The optical properties of a thin holographic optical element (HOE) have been investigated. The Cardiff goniospectrometer was used to measure the spatial and spectral distributions of light transmitted by a HOE in the wavelength range 300 to 2100 nm. The spectrally resolved bi-directional reflection and transmission function (BRTF) was obtained for different angles of incidence. The angular resolution of the measurements is ±0.5°. The large amount of raw data was analysed and selected data are presented. The light transmitted through a HOE is split into a forward-scattered component and a colour image that is deflected through about 45°. The two components each contain different colours of the light spectrum depending on the angle of incidence. For a given angle of incidence, the spectral distribution of the transmitted light varies rapidly with the angle at which it is observed, especially in the colour image. The possible inaccuracy in the quantitative estimates of transmittance, caused by polarisation and second order effects are discussed. The results indicate that HOEs provide a method of separating the bulk of the visible light from the infrared part of the solar spectrum, thus providing a means of controlling daylighting and solar gain separately.