adder 22 more particularly the data r from the multiplier 16 is produced as an r 2 signal the multiplier 17 multiplies the data r 2 from the multiplier 16 by the data r from the multiplexer 15 to produce a data r 3 the multiplier 18 multiplies the data r from the multiplexer 15 by the data r 3 from the multiplier 17 to produce data r 4 the output data r 2 to r 4 from the multipliers 16 to 18 are applied to the multipliers 19 to 21 respectively constants a1 to a3 corresponding to selected channels which are read out from a memory 23 and have been inputted to the multipliers 19 to 21 respectively these multipliers 19 to 21 multiply the input data by these constants to produce data a1r 2 to a3r 4 respectively these data from the multipliers 19 to 21 are applied to the adder 22 the channel data r is also applied to the adder 22 the adder 22 sums these data to produce data r 39 how to derive the constants a1 to a3 will be now described according to the poisson distribution a probability p of arrival of one or more photons after an occurence of a photon is given by the following equation 2 equ1 where m is the number of arrival photons and m an average rate of arrival photons for t sec w m is a probability that photons of m arrive the p also includes a false count rate and hence the counting coefficient k is given equ2 where n is the total number of photons arriving within the period t sec and m t t u00d7 n a relationship between the number of true photons and.