Window glass, termed thermotropic glass, and consisting of two panes with a temperature-responsive polymer gel between, was evaluated for the provision of natural lighting in an office environment whilst controlling direct solar radiation. The low temperature, sol form of the polymer was transparent, whilst the high temperature, gel form was translucent. Double glazing was used, comprising six combinations of float glass and thermotropic glass, with and without low emissivity coating. Glass surface and room temperatures were measured, and the office occupants completed a questionnaire to establish their opinion of the changes. The system was also subjected to mathematical modelling. Large energy savings were achieved by shading direct solar radiation and making efficient use of natural light, and the majority of the occupants considered that the office comfort was improved. 10 refs.