All radiation measuring instruments consist of a radiosensitive detector and a means of recording the effects of radiation on the detector (i.e. the `response' of the detector) . Detectors respond to radiation by producing various physical effects which can be measured. Ionisation is one of these effects. The ion pairs can be collected to give an electrical signal which is related to the intensity of the radiation . Some detectors will emit light pulses in response to radiation and by counting the pulses the intensity of the radiation can be found. Others will store the effects of radiation over a long periods and can then yield the information at a later time. All of these devices in one way or another respond to the energy deposited in them by the radiation.
Instruments can be designed to indicate either the rate at which the radiation is being received, or the integrated amount over a certain time.
An ideal instrument would respond to all types and energies of radiation in terms of equivalent dose or equivalent-dose rate, i.e. it would be a universal equivalent dose meter. In practice it is difficult to realise such a device and most dose measuring instruments are compromises with some limitations on what type or energy of radiation they can measure. A good instrument should have the following characteristics
By using different types of shielding on the detectors and specially designed electronic circuits, instruments can be made to approximate the above characteristics fairly well.