Most tablet counters use a light beam and simple photosensor to count individual parts. These counters increment by one each time the beam is interrupted by a falling tablet etc.
Errors occur if two or more tablets or capsules are in contact when they break the beam of light.
Kirby Lester tablet counters incorporate advanced photosensing to count clusters of tablets etc. These 'intelligent' systems can also recognize broken chips and fragments that are eliminated from the total. Some counters work on a weight principle, sampling five or ten parts that are manually counted and placed on a scale before the prescription is counted. The average weight of these parts is used to calculate the total count. Weight counters are generally less consistently accurate than optical counters because of the variable weight of each part.