This report describes an analog computer designed for calculations of transient absorption from photographed recordings of the oscilloscope trace of the transmitted light intensity. The computer calculates the optical density OD, the natural logarithm of OD, and the natural logarithm of the difference between the limiting optical density OD∞ and the actual optical density, and the reciprocal of the optical density. The calculated values are displayed on a digital voltmeter or recorded on an XY-recorder. Accuracy and linearity of the individual computing circuits are discussed and the overall performance of the system is demonstrated. The computer was especially developed for analysing oscilloscope photos from radiation-chemical pulsed radiolysis.