An improved transmissometer of the type which has a visible light source and is responsive to peak and mean spectral responses between 500 nm and 600 nm and optical assembly therefor contains a solid state light source preferably a light emitting diode. The light source emits a low level light beam that is split, part of which is passed through a gaseous sample then directed to a detector while the other part is directed to a second detector. Electronic components are provided to amplify the signal that has passed through the sample providing high gain, low noise amplification and use signals from the detectors to control the light source and determine opacity of the sample.