34 hard talk
A measure of
free trade
Robert Yeo,
to foster free trade. While the UK completed be life threatening. However, we do often see
its legal transition to SI units in 1995, a few misunderstandings in the parameters used to
partner at Pro- imperial units are still officially sanctioned. For describe the output of light sources. We get
Lite, on dealing
example, draught beer must be sold in pints, asked about measuring the ‘brightness’ of an
the distances shown on road signs must be in LED, but the trouble is that brightness in this
with conflicting
yards and miles and road speed limits must be context has no specific meaning. We could
given in miles per hour. take brightness to mean the total luminous flux
measurement The importance of a uniform system of of the emitter (which is measured in lumens),
units in photonics
weights and measures was recognised in the or we could assume that we are talking about
United States as long ago as 1790. For a variety the luminous intensity (candelas). Maybe the
of political as well as practical reasons, the US customer has a totally different understanding
A 250g piece of Roquefort purchased in has retained the British Imperial system to this altogether. Surprisingly, this seems to be
France should weigh exactly the same as a day. a widespread problem which probably
250g piece bought in New York. Of course, For those of us who routinely work with explains why we have seen so many scientists
the American version will be labelled as US suppliers, we become adept at converting and engineers enrol on our Practical Light
weighing 8 ounces and will be slightly more between Imperial and metric or SI measures. Measurement training course which we run in
expensive thanks to the punitive 300 per cent However, things can (and sometimes do) go association with the Photonics Cluster.
import duty imposed by the outgoing Bush disastrously wrong. Now a plea to the lighting industry:
administration. The point of this (somewhat Why do we persist in using a convoluted
cheesy) reference is that sharing specifications
‘Sharing specifications in an
correlation to specify the output of ‘energy
in an unambiguous way is a basic tenet of efficient’ lighting? We will all be familiar with
modern business, but this was not always so. unambiguous way is a basic the specifications printed on the packaging
For the origins of modern commerce we must of a compact fluorescent lamp (CFL), which
look to Napoleon Bonaparte, whose actions –
tenet of modern business’
reads something like: ‘This 12W CFL has the
in part – laid the foundations for free trade and same power as a 60W light bulb.’ Well, in
the modern day European Union. Take the case of the Mars Climate Orbiter. the first instance, this is factually inaccurate.
Further to developments originating with The spacecraft was lost in 1999 when it The 12W CFL has the same luminous flux
Bishop John Wilkins and the British Royal crashed onto the surface of Mars as a result of as a 60W incandescent. Second, there is a
om
Society in 1668, the metric system began in confusion between Imperial and metric units. perfectly good unit called the lumen that we
earnest in 1793, when a commission of the The navigation software for the spacecraft could use directly. In other words, a 12W
ooptics.c
French Academy of Sciences declared that the was developed at the NASA Jet Propulsion CFL produces the same number of lumens
ectr
standard unit of length would be called the Laboratory and assumed that the thruster as a 60W incandescent. So why not specify
.el
‘metre’, a word which derives from the Greek impulse data would be given in the metric light bulbs in the more meaningful unit of
www
metron, meaning ‘a measure’. The Academy Newton-seconds. However the contractor who lumens?
of Sciences recommended that the definition built the Orbiter provided the values in the So, a photometric panacea would be
be equal to one ten millionth of the distance Imperial units of pounds-force seconds, which when everyone adopts the metric system,
ch 2009
on a meridian between the North Pole and the caused the impulse to be interpreted as about a stops talking about the brightness of lamps
equator. quarter of the required value. and we can buy 2000 lumen bulbs from the
Britain had adopted the standard Imperial This brings me to photometry and the supermarket.
f
ebruary/mar
unit system in 1845 and the move to a metric measurement of light, an area in which my In conclusion, I quote Lord Kelvin who
system was agreed as long ago as 1965. The business specialises. We don’t often see said: ‘When you can measure what you are
European Union issued the Units of Measure problems resulting from confusion between speaking about and express it in numbers, you
o
optics
Directive in 1979, which sought to achieve Imperial and metric units and, in any case, know something about it.’ Just make sure you
ectr
el
a common system of weights and measures the consequences of a mistake are unlikely to stick to the metric system.
EOfeb09 pp34 Hard Talk.indd 34 17/2/09 16:54:30
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