 systems and are also commonly referred to as quantum detectors. DETECTOR METRICS There are two basic types of semiconductor based photodetectors, photoconductors and photodiodes, both of which are commonly characterized by the same general metrics. Those metrics include their responsiveness, noise level, response time, and detection sensitivity. Since these metrics are the same for photoconductors and photodiodes, let's identify them before examining each type of detector. Light Sources and Detectors RESPONSIVENESS The responsiveness of a detector represents the ratio of output in terms of voltage or current to the input in terms of the opti- cal power incident on the detector. Responsiveness is normally expressed in terms of volts per watt (V/W) or amperes per watt (A/W). As you might expect, a higher level of responsiveness is preferred. NOISE There are two types of noise associated with a photodetector: white noise and 1/f noise. White noise represents background noise gener- ated by the movement of electrons and has a uniform spectral power density. Some detectors have an additional noise component referred to as 1/f noise, whose spectral power density is inversely proportional to fre- quency. Because a higher signal-to-noise ratio (S/N) is more advanta- geous, it is important to obtain the former. To do SO requires a reduction in bandwidth that is normally accomplished by the use of filters. Because 1/f noise decreases with increasing frequency, this permits the S/N ratio to be maximized via light detection at a frequency sufficient- ly high that 1/f noise can be minimized. RESPONSE TIME The response time represents the time required for a detector to respond to a significant change in input optical power. A lower response time enables the detector to operate at a higher data transmission rate. Now that we have an appreciation for the basic met- rics associated with the performance level of a photodetector, let's dis- cuss each type of detector. PHOTOCONDUCTORS A photoconductor represents a type of semi- co