3 types of EDM instruments
- Microwave instruments
- Visible light instruments
- Infrared instruments
Microwave instruments
These instruments come under the category of long range instruments ,where in the carrier frequencies of the range of 3 to 30 GHs(1GHz=10^9)enable distance measurements upto 100 km range .Tellurometer come under this category.
phase comparison technique is used for distance measurement.this necessitates the erection of some form of reflector at the remote end of the line.if passive reflector is placed at the other end,a weak signal would be available for phase comparison.hence an electronic signal is required to be erected at the reflecting end of the line.
2.visible light instruments :
These instruments use visible light as carrier wave,with a higher frequency,of the order of 5×10^14Hz.since the transmitting power of carrier wave of such high frequency falls off rapidly with the distance,the range of such EDM instruments is lesser than those of microwave units.A geodimeter comes under this category of EDM instruments
3.infrared instruments
The EDM instruments in this group use near infrared radiation band of wavelength about 0.9micrometer as carrier wave which is easily obtained from gallium arsenide infrared emmiting diod.these dood can be very easily directly amplitude modulated at high frequencies,thus modulated carries wave is obtained by an inexpensive method.due to this reason ,there is predominance of infrared instruments in EDM.wild distomats fall under this category of EDM instruments.
