WHAT could make tomatoes grow bigger, cameras take better pictures and cars
run more efficiently? A new polymer film that blocks infrared light, says 3M in
St Paul, Minnesota.
Contrary to popular belief, letting hothouses get hot makes life difficult
for horticulturists. When infrared light passes through the glass the
temperature rises and forces the grower to open the windows to cool down the
greenhouse. Besides being a nuisance, this also loses all the carbon dioxide and
moisture the grower has just pumped in to help the plants grow.
Paul Hadley at Reading University鈥檚 School of Plant Sciences has been testing
3M鈥檚 new plastic film on tomatoes. Placed over the greenhouse glass, it reflects
back certain infrared wavelengths. Early results show an increase in the size of
fruit鈥� probably due to their ripening in a cooler environment with more
CO2.
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Another interesting thing happens when you block infrared, says Hadley.
Plants use far-infrared light to detect other plants in their vicinity. If they
don鈥檛 sense light in this range, it usually means other plants have filtered it
out. To compete for available light the plant grows faster and taller.
This very specific range of light, between 730 and 740 nanometres, elicits
what鈥檚 called a 鈥減hytochrome鈥� reaction in plants, says Hadley. 鈥淚f you remove
light around that range, you cause the plants to be dwarfed much more.鈥� 3M鈥檚
film does precisely this at low levels of light, which could also explain the
tomatoes鈥� size. There is a huge potential in films with these characteristics,
he explains, because it would avoid using chemical growth regulators.
3M鈥檚 new film is made of hundreds of very thin layers of perspex and
polyethylene terephthalate pressed together. 鈥淭he number of layers determines
the range of frequencies reflected, while their thickness determines the
specific frequencies,鈥� says 3M polymer scientist Andrew Ouderkirk. Different
layers reflect different light frequencies at their boundaries, producing
constructive and destructive interference.
Other types of infrared reflectors, says Ouderkirk, have far less control
over which frequencies are reflected. 鈥淭here is no other way of blocking
infrared without blocking visible light,鈥� he says. 鈥淢aking the layers with the
kind of precision that we need is not simple.鈥�
Besides horticulture, the film, which is slightly thicker than food wrap,
could also make a good laminate for car windshields, says Ouderkirk. If the
amount of infrared inside the car were reduced, drivers would use their air
conditioning less and get better fuel efficiency.
Silver coatings on layers of thin glass have similar properties, says Charles
Carniglia of Optical Coating Laboratories in Santa Rosa, California. But he says
that such products don鈥檛 scale up easily for broad applications. 鈥淏ecause they
can fabricate this new material in a unique way, it鈥檚 an important product,鈥� he
says.
The film will also be good on cameras, says Ouderkirk. Cameras use light
sensors to adjust their aperture. 鈥淒uring the day, this isn鈥檛 a problem,鈥�
explains Ouderkirk. 鈥淏ut if you鈥檙e taking photos at dusk or dawn they can easily
be fooled.鈥� This is because there is usually much more infrared light than
visible light then. 鈥淪o the camera is fooled into thinking there is more light
than there actually is.鈥� This makes it set the exposure for a higher light
level, producing underexposed pictures.