Goniospectrometer measurements of the optical performance of a holographicoptical element

The optical properties of a thin holographic optical element (HOE) have bee n investigated. The Cardiff goniospectrometer was used to measure the spati al and spectral distributions of light transmitted by a HOE in the waveleng th range 300 to 2100 nm. The spectrally resolved bi-directional reflection and transmission function (BRTF) was obtained for different angles of incid ence. The angular resolution of the measurements is +/-0.5 degrees. The lar ge amount of raw data was analysed and selected data are presented. The lig ht transmitted through a HOE is split into a forward-scattered component an d a colour image that is deflected through about 45 degrees. The two compon ents each contain different colours of the light spectrum depending on the angle of incidence. For a given angle of incidence, the spectral distributi on of the transmitted light varies rapidly with the angle at which it is ob served, especially in the colour image. The possible inaccuracy in the quan titative estimates of transmittance, caused by polarisation and second orde r 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 sol ar spectrum, thus providing a means of controlling daylighting and solar ga in separately. (C) 2000 Elsevier Science Ltd. All rights reserved.