A cell counting device for estimating the number of photoactive cells in a sample, the device comprising: a chamber for receiving a sample; at least one light source to generate a plurality of pulses of light and to emit the plurality of pulses of light towards a section of the chamber, wherein the section of the chamber comprises a sub-sample of the sample; a light detector to receive light emitted from the sub-sample at time intervals less than the duration of each pulse of light to form a light signal in response to receiving the at least one pulse of light and to generate an electronic signal associated with the received light; and a controller configured to: calculate and store a variable fluorescence [F v] value of each sub-sample in the predetermined number of sub-samples, by estimating a minimal fluorescence [F o] of each sub-sample using regression analysis of a first part of the light signal, estimating a maximal fluorescence [F m] of each sub-sample using regression analysis of a second part of the light signal, and subtracting F o from F m for each sub-sample; calculate the mean of the stored F v values, and store the distance from the mean of each F v value to obtain a plurality of distances; and integrate the distribution of the plurality of distances to provide an estimate of the number of photoactive cells in the sample. a chamber for receiving a sample; at least one light source to generate a plurality of pulses of light and to emit the plurality of pulses of light towards a section of the chamber, wherein the section of the chamber comprises a sub-sample of the sample; a light detector to receive light emitted from the sub-sample at time intervals less than the duration of each pulse of light to form a light signal in response to receiving the at least one pulse of light and to generate an electronic signal associated with the received light; and a controller configured to: calculate and store a variable fluorescence [F v] value of each sub-sample in the predetermined number of sub-samples, by estimating a minimal fluorescence [F o] of each sub-sample using regression analysis of a first part of the light signal, estimating a maximal fluorescence [F m] of each sub-sample using regression analysis of a second part of the light signal, and subtracting F o from F m for each sub-sample; calculate the mean of the stored F v values, and store the distance from the mean of each F v value to obtain a plurality of distances; and integrate the distribution of the plurality of distances to provide an estimate of the number of photoactive cells in the sample. calculate and store a variable fluorescence [F v] value of each sub-sample in the predetermined number of sub-samples, by estimating a minimal fluorescence [F o] of each sub-sample using regression analysis of a first part of the light signal, estimating a maximal fluorescence [F m] of each sub-sample using regression analysis of a second part of the light signal, and subtracting F o from F m for each sub-sample; calculate the mean of the stored F v values, and store the distance from the mean of each F v value to obtain a plurality of distances; and integrate the distribution of the plurality of distances to provide an estimate of the number of photoactive cells in the sample.