A portable hand held instrument is capable of making accurate colorimetric measurements in the field. The instrument has a body which houses the electronic circuitry and a door which allows a vial having a solution to be measured to be placed therein. The vial is illuminated lengthwise by a light beam of two wavelengths, one of which serves as a reference. The light emerging from the top of the vial is spatially integrated and directed to a detector. The detector measures the intensity of the transmitted light, and the electronic circuit determines the difference between the logarithms of the detected signals. This difference is multiplied by a constant to produce a signal representative of the concentration and is displayed. The resolution of the measurement is automatically increased when the processor senses that the signals are too small.