Abstract: A method of measuring the efficiency of an LED (10) includes illuminating the LED with light and measuring a response of the LED (10) to the absorption of light in order to measure the quantum efficiency of the LED (10). The spectrum of the light at least partially overlaps the absorption spectrum of the LED. An LED assembly (40) includes first (42) and second (44) LEDs, and a driver (43). An absorption spectrum of the first LED at least partially overlaps an emission spectrum of the second LED. The driver includes a monitor(45) for monitoring a response of the first LED to the absorption of light emitted by the second LED; a current supply (47) for supplying current to the first LED;and a switch (49) for switching between (a) operating the first LED in an absorbing mode and the second LED in an emitting mode, and (b) operating the first LED in an emitting mode.

Abstract: An LED assembly (50) includes a plurality of LEDs including a first LED (52, 56, 60) of a first colour and a second LED (54, 58, 62) of a second colour. The assembly also includes a driver (53). The driver (53) includes a monitor (55) for monitoring a response of an LED, the absorption spectrum of which at least partially overlaps the emission spectrum of the first LED, and thereby obtaining an indication of the light output of the first LED. The assembly also includes a current supply (57). The driver (53) is configured to control the current supply to supply current to at least one LED of the plurality of LEDs in dependence upon the indication of the light output of the first LED whereby to maintain a desired colour mix of output light from the LED assembly (50).

Abstract: A substrate includes a diffracting structure providing a hologram (20, 6). The diffracting structure encodes a holographic image so that thatholographic image is produced in response to reference light being incident on a major surface of the substrate at an angle of incidence with respect to the said major surface of the substrate, wherein the angle of incidence is no more than 20°.

Abstract: A system for obtaining a propagation factor for determining the performance of a light beam (32) includes a light sensor (10), a lens element (30) operable to focus a beam from a light source to be tested towards the sensor element (10); wherein the lens element is a variable focus lens (30).

Abstract: A substrate includes a diffracting structure providing a hologram which encodes one or more holographic images. Every image of the one or more holographic images has a holographic location at either a finite or substantially infinite effective optical distance. The hologram is to be used as an ophthalmic eye chart or an eye exercise device or to provide a scale for measuring distances.

Abstract: A system for obtaining a propagation factor for determining the performance of a light beam (32) includes a light sensor (10), a lens element (30) operable to focus a beam from a light source to be tested towards the sensor element (10); wherein the lens element is a variable focus lens (30).