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Measuring light in lumens tells how bright a bulb will be

National Physical Laboratory (NPL) : 05 September, 2010  (Technical Article)
On 1 September 2010, EU legislation comes into force requiring lighting to be labelled in terms of lumens, instead of electrical watts.
An incandescent or tungsten bulb, a CFL (compact fluorescent lamp) bulb and an LED (light-emitting diode) bulb.

All three of these bulbs emit approximately the same lumens but require very
different watts. NPL also measures other properties of lamps, including the spectral colour components of white light it emits. The lumen is a measure of visible light produced by the lamp, whereas the watt measures the electrical power supplied. This change is a result of the EU's Eco-design Directive for Energy-using Products (EuP).

Watts vs lumens

On the packaging of light bulbs watts are a measure of the electrical power supplied to a light, whereas the amount of light (visible to the human eye) produced is measured in lumens.

Why change from watts to lumens?

Because it tells us what we actually want to know - how bright a light will be. Lumens give us an easy way to compare the actual amount of visible light produced by any given bulb. For example, you could buy a 10 watt LED light bulb which would look much brighter than a 10 watt tungsten light bulb. This difference in brightness can only be characterised by the bulbs' lumen output, i.e. how much visible light they are actually producing.

Recent years have seen leaps forward in energy efficient lighting technology. Lighting accounts for approximately 20% of global electricity usage, so these advances mean a dramatic reduction in carbon emissions, and new innovations will only improve the efficiency and quality of lighting.

However there has been confusion over how new products compare to old in terms of brightness, which could be impeding their uptake. This is partly due to brightness being based on the electrical power input, or wattage. When incandescent bulbs were all pervasive, 100 watt equated to a particular light output (approximately 1400 lumen), which people became used to, so it became a meaningful measure.

With various new light sources being developed and incandescent bulbs being phased out, this measure is no longer useful. A similar light output can be achieved from a 13 watt compact fluorescent lamp (CFL) as from a 60 watt incandescent bulb, the CFL only uses approximately a quarter of the energy. As we move towards LED lighting and other new sources, which produce light in different ways, it makes sense to label light according to its lumen output, so we can clearly compare different technologies.

What does this mean for consumers?

The shift to lumen labelling makes consumer choices, both on brightness and energy efficiency clearer. It also facilitates the introduction of new energy efficient products to market, as consumers have a meaningful way of comparing new products with old. This will help reduce emissions from lighting and have positive impact on the innovative UK lighting business.

Why is the lumen better?

There are those that argue that changing the system is confusing for those used to watts, and that the lumen is something we can't relate to. Naturally there will be an adjustment period, but the lumen offers advantages to make this worthwhile:

1.Clarity: the lumen provides a clear comparable system for understanding brightness and energy usage of all different light sources, and helps consumers make an informed decision about their purchase.

2.Adjustment period: Whilst we adjust to using lumens, the labelling will continue to use watts as a secondary measure, so people can still use the system they are used to if they want.

3.User-friendly: the lumen measures light output, which is easier for us to relate to than electrical input.

How does this affect lighting manufacturers?

Lighting manufacturers already measure their products in lumens, which is derived from the SI unit for luminous intensity - the candela, so current products are likely to continue to be manufactured as they are. However the new rules will make it easier to bring new products to market.

How does NPL fit into this?

NPL, as the UK's National Measurement Institute, has been developing and applying cutting edge measurement science since 1900. In the 1920s NPL, along with other laboratories, performed some pioneering work into how the human eye sees light. This work involved measuring how about 200 people's eyes responded to light. This was then averaged out and the eye's visual response to light defined in terms of the V(λ) function.

Fast-forward to the present day, and now lighting manufacturers need accurate measurements of their products' brightness in order to meet safety standards and regulations. For example, ensuring that there is the recommended level of lighting in a laboratory or classroom, adequate illumination of road signs, and the correct light output for emergency lighting. It is also necessary for ensuring the product fits its description.

NPL helps manufacturers do this through its photometric calibration services, which measure luminous intensity, as well as other metrics such as illuminance, luminance and correlated colour temperature. Measurements are performed using a high accuracy photometer, which emulates a standard human eye using a filter with a transmittance which maps onto the V(λ) function. NPL also supplies lamps to enable in-house calibrations. Another system allows the spectral output of light sources to be measured in all directions - this goniospectroradiometer is the basis of our measurement of lamps in lumens. All of this is traceable to the candela - the SI unit for the luminous intensity of light.

Ensuring traceability of light measurement to the candela also allows light companies to guarantee that their measurements are accurate and that their definition of the lumen is the same as everyone else's around the world, facilitating international trade.
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