Metrological support to IR flash photometry
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UDC 535.24.15:621.378.325
Various equipments employ pulsed IR lasers (1.5-10.6 ~), and these require metrological support as regards the parameters and characteristics of these light sources, in particular the optical pulse shape. It is impossible to use the external photoelectric effect in transducers, in contrast to the visible and near-infrared wavelength ranges, while there are virtually no photodiodes of cutoff wavelength greater than 1.5-2 ~, so various photoresistors are employed, which are far inferior in lag to the devices used in the visible and near-IR ranges (photomultipliers, photodiodes, and so on). Also, photoresistors tend not to be interchangeable on account of details of the manufacturing technique, while the lag is substantially dependent on the working conditions, and in addition any photoresistor showing photoconduction will show some response from any exposure to visible light, and so on. Consequently, there is no metrological support to means of measuring the inertial characteristics of photoresistors, which has substantially retarded the development of IR techniques. Modern science and technology impose increasingly rigid specifications on the response rates of measuring equipments. It is already necessary to provide for measuring the parameters of IR laser pulses in the submicrosecond or nanosecond range (pulse lengths of i0 -~i0 -*~ sec), and this must be done with high accuracy. The time parameters of the optical pulses are comparable with the lag parameters of most photodetectors. The working time range can be extended by measuring the complex transfer coefficient [K(jm)] and afterwards recovering the input optical signal from the measured output signal. This involves processing a fairly large volume of data, which has to be presented in digital form; in such a case, one does not determine the individual parameters of the input signal but an entire set of values representing the shape of the input pulse. As a rule, the output signal from such a device is in analog form; it is virtually impossible to digitize this directly at the present time, since the nanosecond range is involved. Intermediate conversion is required, e.g., stroboscopically, which is applicable for repetitive signals, or else filming with subsequent interpretation (for one-shot or rarely repeated signals). The latter type of signal has recently been recorded with storage tubes, where the subsequent readout can be in digital form, while vidicon transmitting tubes with digital output to computers have also been employed. The processing must be performed by computer, which complicates the measurement considerably. However, the rapid advances in computing have led to the production of small fourthgeneration computers, while dedicated computing devices will soon be incorporated into many means of measurement to provide automatic data processing. In that case, the increased complexity of the means of measurement does not result in any essential extra complexity in the measurement procedure, but instead will facilitate simplif
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