MadSci Network: Physics

Re: how does a heat lamp work, how does the red bulb differ from regular bulbs?

Date: Mon Feb 14 09:59:25 2000
Posted By: Todd Jamison,
Area of science: Physics
ID: 949363629.Ph


Light and radiant infrared heat are both forms of electromagnetic radiation.  
Radiant heat is 
often referred to as "infrared" radiation, because it is invisible and 
"below" the red that we see.  Other forms of electromagnetic radiation 
include x-rays, ultraviolet rays, radio and television signals.  Each form 
of radiation can be characterized by its wavelength, which is the distance 
between the peaks of the electromagnetic wave.  (This is similar to 
measuring the distance between the peaks of an ocean wave, except that you 
canít touch or see the electromagnetic wave, itself.)  Almost all things 
give off electromagnetic radiation.  The radiation given off by an object is 
related to its temperature.  The higher the temperature, the shorter the 
wavelength.  X-rays have the shortest wavelength, followed by ultraviolet, 
then visible light, then infrared, and finally radio waves.  The collection 
of all wavelengths is known as the "spectrum".  There is a nice web 
encyclopedia entry that describes infrared radiation at:

this site

Incandescent lamps (which you would know as standard light bulbs) work by 
heating up a material, called a filament, to a very high temperature.  The 
high temperature is needed in order for it to emit radiation in the visible 
light part of the spectrum (remember that we said the wavelength is shorter 
for higher temparture).  These bulbs are "optimized" so that they give off 
as much energy as possible in the visible part of the spectrum.  However, in 
addition to light, standard light bulbs also give off significant energy in 
the infrared part of the spectrum, which you would know if you have ever 
burned yourself on a light bulb.  This is because objects give off energy in 
a whole range of wavelengths, not just a small portion.  

In a "heat lamp", or infrared lamp, the filament is optimized to give off 
most of its energy in the infrared portion of the spectrum, rather than the 
visible light portion.  Optimization is usually done through selection of 
the filament material.  So an infrared lamp might use quartz, for example, 
instead of tungsten that is used in light bulbs.  An infrared lamp will 
sometimes have a slight orange or red glow to it, but not be very bright.  
That is why you might think of a heat lamp as a "red" light bulb.  (These 
are different from darkroom "red" lights used for photography.)  If you have 
ever looked at an electric stove element that is on high, you will sometimes 
see that when it gets very hot, it starts to glow orange.  This is because 
at the higher temperature, it starts to give off visible light.  Again, the 
stove is optimized for heat, not light, so the glow is very dull.

Another web site that you might look into contains notes from a college 
course in physics.  Donít be worried, though, it is quite easy to 
understand.  Here is the link:

Good luck with your investigation!

[note added by MadSci Admin:  Some, especially older or less
expensive "heat" lamps are actually just regular spot (or
flood) lamps that have a red filter on them that blocks a
lot of the visible light and allows the longer wavelength
infrared energy through.]

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