In atomic spectrophotometers comprising a source of radiation characteristic of an element to be detected in a sample, a monochromator for selecting a desired spectral line from radiation received from the source by the sample, and a detector for producing an output signal from the monochromator output radiation, the monochromator is accurately tuned to the desired line to obtain a maximum detector signal, and simultaneously, the gain of the detector is adjusted to keep the signal magnitude within the best working range of subsequent signal processing circuits. In accordance with the present invention, both adjustments are made simultaneously by comparing an amplified detector signal with a desired reference signal level corresponding to the best signal magnitude, and using the difference between them to adjust the gain of the amplified detector signal in a closed loop control system to continuously equalize the amplified detector signal with the reference level. The monochromator is tuned, preferably by an automatic device, to seek a minimum value of amplifier gain, at which the detector output will be a maximum at the peak of the spectral line with the amplified detector output being simultaneously equal to the desired reference level. The amplifier gain is then set to the values so obtained by a digital-analog converter, and absorption measurements are made with the control loop open.