Robert Yeo, partner at Pro-Lite, on dealing with conflicting
measurement units in photonics
A 250g piece of Roquefort purchased in France should weigh exactly the same as a 250g piece bought in New York. Of course, the American version will be labelled as weighing 8 ounces and will be slightly more expensive thanks to the punitive 300 per cent import duty imposed by the outgoing Bush administration. The point of this (somewhat cheesy) reference is that sharing specifications in an unambiguous way is a basic tenet of modern business, but this was not always so. For the origins of modern commerce we must look to Napoleon Bonaparte, whose actions – in part – laid the foundations for free trade and the modern day European Union.
Further to developments originating with Bishop John Wilkins and the British Royal Society in 1668, the metric system began in earnest in 1793, when a commission of the French Academy of Sciences declared that the standard unit of length would be called the ‘metre’, a word which derives from the Greek metron, meaning ‘a measure’. The Academy of Sciences recommended that the definition be equal to one ten millionth of the distance on a meridian between the North Pole and the equator.
Britain had adopted the standard Imperial unit system in 1845 and the move to a metric system was agreed as long ago as 1965. The European Union issued the Units of Measure Directive in 1979, which sought to achieve a common system of weights and measures to foster free trade. While the UK completed its legal transition to SI units in 1995, a few imperial units are still officially sanctioned. For example, draught beer must be sold in pints, the distances shown on road signs must be in yards and miles and road speed limits must be given in miles per hour.
The importance of a uniform system of weights and measures was recognised in the United States as long ago as 1790. For a variety of political as well as practical reasons, the US has retained the British Imperial system to this day.
For those of us who routinely work with US suppliers, we become adept at converting between Imperial and metric or SI measures. However, things can (and sometimes do) go disastrously wrong.
Take the case of the Mars Climate Orbiter. The spacecraft was lost in 1999 when it crashed onto the surface of Mars as a result of confusion between Imperial and metric units. The navigation software for the spacecraft was developed at the NASA Jet Propulsion Laboratory and assumed that the thruster impulse data would be given in the metric Newton-seconds. However the contractor who built the Orbiter provided the values in the Imperial units of pounds-force seconds, which caused the impulse to be interpreted as about a quarter of the required value.
This brings me to photometry and the measurement of light, an area in which my business specialises. We don’t often see problems resulting from confusion between Imperial and metric units and, in any case, the consequences of a mistake are unlikely to be life threatening. However, we do often see misunderstandings in the parameters used to describe the output of light sources. We get asked about measuring the ‘brightness’ of an LED, but the trouble is that brightness in this context has no specific meaning. We could take brightness to mean the total luminous flux of the emitter (which is measured in lumens), or we could assume that we are talking about the luminous intensity (candelas). Maybe the customer has a totally different understanding altogether. Surprisingly, this seems to be a widespread problem which probably explains why we have seen so many scientists and engineers enrol on our Practical Light Measurement training course which we run in association with the Photonics Cluster.
Now a plea to the lighting industry: Why do we persist in using a convoluted correlation to specify the output of ‘energy efficient’ lighting? We will all be familiar with the specifications printed on the packaging of a compact fluorescent lamp (CFL), which reads something like: ‘This 12W CFL has the same power as a 60W light bulb.’ Well, in the first instance, this is factually inaccurate. The 12W CFL has the same luminous flux as a 60W incandescent. Second, there is a perfectly good unit called the lumen that we could use directly. In other words, a 12W CFL produces the same number of lumens as a 60W incandescent. So why not specify light bulbs in the more meaningful unit of lumens?
So, a photometric panacea would be when everyone adopts the metric system, stops talking about the brightness of lamps and we can buy 2000 lumen bulbs from the supermarket.
In conclusion, I quote Lord Kelvin who said: ‘When you can measure what you are speaking about and express it in numbers, you know something about it.’ Just make sure you stick to the metric system.