Abstract: A holographic grating spectrophotometer for detecting ozone and sulphur dioxide in the atmosphere is described which provides automatic calibration and which provides automatic linearity correction for the photomultiplier tube. Automatic calibration is provided by using a computer to control a stepper motor to move the grating so that the photomultiplier tube receives maximum intensity at the calibration wavelength of 302.1 nm from a mercury source. Automatic linearity correction is obtained by cycling a wavelength selection mask across exit slits located in the focal plane of the device and firstly combines separately taken counts of two different wavelengths and comparing this sum with the sum of counts of these wavelengths taken simultaneously. The difference is used to calculate photomultiplier tube deadtime and improve accuracy of the results.