[DJ]

DJ asked the Naked Scientists:
   
I was walking on a railway platform on a bitterly cold winter day when I stepped beneath a bank of infrared lights.

I immediately felt warm, almost balmy. The light bulbs appeared to be incandescent bulbs.

How are these lightbulbs designed to produce such larges amounts of infrared light?

What do you think?

[Soul Surfer]

Most incandescent bulbs work just below 3000 dec C  this is very much lower than the sun at 6000 deg C where the peak of radiation is around yellow in the spectrum.  The peak energy from a light bulb is in the infra red and the light strongly biassed towards red this is why you feel it strongly as heat because it IS heat.

[Bored chemist]

Even an ordinary bulb only converts about 3% of the electricity to light- the rest is lost as heat.

[LeeE]

The frequency (actually a range of frequencies for 'real' materials) of thermal radiation, which is what these types of heat lamp produce, depends upon the temperature of the body emitting the thermal radiation; the hotter the emitting body, the higher the frequency of the emitted radiation.

The incandescent elements in heat lamps operate at relatively low temperatures, as SoulSurfer suggests, and this means that nearly all of its output is in the IR range.

The effect, where the frequency of the emitted radiation increases as the emitting body gets hotter, can be seen with an electric hob; at low settings the hob will not give off any visible light at all but the IR it produces can still be felt if you hold your hand above it.  When you turn the hob up to its maximum setting it will start to produce visible light and you can see it glowing 'red-hot'.  Typical incandescent light bulbs, of the sort now being widely phased out, operate at an even higher temperature than the hob and the upper frequency of the light they emit increases as a consequence, with the result that the emitted radiation includes higher frequency 'yellow' light.

However, it's not strictly true to say that these lights radiate 'heat', for strictly speaking, heat is a measure of the kinetic motion of the atoms or molecules in a medium and the IR radiation produced by incandescent lights can travel trough a vacuum, where there is no medium, or through the air without heating it to any noticeable degree (because ordinary air is largely transparent to IR i.e. the IR radiation is not absorbed by air).

The fact that air absorbs hardly any IR is why these outdoor heat lamps actually work, for if the air absorbed a significant fraction of the emitted IR then it would be heating the air up, rather than you.  It also means that these types of heat lamp also work when it's quite windy, although there will then be an additional chill-factor.

The bottom line is that the 'heat' produced by these lamps doesn't actually become heat until your body absorbs the IR radiation, which then increases the kinetic motion of the atoms and molecules in your body.