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Infrared element glowing red on a halogen ceramic cooktop

Halogen cooktops (infrared hobs)

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.

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  1. Why cook things anyway?
  2. How does heat move?
  3. Infrared radiation is hot light
  4. How to cook with light
  5. Cooking with halogen
  6. What's inside a halogen cooktop?
  7. Advantages and disadvantages
  8. Do halogen cooktops damage your eyes?
  9. Find out more

Why cook things anyway?

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.

Animation showing how heat circulates in convection

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 of lentil soup on a cooktop.

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.

Electromagnetic spectrum by NASA.

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.

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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.

What's inside a halogen cooktop?

Components of a typical halogen cooking element from US patent 4,864,104.

Artwork from US Patent 4,864,104: Heating assembly using tungsten-halogen lamps by Peter W. Crossley et al, Thorn EMI, 1989, courtesy of US Patent and Trademark Office.

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:

  1. 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.
  2. Microporous insulating material (typically Microtherm®) on the base of the tray.
  3. Right supporting flange.
  4. Left supporting flange.
  5. Right flange end.
  6. 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.
  7. 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.
  8. Ceramic fiber packing.
  9. Molded ceramic end cap.
  10. Lamp holder tab.
  11. Metal thermostat ("thermal limiter") expands and contracts as it heats and cools, pressing against microswitch.
  12. 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]

Cleaning a ceramic cooktop.

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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  1.    Cooking with Halogen by V. Elaine Gilmore, Popular Science, May 1989, p.143.

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Text copyright © Chris Woodford 2007, 2021. All rights reserved. Full copyright notice and terms of use.

Microtherm® is a registered trademark of Microtherm (GB) Ltd.

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