An opacity measuring apparatus and method for monitoring the density of effluent flowing through a furnace stack includes a collimated light source directed in a first light path through the stack and the effluent flowing therein, which impinges upon a photoelectric detector providing a first electrical signal related thereto. A second light path, provided by a light pipe disposed in a path external to the stack, directs the light to the photoelectric detector which provides a second electrical signal related thereto. Circuit means provide a third electrical signal related to the ratio of the first electrical signal to the second electrical signal. Also included is an electrical correction signal related to the drift of the electronic circuitry and unwanted contamination occurring in the first light path which is subtracted from the first and second electrical signals prior to obtaining their ratio and circuitry to display and automatically calibrate the apparatus over relatively long periods of time.

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