Lighting in Design

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19 LiD FEB/MAR 2016
and – as an imperfect dark body emitter – radiates
energy across a continuous spectrum. Only a very
small part of that emission (about 2 to 3 per cent)
is as visible light. The rest we experience as heat.
And it is that lack of efficient visible light produc-
tion that makes incandescent lighting so wasteful
of electricity.
There are two low-energy alternatives, only
one of which you probably see in people's homes.
Fluorescent lighting is produced by a tube of
mercury-vapour which releases ultraviolet light in
response to an electrical current.That UV causes a
phosphor coating on the inside of the glass to glow.
They’re about 7 to 15 per cent efficient.
LEDs are a solid-state p-n junction diode which
emits light as a result of an electric field applied to
a semi-conductor.They’re about 5 to 20 per cent ef-
ficient and remain expensive.There’s still a great deal
of lost energy in each of these alternative solutions.
In the world of solar panels (really just the re-
verse of a light bulb – using broad-spectrum light
energy to create electricity), researchers have been
developing mechanisms to improve the range of en-
ergy wavelength that can be absorbed by solar cells.
Perovskite-structured materials increase energy
absorption by over 20 per cent.These aremethylam-
monium lead halides dissolved in a solvent and then
coated onto a substrate using vapour deposition.
Then there’s lanthanide doping to improve conver-
sion of infrared photons into higher energy photons.
Or light-absorbing dyes such as ruthenium.
Only one problem: all these specialist materials
start to fail after 1000 K. And our incandescent is
most productive much closer to 3000 K.
Professor Marin SoljaČiĆ and his team began
working on interference systems sandwiched
around the incandescent emitter (for want of bet-
ter terms, they refer to these as the cold and hot
sides respectively).
‘In general,’ they declare, ‘the emissivity of a
high-temperature emitter depends on temperature
and wavelength.’ By surrounding the emitter with
an interference structure designed to transmit vis-
ible light and recycle infrared light across a wide
range of emission angles they hoped to improve
the efficiency of the system.
Simply put, all that radiated non-visible energy
can be transformed into visible light if we can only
absorb it in some way and recycle it back into the
emitter to reduce the initial energy required to heat
the light source. As a very bad analogy, consider how
much less energy it takes to heat water in a thermos
versus when the water is flowing past in a pipe.
‘An ideal lighting source would then be a hypo-
thetical high-temperature black body that emits only
visible wavelengths. Such an emitter would have
a luminous efficiency of approximately 40-45%,’
says SoljaČiĆ.
The problem, as mentioned earlier, is that exist-
ing incandescents are not ideal black body emitters
and the current approaches to absorption fail at
high temperatures.
What SoljaČiĆ and his teamwere looking for was
an interference stack, like a thin film, that could ab-
sorb a wide range of wavelengths and angles (since
incandescent light is not particularly linear), that is
able to efficiently reabsorb non-visible energy radia-
tion while being subjected to resistive heating, and
still be compatible with thermal emitters.
They selected four materials: SiO2, Al2O3, Ta2O5
and TiO2.
The next step was to create a film that would
act appropriately.They chose what is called a rugate
filter, which is based on a dielectric coating where
the refractive index varies continuously as part of
its structure. Dielectric coatings themselves are
thin-film or interference coatings composed of
sub-micron layers of transparent dielectric materi-
als. These can be placed by vapour deposition on
a substrate (like glass).
They are used to modify the reflective proper-
ties of a surface by exploiting the interference of
reflections from an optical source. At its simplest,
rugate filters have a sinusoidal oscillation of the re-
fractive index.This creates a notch filter (or multiple
notches), blocking a limited wavelength range and
reflecting it back onto the emitter.
There is a great deal of mathematics in the
SoljaČiĆ paper since, as you can imagine, fine-tuning
the properties of the materials and the rugate filter
(and its numerous notches) is supremely complex.
SoljaČiĆ and co then went ahead and built a
proof-of-concept. It’s actually quite pretty.
The rugate filter consists of ninety layers of
SiO2 andTa2O5 and the usual tungsten filament was
made to maximise reabsorption (making it into a
highly-polished and flattened radiator-like structure).
They then measured the luminous efficiency as
being 6.6 per cent, already significantly better than
existing incandescents and reaching the levels of
low-end LEDs.
Theoretically, they could achieve 40 per cent.
The more efficient the reflection of non-visible light
back to the tungsten filament, themore heat ismain-
tained in the system so the more energy-efficient it
becomes at converting heat into visible light.