Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1987-08-17
1988-05-17
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250337, 2504841, G01T 111
Patent
active
047452730
DESCRIPTION:
BRIEF SUMMARY
The object of the present invention is a method and a circuit arrangement for luminosity measuring, over a range of at least six orders of magnitude, advantageously in "TLD" equipment.
In the field of luminosity measurements, photomultiplier tubes (PMT) are very often applied in scientific and technical practice.
For their operation, such multiplier tubes require high voltage (500-1500 V) power suppllies. The sensitivity of the tubes depends considerably on supply voltage, but in the case of a stable supply voltage the luminosity to current conversion ratio of the multiplier tube - that is, its sensitivity - is also stable (if constant temperature is assumed).
Other well-known devices are available for measuring luminosity, for example vacuum photodiodes, solid state photodiodes and phototransistors, but their sensitivity is 3-4 orders of magnitude lower than the sensitivity of multiplier tubes. Therefore, to detect very low level luminosities - for example 10.sup.-12 -10.sup.-13 lumen intensity - only multiplier tubes are suitable and applied at present. Luminosity levels such as this must be processed in scientific and technical practice, e.g. in the field of thermoluminescent dosimetry (TLD) or measuring signals with scintillation detectors.
But in these fields, not only do too low luminosity levels occur, but levels 8 orders of magnitude higher, say for example 10.sup.-4 -10.sup.-5 lumen magnitude, also occur. If a multiplier tube is set to sense the lower luminosity levels - which means greater sensitivity - it may become saturated in the case of too great a light intensity, that is, it "goes blind".
Moreover, because of the large output current, we have to apply a high current resistor-network, which means a needlessly high load for the high voltage power supply in most cases, and this heats the multiplier tube thereby reducing its stability.
The current output by the multiplier tube is usually displayed in digital form by means of analog to digital converters, which may be luminosity level sensing, but can also operate by summing the luminosity over a certain time interval, such as the method developed in thermoluminescent does measurements.
In the case of a digital display, generally 3-4 digits of readout are required because of the necessary 0.1% measuring accuracy. For a 3-4 digit display, we need to use a decimal point and display the exponent, in order to cover a measuring range of 6-8 orders. In view of this, the operation of the signal processing circuit must be appropriately modified. A convenient way of doing this is to change the conversion factor of the analog to digital converter; but this method does not prevent saturation of the multiplier tube if higher luminosity levels are applied.
The present invention seeks to solve the problem in "TLD" measurements by eliminating the saturation of multiplier tubes in the case of greater light intensities on the one hand, and by making possible the measuring of luminosity over more orders of magnitude on the other.
Thus the task to be solved by this invention may be considered to be the creation of a method and circuit arrangement to reduce the saturation or sensitivity of the multiplier tube by a well-defined factor, thus rendering it suitable for measuring light quantities over more orders of magnitude.
The invention is based on the concept that the sensitivity of the multiplier tube may be very precisely set by changing the input voltage, and it can be changed over more orders of magnitude, and in the case of switching from a higher voltage or sensitivity to a lower one, the multiplier tube can rapidly adjust to the lower sensitivity level. For example in thermolluminescent dosimetry the time required for light quantity measurements is about 10-30 seconds. The time required for switching is 10-20 milliseconds. If the worst case is presumed, the error caused by this is less than 0.1%. For example this error is invisible in the case of three-digit display and on the other hand the probable accuracy of "TL" dosimetry is not better than 0.5% so an
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V. V. Matveev et al., "Pribory Dlya Izmerenia Ionizirujuschikh Izlucheny", 1967, Atomizdat (Moscow), see pp. 394, 395, 398, 399.
G. D. Bakhtiarov et al., "Analogo-Tsifrovye Preobrazavateli", 1980, Sovetskoe Radio (Moscow), pp. 232-233.
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Feher Istvan
Szabo Bela
Vagvolgyi Jeno
MTA Kozponti Fizikai Kutato Intezete
Westin Edward P.
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