The present invention relates to an illumination assembly.
In many applications, the spectral characteristics of a lighting system are critical and may be required to meet certain specifications. One particular example of such an application is medical lighting. A large number of devices exist for medical lighting ranging from large aperture operating theatre lights to lights for general examination and simple tasks. The specifications of these devices are the subject of International standard IEC 60601-2-41:2000. The precise characteristics of medical lighting devices are important to a user, such as a surgeon, doctor or nurse.
Until recently, the characteristics required of medical lighting devices have been provided using light configurations based on, for example, tungsten halogen bulbs. These bulbs are usually used in combination with reflector elements to gather the light from the source and project it into a spot or well defined beam 0.5 m-1 m in front of the reflector aperture. In addition, by using heat filter elements in front of the reflector aperture and/or incorporated into the reflector coating, the majority of the infra-red component of the beam can be removed. Colour shift filters are also used to produce specific colour temperatures. For example, Schott Glass type KG1 can be used to shift a tungsten halogen source at a colour temperature of about 3200K up to a colour temperature of ˜4300K.
More recently, a number of manufacturers have started to produce medical lighting devices using high brightness light emitting diodes (LEDs). Commercial examples of these include the Iled® (Trumpf) which uses white, green and blue LEDs and the PENTALED® (Rimsa) which uses a small number of high power, high lumen output cold white LEDs. Other commercial devices use LEDs to mix in warm white, but the lumen output is low. Typically, however, these devices require a large number of LEDs to produce the requisite light output for medical lighting (e.g., typically 150 LEDs but often up to 300 LEDs for an operating theatre light). Moreover it is difficult to achieve a good colour rendering index (Ra and R9 in particular are usually low) because of the non-uniform spectral output (i.e., the spectrum has wavelength gaps). As a result of the large number of LEDs and associated hardware, the devices tend to be expensive with poor optical design and inefficient use of the LED light.
U.S. Pat. No. 6,636,003 discloses an LED arrangement which produces white light with an adjustable colour temperature. The arrangement includes one or more white LEDs and one or more coloured LEDs (e.g., amber or red and yellow) to produce an output with a desired colour temperature in the range 2500-5000K. The desired colour temperature is adjusted using first and second driver circuits to control the output of the white LEDs and coloured LEDs, respectively.
WO-A-01/36864, EP-A-1462711, US-A-2006/285323, EP-A-1568935 and Chenhua et al., Optical Engineering, vol. 44, 11 (1 Nov. 2005), 111307-1-111307-7 disclose various systems and methods for generating and modulating illumination conditions provided by lighting fixtures with a plurality of LEDs.