Colorimeter having an LED light source

A portable colorimeter, eg for analysing the haemoglobin content of blood samples, comprises a housing having an LED light source (20), a light detector (50), a sample chamber (120), an output display, and a power supply eg battery and/or mains. The light beam (10) from the source passes via an interference filter (180) through a solution (30) in a cuvette (40) to the detector (20). The detector converts the detected light Intensity into a corresponding frequency of electrical pulses which are fed to microcontroller. A further detector (80) detects any variation in the source output, eg due to temperature changes, and adjusts the current supplied to the LED via a servo loop to keep the source intensity constant. The sample chamber is closed by a removable plug (130) and a hinged lid (170).