the single wavelength light beam 22 the intensity and wavelength of the light beam 22 are controlled by computer 20 through a monochromator controller 32 such as may be constructed from a slosyn model m061 ff 411c3a stepper motor and modulynx iod010 motion controller both manufactured by superior electric co 383 middle street bristol conn 06010 the fractional transmission of light spot 30 through the gaas wafer 10 is measured by a cooled germanium detector 40 and coupled to a detector readout amplifier 42 for input to computer 20 the cooled germanium detector 40 is a 3 mm diameter planar p n germanium junction mounted to a to 5 header with an infrared transmitting window and a peltier junction cooler to reduce thermal noise the combination of germanium doping materials temperature and window material can give a response that is linear with intensity over six decades of intensity to enhance the stability of the measurement the infrared light beam 22 may be passed through a mechanical chopper before passing through the gaas wafer 10 so that narrow band synchronous amplification can be used this also minimizes noise the relationship between transmission and etch pit density is complicated by the fact that the etch pits can overlap especially if the pits are large and their density is high the relationship can be expressed mathematically as.