Abstract: An arrangement in vehicle headlamps which are intended to emit a significant proportion of ultraviolet light. The invention is characterized by a control circuit (1) for adjusting, via means (3; 5, 9, 10), the intensity of the ulraviolet light emitted by the headlamp (6); vehicle speed sensing means (2) intended to deliver to the control circuit (1) an electric signal corresponding to prevailing vehicle speed; and further characterized in that the control circuit (1) is constructed to decrease said intensity to a predetermined, lower level when the speed lies beneath a predetermined value, and to increase the intensity to a predetermined, higher level when the speed lies above a predetermined value.

Abstract: The sensitivity of an opto-acoustic absorption detector is increased to make it possible to measure trace amounts of constituent gases (approaching 0.01 parts per billion) in a sample by creating a second beam radiation path through the sample cell identical to a first path except as to length, alternating the beam through the two paths and minimizing the detected pressure difference for the two paths while the beam wavelength is tuned away from the absorption lines of the sample. Then with the beam wavelength tuned to the absorption line of any constituent of interest, the pressure difference is a measure of trace amounts of the constituent. The same improved detector may also be used for measuring the absorption coefficient of known concentrations of absorbing gases.