Edison's Incandescent bulb
To understand an LED it's important to have an understanding of the original Edison incandescent bulb.
The incandescent bulb is powered by Alternating Current (AC) mains power. In AC current the electrons carrying the power move quickly backwards and forwards in a juddering action that conveys power along wires.
This is useful for incandescent bulbs because it is the friction (electrical resistance) of the electrons vibrating in the extremely fine tungsten filament that generates the heat required to glow red hot and emit light. This is also its downfall. The majority of the vibration energy (>95%) is emitted as heat and so is wasted, leaving only a tiny amount of useful energy to provide light.
Light Emitting Diodes (LEDs) work very differently. They are driven using Direct Current (DC) where the electrons travel along a wire in a continuous stream. In an LED there is an area that has a surplus of electrons and an area that has a deficiency. These positive charges are called “holes” as they represent the absence of an electron. Like two sides of a magnet being pulled together, the negative electrons and positive holes attract one another and so are drawn from their respective areas. They meet at the junction between the electron rich area (negative type material, n-type) and the electron poor area (positive type material, p-type).
At the so called p-n junction the electron and hole combine, the electron fills the hole's position and in doing so becomes less energetic. As it's energy level lowers, the lost energy is emitted as a visible photon creating the light we see.
The Light Emitting Diode
Creating white light
The colour of the light emitted depends on the semiconductor material used in the p-type and n-type areas. Red was created in the early 60’s, followed by yellow, green and finally blue in 1979. By the 90’s red LED had become efficient enough for use in car brake lights, and red, green and blue LEDs were sufficiently developed to allowed them to be combined, creating the first white LED light source.
To bring the efficiency of LEDs up to their current standard, a new method of creating white light has been invented. In the late 90’s phosphor layers were developed, which converts the light from highly efficient blue LEDs into a white light that can be tuned to suit the purpose, from a warm 2000k candle light to 8000k daylight.
Colour Rendering Index (CRI)
The colour rendering index (CRI) is a measure of how true the light emitted by a bulb is to the light of the sun. This is assessed by measuring the difference between the tested bulb and a reference bulb in terms of how they render eight colour samples.
A perfect score of 100 CRI is a light source that has exactly the same rendering of all colours as the sun. Tala bulbs all score above 95 CRI. This is due to the phosphor coating applied to the LED filaments that absorbs the few wavelengths of light emitted by the LED chips and re-emits this light across a tuneable wide rage of wavelengths, hereby achieving a light output that is close to the quality of light emitted by the sun. For more information on phosphors, click here.
Colour temperature (CCT)
Colour temperature is not to be confused with CRI. CRI is the quality of the colour components of the light emitted, while the colour temperature is the overall colour of the light as a whole. The phosphor filaments in Tala bulbs have been tuned to be provide a very warm 2200K, warm 2500K and brighter 2700K light. This covers the range of settings the bulbs are designed to be used in.
Typically LEDs have been manufactured to only 2700k. We think that to really capture the warmth of a dimmed Edison bulb it is important to bring this colour temperature down to 2200k for the perfect, dimly lit bar setting, and 2500k for a warm reading light.