Castor and Pollux - Gemini LEDs

Colour temperature of illuminating white light is the most important parameter affecting the visual appearance of illuminated objects. Natural sunlight from a cloudless sky at noon has an approximate colour temperature of 5500 Kelvin. This, together with the spectral make up of sunlight, is a benchmark against which man-made light sources are compared. White light from LEDs is generated by exciting a phosphor with a blue-emitting LED chip. Properties of both the phosphor and the chip determine the spectral characteristics and colour temperature of light emitted by such LEDs. Traditionally, white LEDs have been available that emit either cool or warm white light. Cool LEDs generate bluish-white light with higher colour temperatures (6500 Kelvin or more) whereas warm LEDs generate yellowish-white light with lower colour temperatures (4000 Kelvin or less). Lighting designers can choose either of these two types of LEDs for designing solid-state lighting systems. The resulting products provide either cool or warm white light suitable for specific applications. There are many areas where a more flexible approach can be beneficial. Retail lighting is an example, where the ability to change the colour temperature can be of great use. This is because different objects are best seen in light of different colour temperatures. Adjusting this parameter can, therefore, help display objects at their highest visual appeal. One such illumination system can replace several different luminaires operating at different colour temperatures. Other areas where variable colour temperature lighting is of great utility include photography, museum illumination and lighting for art galleries. The pictures below show the striking effects obtained by changing the colour temperature of white light illuminating a painting. The picture on the left was taken with illumination set at 6800 Kelvin while the picture on the right was taken with the same light source set at 2800 Kelvin. The cool white illumination causes the little girl’s dress and the sky to get emphasis (picture on the left) whereas the warm white illumination shifts the emphasis to the flowers in the field (picture on the right).

This ability to highlight different visual elements in real world objects is a powerful tool for enhancing and accentuating the visual appeal of almost any illuminated object. Variable colour temperature lighting can also be used with advantage in in-door building lights. The flexibility of varying colour temperature can have great consumer appeal as it opens up another dimension in light control that goes beyond just the ability to change the intensity of light.

Stereochromic lighting systems that use two channels of white light to synthesize light of different colour temperatures can be made from a combination of cool and warm LEDs. The change in colour temperature is brought about by changing the proportion of light from each type of LEDs. With appropriate control it is possible to alter the colour temperature with minimal change in brightness. Electrospell’s Gemini LEDs are particularly suitable for this task. The Gemini set comprises a pair of matched LEDs specifically designed for stereochromic illumination. The two Gemini LEDs are called Castor and Pollux. Castor is a cool white LED that emits light at approximately 2800 Kelvin. Pollux is a warm white LED that emits light at approximately 6800 Kelvin. Together this pair can be used to generate most colour temperature points in between these limits with very small change in apparent brightness. The Gemini LEDs will be available from July 2012 in both power LED and low power SMD 3528 packages.