How coloured and RGB LEDs create colour

A coloured LED makes its colour with the semiconductor itself, not with paint. RGB lamps mix three primaries. How both approaches work and where their limits lie.

The difference between a white LED lamp and a coloured one is not just the shade; it is how the light is produced. With an incandescent bulb, tinting the glass was enough to get coloured light: the unwanted part of the white light was simply absorbed. An LED, in most cases, produces the colour you want directly. That difference shapes both the efficiency and the look of coloured light.

Single-colour LEDs: the colour is in the material

An LED is a semiconductor crystal. When current flows, electrons drop to a lower energy level and release the difference as a quantum of light. The size of that energy step, the band gap, is set by the chemical composition of the crystal. The larger the energy, the shorter the wavelength, so the colour shifts toward blue.

In practice, different material families are used for different colours. Blue and green LEDs are mostly based on indium gallium nitride, while red, orange and amber LEDs are based on aluminium indium gallium phosphide. Adjusting the proportions within a family fine-tunes the shade.

Light from such an LED has a narrow spectrum; it emits almost a single pure colour. That is why a single-colour LED produces highly saturated, vivid colour without any filter.

White and softer shades: the role of phosphor

Most white LEDs are really blue LEDs. A yellowish phosphor layer is applied over the crystal: part of the blue light is converted in the phosphor into yellow-green and red, the rest of the blue mixes in, and the eye reads the result as white.

The same principle is used for coloured light too. Some coloured LEDs, certain green and amber versions for example, emit phosphor-converted light. Their spectrum is wider than that of a direct single-colour LED, and the colour looks a little softer. There are also lamps that take a cheaper route and place a coloured diffuser in front of white LEDs; part of the light is then absorbed, and the lamp is dimmer at the same power.

RGB: thousands of shades from three colours

An RGB lamp has three separate crystals in one module: red, green and blue. Because the eye senses colour through three types of receptor, mixing these three primaries in different proportions creates a wide palette. This is additive mixing:

• red plus green looks yellow;
• green plus blue gives cyan;
• red plus blue gives magenta-pink;
• all three together give light close to white.

The brightness of each channel is usually controlled by PWM (pulse-width modulation): the LED switches on and off many times per second, and the share of "on" time sets the brightness. At a high enough frequency the eye sees steady light; at a low frequency, flicker and bands on a phone camera can appear.

White made from three colours may look white to the eye, but its spectrum has gaps. Under such light the colours of objects, skin and food look distorted. Better lamps therefore add a fourth, separate white crystal; these RGBW lamps can give both coloured light and proper white light.

Why coloured light looks dimmer

Put a white lamp and a coloured lamp of the same power side by side, and the coloured one, especially blue or deep red, often looks dimmer. There are two reasons.

The first is the sensitivity of the eye. In daylight conditions the human eye is most sensitive to yellow-green light and much less sensitive to the edges of the spectrum, blue and deep red. The lumen unit takes exactly this sensitivity into account, so blue light of the same energy has fewer lumens than green light.

The second is how light spreads around a room. White light bounces off every surface. Coloured light is absorbed by surfaces of a different colour: under red light, a blue wall looks almost black.

Where and how to use coloured light

Coloured and RGB light is for mood and accents, not for general lighting. Practical advice:

• keep white light for reading, cooking and the desk, and treat coloured light as an extra source;
• festive decorations, bar counters, shop windows and facade details are where coloured light fits best;
• consider the colour of the surface being lit, since white and pale surfaces show colour best;
• the separate crystals of an RGB lamp can leave coloured fringes at the edges when seen up close; models with a frosted diffuser blend colour more evenly;
• for a night light in a child's room pick warm, soft shades; harsh blue light is a poor choice before sleep.

Besides its white-light range, Zəfər İşığı makes a coloured LED lamp meant for decoration.