A calibration/zero control is combined with an electro-optical measurement instrument in a time-multiplex fashion to provide a self-diagnostic in situ measurement system and automatic compensation of the entire electro-optical train without interruption of the measurement process. The electro-optical instrument includes a laser which provides a source of substantially monochromatic electromagnetic radiation (laser beam) and optics for collection and focusing radiation onto a detector. The radiation is directed through a sampling volume where it impinges upon an object whose properties or physical characteristics are to be measured by scattering techniques. The calibration/zero control includes an element rotatably positioned in the path of the electromagnetic radiation, optically upstream of the sampling volume, in having an axis of rotation perpendicular to the optical axis such that the radiation is periodically blocked and sampled. The laser source is synchronously pulsed to fire in accordance with the position of the calibration/zero element. The calibration/zero element is constructed such that in one blocking position, a predetermined fraction of the laser beam is allowed to penetrate and pass through the element to the collection optics and detector. The analog voltage of the detector is time-multiplexed to develop diagnostic information including a calibration signal, a zero radiation signal and a measurement signal. All signals are developed under the control of a timing signal derived from the calibration/zero control.