An improved system and method for calibrating spectrophotometric instruments for the effects of light source drift. An instrument includes both received light and reference light paths to a light sensor where light signals are converted to electrical signals to be used by a processor. A shutter is used to block or allow passage of one of the light signals to the sensor. When the selected light signal is blocked, the sensor produces an electrical signal representative only of the other signal. When the selected light signal is allowed to pass, the sensor produces an electrical signal representative of both light signals. By knowing the value of one of the signals by itself, the value of the other signal can be determined. The individual values are then processed to produce an output signal that is a function of both light signals. Further, because only one light path must be blocked, simpler structures for blocking the light may be used.