A compact, diode-array spectrophotometer has been designed to make measurements of the downward spectral flux on a horizontal surface, the limb brightness and the apparent brightness in the nadir direction from on-board the NASA ER-2 research aircraft. The instrument was included as part of the photochemical payload used for the Stratospheric Photochemistry, Aerosol and Dynamics Expedition (SPADE) of the NASA High-Speed Research Program. The spectrophotometer is based on a 1024-element, randomly-addressable Reticon photodiode array and makes measurements covering the 300 to 775 nm spectral region at a resolution of about 1 nm. Some aspects of the design and the performance of the instrument during SPADE will be presented. The direct-beam, spectral measurements are in good agreement with modelled data (MODTRAN).