It takes a mere three minutes to boil
an egg—but, ironically, it can
take two or three times longer than that for your stove to get hot
enough for cooking
in the first place. That's the trouble with electric
cookers: unlike gas, which makes a flame as soon as you turn it on, electric cooktops (hobs)
take quite a bit of time to get hot. Halogen cooktops give you the clean
convenience of electric cooking with all the speed and power of gas.
They make heat with a bright burst of red light—but how exactly do they work? How can you cook something with nothing but light?
Photo: An infrared halogen cooktop in action. You can
see a bright red light, but most of the energy this halogen "ring" is
pumping out is actually heat (infrared radiation).
There are four tungsten halogen tubes in a single "ring" like this, running in parallel
from one side to the other. (You can just see one at the top, currently unlit, running from top left to top right.) The thin white bar you can see running from top right to
bottom left is a metallic thermostat that switches the tubes on and off to keep the "ring"
at a steady cooking temperature.
Have you ever stopped to think about the science of cooking?
Probably not—but let's think about it now. We cook food to kill harmful
bacteria that may be lurking inside it, so the food becomes safe for us
to eat. Often food tastes much better cooked too; a light and fluffy
sponge cake tastes much more scrumptious than a bowl full of eggs,
butter, and flour! Pretty obviously, cooked food is usually hot, while
uncooked food is cold. The essence of cooking is to heat food over a
period of time that's long enough to kill the bacteria and transform
the food's structure or appearance. In short, cooking is all about
moving heat from the cooker into the food—but how do red lights
move heat?
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How does heat move?
If you know anything at all about heat, you'll probably know that it
doesn't like to stay in one place. Hot things tend to pass their heat
onto other things nearby—and they do this in one of three different
ways called conduction, convection, and radiation.
If you touch a car that's been standing out in the sun on a summer's
day, you'll feel the heat instantly. The heat from the hot metal flows
instantly into your hand by a process called conduction.
You may have heard the word conduction linked with electricity and
heat conduction is very similar: heat can flow through a material much
like electricity can. If you push a metal poker into a red-hot fire,
heat will flow into the poker from the fire by conduction. And it will
keep flowing into the poker until it's as hot as the fire around it.
Take the poker out of the fire and plunge it into a bucket of water and
you'll see a great whoosh of steam. Now heat flows from the poker to
the water, also by conduction. The process of conduction transfers heat
between two things that are in direct contact.
Not all heat moves by conduction. If you've ever heated a pan of
soup on top of your stove, you'll have noticed how it soon starts
bubbling. That's because heat is flowing through it by another process
called convection. The soup at the bottom of
the pan is closest
to the hot stove. It warms up and, because hot materials are less dense
(effectively lighter) than colder materials, it starts to rise upward.
When it gets to the top of the pan, it cools and falls back down.
Meanwhile, more soup is starting to rise up from the bottom of the pan
and take its place. So there's a constant pattern of warming and
cooling that gradually moves heat throughout the pan. This convection
process is how heat travels through fluids (liquids and gases) that are
near to something hot. The fluids carry the heat systematically away
from the heat source a bit like a conveyor belt.
Picture: Convection pumps heat into a saucepan
like a beating heart. The pattern of warming, rising soup (red arrows)
and falling, cooling soup (blue arrows) works like a conveyor belt that
carries heat systematically from the hot stove into the cooler soup
(orange arrows). This circulating pattern of liquid is called a
convection current.
There's a third way heat can move and it's called radiation.
If you've ever sat near a camp fire, you'll know heat beams out from
the fire, toward your face, in a direct line. The closer you sit to the
fire, the warmer you feel. If anything blocks the direct path between
the fire and your face—for example, if someone walks in front of
you—you'll notice the difference straight away. Heat radiation is
unlike both conduction and convection. It's unlike conduction because
you don't have to be touching the heat source (the fire) to feel the
radiated heat. And it's unlike convection because there doesn't have to
be any liquid or water in between to carry the heat toward you. We can
feel radiant heat from the Sun even though most of the vast distance
between us and that blazing star is empty space.
Photo: When you heat soup in a pan on a cooktop, you're seeing heat move by conduction, convection,
and radiation. Heat conducts from the hot pan into the soup. Heat moves from one part of the soup to another by both
conduction and convection. Stand anywhere near the pan and you'll feel heat being given off by radiation.
Infrared radiation is hot light
Why does sunlight feel hot if the Sun is sending out light? Sunlight
is actually a mixture of light and heat—of cool light we can see and a
kind of "hot light" we can't see called radiation.
All hot objects give off radiation in this way. A fire feels hot because a
steady stream of infrared radiation beams
out from the burning wood and coal and hits our face. Infrared radiation is similar to
visible light, but it has a longer wavelength
and a lower frequency.
In other words, the waves that carry it through the air or space are
bigger than light waves and arrive less often. Infrared radiation and
visible light (the light we can see) are two kinds of what we call electromagnetic
radiation. They're types of energy that travel out as an
up-and-down, wave-shaped pattern of electricity and magnetism. X-rays, radio waves, microwaves, and gamma
rays are other kinds of
electromagnetic radiation. Together, all these things make up what's
known as the electromagnetic spectrum.
Artwork: Infrared radiation is a type of light with a slightly longer wavelength and lower frequency than the light we can see (visible light). Artwork courtesy of NASA (follow the link for a bigger version of this image).
What makes one type of electromagnetic radiation different from
another? Like light waves, waves of infrared travel at the incredibly
fast speed of light: 300,000 km (186,000 miles) per second. It takes
just over 8 minutes for the Sun's light and heat to reach Earth, even
though it has to travel 149 million kilometers (93 million miles) to
get here! There's no real difference between red light and infrared
radiation. It's pretty much the same stuff. The difference is simply
that our eyes have evolved to see red (and other colors) of light, but
they cannot detect the lower frequencies in infrared. Other creatures
are built differently. Snakes have special pits in their face that can
detect infrared radiation. That means they can hunt at night, even when
there's little light about, by detecting the heat that nearby animals
give off.
How to cook with light
The Sun makes Earth warm and light by sending out a mixture of
visible light and invisible radiation. Electric light bulbs work the
same way. Old-style, incandescent lamps make light when electricity
flows through a thin, coiled wire called a filament.
The filament gets so hot that it glows white hot and gives off a bright
light. This is actually a very inefficient way of making light, because
most of the electrical energy the bulb uses is given off as heat and
wasted. Energy-efficient
lamps work an entirely different way:
they use fluorescence to make cool light by
crashing atoms
together. They use only a fraction of the energy of incandescent lights
because they produce very little waste heat.
Cooking with halogen
When you switch on a halogen "ring," the first thing you notice is
a dazzling, bright red light as the four tungsten halogen tubes
inside start pumping out a mixture of invisible infrared radiation and visible red light
toward your food. (Typically, depending on the setting, this gives cooking power between about 150W and 2000W and a temperature of about 700°C or 1300°F—plenty hot enough to cook food. but not so hot that it damages the glass ceramic
cooktop.)
Unlike with a normal incandescent lamp, where the light is the important thing and
the heat is wasted, it's the invisible, radiant heat we're interested in
harnessing here. The visible red light is the wasted bit,
although it does indicate, very usefully, the power level
of the cooking "ring"—the brighter the light, the more infrared heat
it's sending, simultaneously, toward your food.
The heat travels out from a halogen "ring" at the speed of light, instantly
beaming through the ceramic glass (vitroceramic) cooktop directly above it.
The glass is specially designed to transmit about 80 percent of the infrared radiation beaming through it,
though it absorbs most wavelengths of visible light (which is why it normally looks dark).
If you stand a pot on the glass, it warms up by a mixture of radiation and conduction: heat radiates into the pot from the filaments in the halogen tubes, but it's also transmitted into the pot by conduction from the hot glass just beneath. If you have soup in your cooking pot, it gradually warms up by convection just like with a conventional stove.
So, while it's true to say that halogen cooktops work using radiation, they
actually cook with a mixture of conduction, convection, and radiation.
Here are the key parts of a typical halogen "ring"—a patented British Thorn EMI design from the 1980s—and the same kind as the one in the very top photo. I'm following the numbering on the original artwork, though I've added colors to make it easier to understand:
Circular metal container tray: Made of aluminum, this is shallow and its upper surface is reflective so it bounces
light up onto the cooking pot above.
Microporous insulating material (typically Microtherm®) on the base of the tray.
Right supporting flange.
Left supporting flange.
Right flange end.
Left flange end. The tray (1) and flanges (3–4) are designed so they can expand slightly when they get
hot, and so they support the halogen lamps without breaking them.
Quartz, halogen infrared lamp with tungsten filament (four in total, each one providing up to 500W of cooking power).
The glass bulb of each lamp has a reflective coating on its underside so any infrared radiation firing downward is
bounced back up toward the cooktop.
Ceramic fiber packing.
Molded ceramic end cap.
Lamp holder tab.
Metal thermostat ("thermal limiter") expands and contracts as it heats and cools, pressing against microswitch.
Microswitch, controlled by the thermostat, turns halogen lamps on and off, keeping the cooktop at a steady temperature.
Advantages and disadvantages
Apart from instant heat, halogen cooktops have lots of advantages over old-fashioned cookers with electric rings. The cooking elements are fixed "inside" (under) the ceramic glass top, so the surface is flat and smooth and therefore notably easy to clean
(an advantage over gas burners). Since you rest your boiling pans directly on the flat cooktop, instead of balancing them (often precariously) on burners, it's also safer.
Cooktops like this are made from ceramics that don't conduct heat very well (they have low thermal conductivity), so although the cooking "rings" get hot, the areas around them stay relatively cool to the touch. The low thermal conductivity of the ceramic glass is also a bit of a disadvantage, however, because it means the cooking "rings" take more heating initially (to get heat traveling through the glass and into your cooking pot). The other big disadvantage is the obvious fragility of glass ceramic. Although it's toughened so it can withstand high temperatures and the weight of heavy cooking pots, if you drop a heavy pan on it, you'll be looking at a crack and a very expensive repair bill. But even if you're very careful, your cooktop can still scratch relatively easily.
How efficient are halogen cooktops? Up above, I talked about how old-fashioned electric lamps waste an awful lot of
energy getting hot: making heat instead of light. A glowing-red cooktop is doing exactly the opposite—wasting
energy making light instead of heat—although it's nothing like as inefficient. According to a 1989 article
in Popular Science, halogen cooktops produce 90 percent infrared and 10 percent visible light,
which would make them (at most) 90 percent efficient.
[1]
Photo: A typical halogen cooktop. You cook on top of a piece of thick glass ceramic (vitroceramic)—so keeping your cooker looking good is a fairly quick operation. You can clean it with an ordinary cream cleaner (slightly abrasive), wiped on and then rinsed off with a clean cloth. Really stubborn burned food can be removed with something like a Stanley knife blade pressed at a very shallow angle to the glass (be careful with your fingers). But be sure to read and follow the manufacturer's cleaning instructions.
Do halogen cooktops damage your eyes?
I've often heard people say that infrared halogen cooktops can damage your eyes, but is
there any truth in it? From the moment we wake up to the time we go to bed, our eyes are exposed to all kinds of light,
visible, infrared, and ultraviolet, and without a carefully controlled scientific study,
it would be hard to prove conclusively that any visual problems you suffer are caused by halogen cooktops; you're much more likely to damage your eyes accidentally staring at the Sun.
A quick search of Pubmed (the definitive database of scientific, biomedical articles)
turns up a couple of articles arguing that exposure to near-infrared radiation (the infrared closest in wavelength to
visible light) can indeed damage the lens of the eye, "denaturing" proteins (damaging them by restructuring), and causing cataracts. (For example, see Effect of infrared radiation on the lens by Eman Mohamed Aly and Eman Saad Mohamed, Indian J Ophthalmol. 2011 Mar-Apr; 59(2): 97–101.) How great is the risk? Bearing in mind that an infrared cooktop is generally covered up by a pan,
flashes on and off intermittently, and isn't something you stare at continuously, probably
quite small. Even so, it seems prudent not to stare at halogen cooktops up close, or for extended periods,
just as you don't—if you have any sense at all—stare straight at the Sun.
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Find out more
On this website
You might like these other articles on our site covering the science of cooking:
The New Kitchen Science by Howard Hillman, Houghton Mifflin, 2003. If you think cooking has a lot in common with science, you're absolutely right; this book explains the real science behind the foods we eat.
What Einstein told his Cook by Robert Wolke, W.W. Norton, 2010
and What Einstein Kept Under His Hat by Robert Wolke, W.W. Norton, 2012. An alternative look at the science of cooking, including the chemistry of ingredients and the physics of things like microwave ovens.
Articles
Israeli Startup Goji Is Planning the Oven of the Future by Iddo Genuth. IEEE Spectrum, February 6, 2017. Halogen hobs cook with electromagnetic waves; so too does a radically new oven, which works a bit like a microwave only using a much more controllable system based on radio waves.
Nathan Myhrvold's Recipe for a Better Oven by Nathan Myhrvold and W. Wayt Gibbs. IEEE Spectrum. June 30, 2014. This is a great review of how science could help us improve our cookers and stoves. It does mention halogen, though mainly in the context of halogen lamp ovens, rather than cooktops.
Efficient Cooking: It's All in the Pot by Tekla Perry. IEEE Spectrum. July 25, 2013. Adding a heat sink to the bottom of a pot greatly improves its efficiency when you're cooking by gas.
Cooking with Halogen by V. Elaine Gilmore, Popular Science, May 1989. A nice "retro" article from PopSci announcing the arrival of infrared cooking in North America homes over 20 years ago.
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