Ian Young, of Kelburn, asks :-

Recent light shows have had strings of large white translucent globes gently change colour through six colour ranges. How is this done?

Andy Edgar, a physicist at Victoria University of Wellington, responded.

Every strand of large white globes on the Telecom Christmas tree was interspersed with three strands of smaller lights, almost certainly made up from individual light-emitting diodes (“LED’s”) which are of fixed colour. They are commonly available in red, green , blue, and amber varieties and used for many lighting purposes. LEDs are based on two semiconductor materials which are in intimate contact (hence the “di” in diode). The semiconductors are not the silicon used in our computers, but chemical compounds of gallium, or indium, with nitrogen or phosphorous, for example. Electrons are injected from the power source into one side of the diode, but lose energy when they cross into the other material at the junction because they enter a different set of chemical bonds. This energy is emitted as light with a fixed relatively pure colour which depends on the particular compound.

The mystery is really how the larger white globes change colour. These cannot be conventional incandescent bulbs, which are based on heating fine tungsten wire with electrical power until the filament glows white hot. The only colour variation that can be achieved is by reducing the power until it glows only red hot, but then the total light output is vastly reduced and the lamp would be very dim.

Instead, it seems likely that the white globes in the telecom lights are of a new generation, with each globe containing multiple LEDs of different colours whose light output can be individually externally controlled by wireless or infra-red signals from a remote control, computer, or smart phone, resulting in a perceived output colour spanning the whole visible spectrum; the colour can be smoothly changed by mixing the proportions of light from each LE, and a pattern can be generated with a computer program. The frosting on the glass globe, and the close proximity of the LEDs in each globe compared to the distance to the viewer’s location, means the human eye cannot resolve the separate LEDs, the eye sees the combination colour, much like one sees with a TV set at the normal viewing distance.