Ever wondered what all those figures and diagrams on light bulb packages actually mean?

We’re here to be your guiding light on all such matters and help you make the right decision about which light bulb to buy so you get just the effect you want. After all you don’t want to end up with a bedroom that’s lit like a dentist’s surgery.  This short guide fills you in on the different scales used to measure the brightness, colour (warmth) and colour rendering ability of your light bulbs as well as showing the different shapes available.


In the old incandescent light bulb days, the watt of a bulb was synonymous with it’s brightness, even though watts are actually a measure of power. So, for larger rooms and living rooms where bright light is needed, the standard was a 100w or even a 150w bulb. 40w and 60w bulbs were used for smaller rooms and those that needed something less bright like bedrooms and also for table and standard lamps.

With the advent of energy-saving bulbs, and new bulb technologies such as halogen, CFLs (compact fluorescent bulbs) and LEDs, a more uniform way of describing brightness has been developed since they use a lot less power to produce the same amount of light. The new scale used is called ‘Lumens’ and the more lumens the brighter the light.

The chart below shows the lumens output for the old-fashioned incandescent bulb and for the three types of energy-saving bulbs.

Lumens Chart

As a rough rule of thumb, 400 lumens is about right for a bedside table lamp, but for a lounge you’ll probably need between 1,500 and 3,000 lumens – they don’t all have to come from the same bulb.

Colour of Light (Kelvin scale)

Along with the brightness, white light also has another quality which is based on its ‘warmth’ or colour. Light bulb manufacturers have come up with a ‘colour-coding’ system (see Colour of White Light)which helps buyers understand the colour of the light emitted by the light bulb.

The scale used to measure this is called the Kelvin scale, which is actually a measure of temperature rather than colour, (hence using the term warmth), but to the human eye a light is warmer if it has more red/orange hue, and colder if it has more green/blue hue. We associated cold colours with the kind of harsh bright light that you get in hospitals and dental surgeries – so not the kind of light we’re looking for at home!

Most of us have been ‘conditioned’ by the warm light given out by the old incandescent light bulb which is 2,700 on the Kelvin scale. The dimmest light we’re all familiar with is probably candlelight – which is around 1,600k. Sunset and sunrise come in about about 2,500k whilst midday sunlight is about 5,500k.

The colour of the light impacts everything from your mood and alertness, to the colour of the vegetables on the chopping board, so it’s important to understand this scale so you choose the right light bulb for the job. The diagram below shows the range:

Kelvin Chart
Notice that the midday sun, whilst very bright has a lower score than a blue-sky day. This is because the Kelvin scale is not measuring brightness (thats the Lumens score) but the warmth of the light. It can be confusing eh?

CRI Value

There’s one last measure that manufacturers use to describe the quality of the light emitted by a light bulb. This is the CRI value.

CRI stands for ‘Colour Rendering Index’ and measures how well the light in question actually ‘renders’ a specific colour. In simple terms this is a measure that will tell you if a light bulb makes a tomato look a proper red colour rather than a bit orange, or a lettuce look a healthy green rather than a yellowish lime colour.

Traditional incandescent light bulbs and halogens score in the high 90s. LEDs and CFLs are more likely to score in the 80s. A score of 80 or over is considered acceptable, although this is likely to be a matter of personal taste. When it matters is when you need more accurate colour rendering. If you are an artist or creative professional you may need to be sure of your colours, likewise if you’re intending to use a light bulb to accent a painting where ensuring the light represents the colours correctly will be essential.

Bulb Shapes

Finally, bulbs can now come in all sorts of different shapes and sizes. Each of the shapes provides a slightly different light diffusion. Contrast the almost 360 degree spread of a globe or golf bulb with the narrow beam of a spotlight. This is where knowing what effect you want to achieve is essential.

Also consider what the light fitting will look like when the light bulb is off – it’s important to make sure that the bulb fits sensibly and doesn’t protrude.