Why coloured LED bulbs look dimmer than white ones

A coloured lamp of the same wattage looks much dimmer than a white one. The reasons lie in how differently the eye responds to colours and in the physics of coloured light.

Red and blue LED bulbs have been bought for a celebration and fitted in place of the usual white ones. The wattage is the same, yet the room suddenly goes dark, and when the blue lamp is on it becomes hard to make things out. The first thought is that the lamps are poor quality. In fact this is a natural property of coloured light, rooted both in how our eyes work and in how the lamps work.

The eye is not equally sensitive to all colours

Our daytime vision peaks at about 555 nanometres, in the yellow-green part of the spectrum. Towards the edges of the spectrum, deep red at one end and deep blue and violet at the other, sensitivity falls sharply. Blue light of the same energy looks far weaker to the eye than green light.

The lumen is calculated with exactly this sensitivity in mind. So even if a lamp converts the same amount of energy into light, the lumen figure is lower when that light is blue or red. In other words, a coloured lamp "looking dim" is not an illusion but a measurable reality.

White LEDs versus coloured LEDs

An ordinary white LED lamp consists of a chip emitting blue light with a phosphor layer on top. Its phosphor re-emits much of that blue across green, yellow and red, giving a broad white spectrum. Much of this spectrum falls where the eye is most sensitive.

Coloured lamps are built differently:

• Single-colour lamps emit a narrow spectrum. The light of a red or blue lamp sits where the eye is less sensitive.
• RGB lamps mix red, green and blue chips. When a saturated colour is produced, only one or two chips are working, so the full power is not used.
• In some coloured lamps light passes through a tinted bulb, which absorbs the unwanted colours and loses part of the light.

Which colours look "weakest"

Under the same conditions, colours are perceived roughly in this order, from brightest to dimmest:

• green and yellow: the brightest, close to the eye's most sensitive zone;
• orange and light blue or turquoise: medium;
• red: noticeably dimmer;
• deep blue and violet: the dimmest and the weakest at lighting a space.

It is also known that saturated colours can look "vivid" and "bright" to the eye, but that does not increase the amount of light available for seeing objects in a room.

Room surfaces play a part too

Coloured light reflects well only from surfaces of the same colour. A white wall returns red light as red, but a green wall absorbs most of the red light and looks dark and grey. In a room with coloured furniture, rugs and curtains, a coloured lamp seems even dimmer.

On top of that, objects lose their own colours under coloured light. A blue book looks black under red light. The eye loses contrast, and that strengthens the feeling of darkness.

How to use coloured lamps well

Coloured lamps are for atmosphere and decoration, not general lighting. To use them effectively:

• Add a coloured lamp alongside the main light rather than instead of it. Let white general light illuminate the room, and let the coloured lamp tint a wall, shelf or corner.
• Light white or pale surfaces. Coloured light on a white wall looks most striking.
• Do not compare wattage with white lamps. A coloured lamp's watts say little about how bright it will appear.
• For celebrations, restaurants, bars or shop windows, think of coloured light as a short-term effect.
• As a nursery night light, a soft orange or amber bulb is gentler than blue or green.

When it really is a quality problem

It is normal for a coloured lamp to look dim, but some signs can point to a genuine problem: two lamps from the same range differ clearly in colour or brightness, the lamp flickers, or its light fades noticeably within a short time. In such cases, check the lamp itself and the conditions it is connected in. If a coloured lamp is connected to a dimmer or an illuminated switch, for example, the cause of flicker may lie in the circuit rather than the lamp.

Zaferoğlu Elektrik's Zəfər İşığı range includes a 9-watt coloured LED lamp; using it as decorative light together with white general lighting gives the best result.