Torr or 0.1–6.7 kPa) and high relative humidity (up to 100%). This was made possible by the development of a secondary-electron detector capable of operating in the presence of water vapour and by the use of pressure-limiting apertures with differential pumping in the path of the electron beam to separate the vacuum region (around the gun and lenses) from the sample chamber. The first commercial ESEMs were produced by the ElectroScan Corporation in USA in 1988. ElectroScan was taken over by Philips (who later sold their electron-optics division to FEI Company) in 1996. ESEM is especially useful for non-metallic and biological materials because coating with carbon or gold is unnecessary. Uncoated plastics and elastomers can be routinely examined, as can uncoated biological samples. This is useful because coating can be difficult to reverse, may conceal small features on the surface of the sample and may reduce the value of the results obtained. X-ray analysis is difficult with a coating of a heavy metal, so carbon coatings are routinely used in conventional SEMs, but ESEM makes it possible to perform X-ray microanalysis on uncoated non-conductive specimens; however some specific for ESEM artifacts are introduced in X-ray analysis. ESEM may be the preferred for electron microscopy of unique samples from criminal or civil actions, where forensic analysis may need to be repeated by several different experts. It is possible to study specimens in liquid with ESEM or with other liquid-phase electron microscopy methods. Transmission SEM The SEM can also be used in transmission mode by simply incorporating an appropriate detector below a thin specimen section. Detectors are available for bright field, dark field, as well as segmented detectors for mid-field to high angle annular dark-field. Despite the difference in instrumentation, this technique is still commonly referred to as scanning transmission electron microscopy (STEM). Color in SEM Electron microscopes do not naturally produce color images, as an SEM produces a single value per pixel; this value corresponds to the number of electrons received by the detector during a small period of time of the scanning when the beam is targeted to the (x, y) pixel position. This single number is usually represented, for each pixel, by a grey level, forming a monochrome image. However, several ways have been used to get color electron microscopy images. False color using a single detector On compositional images of flat surfaces (typically BSE): The easiest way to