, so the beam current density at the specimen is obtained directly if the magnification is known. The Faraday cup has also been used in an AEI EM6G and in a Cambridge Instruments Mk 1 Stereoscan.