 No commercial instrument is as yet (by 2009) available in conformity with all the principles of an optimal design, so that any further limitations listed are characteristic of the existing instruments and not of the ESEM technique, in general.The ESEM can also be used in transmission mode (TESEM) by appropriate detection means of the transmitted bright and dark field signals through a thin specimen section.This is done by employing solid state detectors below the specimen, or the use of the gaseous detection device (GDD).