System and method of extending the linear dynamic range of event counting
A method and apparatus for photon, ion or particle counting described that provides seven orders of magnitude of linear dynamic range (LDR) for a single detector. By explicitly considering the log-normal probability distribution in voltage transients as a function of the number of photons, ions or particles present, the binomial distribution of observed counts for a given threshold, the mean number of photons, ions or particles can be determined well beyond the conventional limit.
1. An apparatus for measuring a large dynamic range of light intensity, comprising:
a pulsed light source;
a detector for receiving light energy;
an analog-to-digital converter having a sampling rate synchronized with the pulsed light source, digitizing a voltage output of the detector;
a processor configured to accept output data of the analog-to-digital converter for a plurality of light pulses and to process the output data so as to compute an average-count-type data value obtained by threshold processing of the detector output, and an average signal-type-data value obtained by averaging the detector voltage output for the same received light pulses,
wherein the processor is further configured to use the average-count-type data value where the average-count-type data value is less than about 1 and to use the average-signal-type data value where the average count-type data value is greater than about 1 as a measure of the light intensity.
2. The apparatus of claim 1 , wherein the transition point is selected based to optimize a signal-to-noise ratio over the range of the count-type data and the average-signal-type data threshold value is a peak voltage output of the detector when there is no light from the light source incident on the detector.
3. A computer program product, stored on a non-transient computer readable medium, comprising:
instructions for operating a processor of a photon counting system so as to perform a measurement of a value of light intensity incident on a detector by performing the steps of:
synchronizing analog-to-digital conversion of a voltage output of the detector with a light source such that analog-to-digital conversion is performed at a time where the voltage output from the detector is expected and outputting analog-digital-conversion data values;
accepting analog-to-digital conversion (ADC) data values;
processing the accepted ADC data values so as to compute an average-count-type data value and an average-voltage-type data value, where the average-count-type data value is the average number of ADC output data values above a threshold value per ADC conversion and the average-voltage-type data value is the average of the ADC output data values above the threshold value per ADC conversion and the average-count-type data value and the average-voltage-type data is computed for the same group of ADC conversion output data values,
wherein the average-count-type data value is used to represent the light intensity value when the average-count-type data value is less than about 1 count per pulse, and the average-signal-type data value is used where the average-count-type data value is greater than about 1 count per pulse; and,
outputting the light intensity value on a display device.
4. The computer program product of claim 3 , wherein a plurality of accepted data is stored prior to performing the process.