The doctor will see you now…

There’s no getting away from it, lighting is a technical subject. But you don’t need a physics degree to understand it all – you need the doctor. Dr Gareth Jones, that is, one of the UK’s top experts. Send your questions to thedoctor@archive.luxmagazine.co.uk. Next!

I’m confused about the difference between lumens and candela – conventional light bulbs seem to be quoted in lumens, whereas MR16 products appear to use candela?

The easiest place to start is with the lumen. It is a measure of the rate that light visible to the human eye comes out from a product. A rate is something that is measured per second. If you switch the lamp on for one second and collected all the light (photons) coming from the product then you have measured the total luminous flux from the product which is given in lumens.

Conventional light bulbs emit their light in all directions and are best described using the lumens value. The candela (cd) is something different and in technical terms is the unit of measure of the intensity of the light visible to the human eye – the amount of light that is packed into a particular direction or angle. The more light that is packed into a particular angle, the higher the intensity or number of candelas.

MR16 lamps tend to emit in a cone of light and it is the intensity or ‘punch’ the lamp provides that is most important and therefore the candela measurement is most suitable. To convert between cd and lm you must know how the light is being emitted from the product. To do this you need a good measurement lab such as ours. The easiest way to cure any remaining confusion is to think of light like water from a hose. The candela is the intensity of the water coming from a hose – the narrower the hose the more intense the experience you get. Anyone who has been to a dodgy prison will know what I mean – that’s not to say I’ve been to one!

It is worth noting that the lumen we are talking about here is the real lumen and not the other lumen which tends to appear on the labels of cheap imported LED lamps. This figure can be up to five times higher than the real lumen value. We shall call these imaginary lumens and we encourage readers to tell us about their experience of these slippery units.

What is binning? I’ve heard some LED fittings providers talk about how they use good bins. Are there bad bins? How can you tell just by looking at aproduct ?

This is a complex area because it involves humans and the way in which they perceive colour. It is also affected by the way in which LEDs are manufactured. The CIE has a wealth of information about colour perception dating back to 1970 on its website cie.co.at

Binning is a major topic of conversation at the moment because a) humans are very good at seeing differences in the colour or chromaticity of light, and b) current LED production techniques produce a wide variety of chromaticities and the manufacturers want to sell as many LEDs produced as possible to avoid scrap.

Because so many LEDs of different chromaticity are produced, they must be parcelled up, or binned, into a selection of chromaticities within which the majority of people will not perceive a noticeable difference. The boundary regions tend to be sized based on a set of MacAdam ellipses and a set of quadrangles – which we can talk about in future editions.

Bins tend to be located around the black body (or Planckian) locus (BBL), which is not a voracious foraging insect responsible for mass devastation across parts of Africa, but a line that denotes pure white light as produced by a glowing body at different temperatures. It is a line in colour space because the hotter the body gets then the colder the light gets – obvious, yes?

Binning is shown schematically in the diagram using the bin structure of a well-known European LED manufacturer, courtesy of Osram Opto Semiconductors, in the CIE 1931 colour space representation. Good bins are close to the BBL. Bins above the BBL appear greenish and those below the line appear pinkish. Therefore putting LEDs from these bins next to each other can results in unpleasant viewing when shone onto a surface, and can give someone the appearance of being rather ill if used to illuminate your face.

I have heard people talk about colour rendering index and Ra of light sources – can you tell me what these terms mean?

The colour rendering index or CRI is a measure of a light source’s ability to render the colours of an object ‘realistically’ or ‘naturally’ compared with a familiar reference source, either incandescent light or daylight.

The colour rendering index is a set of ratios that provides a quantitative measure of the capability of a light source to reproduce the colours of various objects faithfully in comparison with an ideal or natural light source, such as the D65 ‘daylight’ illuminant, which is used to express the spectrum of light that corresponds to open air daylight under clear skies in western/northern Europe. This index consists of a set of 14 ratios as defined by CIE (15 with Asian skin tone also included as defined by JIS) which represent agreed test colour samples. The Ra value is the average of the first eight sample ratios that provides a single value approximation of the colour quality of the light source.

So, under the worst types of light source for colour rendering, such as the low-pressure sodium lamp, most colours are not rendered well at all resulting in a poor illumination quality.

Gaps in the spectra of a light source will lead to poor colour rendering. For example, shining orange low-pressure sodium light on a green apple will make the apple look grey because there is a shortage of colour in the light source that can reflect the green part of the spectrum from the apple. There is much debate about the use of the CRI scheme for new light sources such as LEDs, and newschemes such as the colour quality scales (CQS) and colour gamut are being explored to represent better the wider range of light sources available now.

Lighting is, however, a very complex set of applications. What represents high CRI lighting – with evenly spread colour rendition – may not be the best light source where accenting of brighter saturated colours might be needed, such asin retail applications.

Everything under control?

Simple on/off switching, dimming or fully networked controls? Dr Jones takes a look at the range of technological options available to specifiers

The wheat and the chaff

When it comes to light measurement, not all photometers are created equal. Which one should you use if you want to improve your luminous metrology?