Abstract: A dual beam spectrophotometer has a blazed concave reflective grating 25, formed using the interference fringes of a coherent light beam reflected back from a spherical convex mirror 73 having a center of curvature at a point 65. The point 65 is off the Rowland circle 79 of the grating 25, so as to provide a different surface of focus 81 at the position of a photodiode array 29 which detects the spectrum formed by the grating 25. This permits the angle of the photodiode array 29 to be selected for good linearity of the distribution of wavelength of the spectrum along the array while maintaining the spectrum in focus over the whole length of the array 29. In order to compensate for remaining non-linearities, the width of barrier zones 93 between adjacent diodes 49 in the photodiode array 29 may be varied, so that the pitch of the diodes 49 varies with position along the array 29.