I think I know the answer to this, but still curious:
As you know the human eye’s perception range of the light spectrum is limited. All textbooks since the time we are out of diapers show this nifty diagrams to the effect that the “visible light” spectrum is very thin, with wild kinds of radiative light waves and other energies on either side of the hertz. Bees can see ultraviolet and perhaps some other creatures can make out the heat signature of infrared (like from those wild Predator movies).
Infrared and ultraviolet are supposedly in this realm, or so we are told? Now of course we can’t easily see these things while in the bright light which is the combo of many different colors as coming straight from the Sun or incandescent sources, etc. But go into any Micky D’s and see some meat warming under—well–INFRARED lights and surely you see the light!
It’s red! Ultraviolet tanning booths have an eerily purple light!
Or do they?
At first my suspicion was that while immersed in OTHER lights these light are hard to make out to the human eye. Perhaps by themselves in a darkened area they can be distinguished, much in the same way we don’t generally see the “blue” of sunlight unless dispersed against or through the sky.
At dawn and dusk we can see the yellows and reds. So far so good.
But then it occurred to me that some —many—kinds of radiation ARE truly invisible to the human eye. X-Ray machines that are on truly don’t let you know you’re being beamed through until or unless the X-ray tech tells you so or when you hear the machine buzz and click.
Now is this simply a matter of two other possibilities:
Manufacturers COULD make a pure infrared heat lamp that shmoes like me use for our reptile exhibits to keep the little lizards warm on a cold day BUT this is extraordinarily expensive so to keep costs down they just have a general coating that filters out all but the RED range in GENERAL, thus the side effect of giving off infrared to boot. Ditto for the UV bulbs to help you have that lofty tan like you’ve been to Aruba over the weekend.
They have a filtering device or coating on the IR or UV bulbs that must be added to let you know the light is on, to avoid having hippies who house dangerous reptiles next to the herbs burn the whole shack down.
After all, I’ve never once seen an IR bulb that looked “off” that gave off heat.
:smack:
Well?
–DC—using heatlamps for warmth outside the Amazon since 2007
OK, two things. The natural lens in the human eye filters out UV light. The retina can see it, but the lens won’t let it have any. Before the days of lens implants for cataracts, the standard treatment was to take out the lens and give the patient thick glasses. Those people could see UV. Present-day cataract patients (like me) get a plastic implant, which filters out UV just like the original did.
Most artificial sources of infrared and ultraviolet also give off visible light. What you’re seeing is not the infrared or ultraviolet. If those lamps gave off only IR or only UV, you couldn’t tell they were on. Tanning junkies would think they were being cheated, if they didn’t see anything.
Lightbulbs that are meant for normal illumination bleed into non-visible ranges. The light you see from IR and UV lights is the bleed from non-visible spectrum into the visible. It also provides a handy-dandy way to tell if the light is on.
For some methods of generating light, it’s hard to produce one wavelength without getting others in there as well. Most infrared lamps work by heating up a filament until it glows. Radiation generated by heating something up is blackbody radiation (I explained blackbody radiation more fully in this post). The thing about blackbodies is that they don’t radiate all their energy at one wavelength of light- you can see some blackbody curves as a function of wavelength and temperature here.
An interesting feature of these curves is that they don’t cross each other- a blackbody at 3000 Kelvin will radiate more energy at every wavelength than one at 2000 Kelvin, even at the peak wavelength of the 2000 Kelvin blackbody. So if you want to generate more infrared radiation, the obvious solution is to raise the temperature of the filament. That will make the filament give off more red light as well.
The blackbody curve is why the visible light you see from ultraviolet lights tends to be blue or purple. Those are the colors nearest the ultraviolet, so that’s what you’re going to see the most of in the light.
There are also ultraviolet fluorescent lights, which don’t work via blackbody radiation. Fluorescent lights have a gas inside the tube, and they work by bumping electrons in the atoms of that gas to a higher energy level. The electrons emit light at certain specified frequencies when they go back down to a lower energy level. There aren’t many gases that only emit in the ultraviolet or infrared (at least not that I know of), though, so you get some visible light as well.
You could treat the glass in the light bulb to block some of the visible light, and this is often done for ultraviolet lights. There’s an obvious safety reason why you wouldn’t want to block it all, though. Infrared and ultraviolet radiation can be dangerous (ultraviolet can cause a burn like a sunburn, and infrared can burn you if you touched the bulb). You want an obvious visual cue for anybody around the bulb that it is running, and visible light provides that.
Actually there are ceramic infared heaters that do not give off any visible light. They’re used in terrariums, for animals that need darkness and warmth.
This type of heater get hot to the touch, so it emits long-wavelength IR. But it’s not so hot that it glows (emits visible light). The problem is, it also means the amount of heat radiated per unit area is small. You need a very large ceramic heater to keep a person warm outdoor. And when you do that, most of the heat is carried away from the heater by air (convection) rather than emitted as IR. This is OK for an enclosed terrarium, but for an outdoor heater it’s much more efficient to use a compact high-temperature filament, hot enough to glow visibly.
As for UV - most lamps you encounter (tanning booths, blacklights, etc) are UVA, which is the long-wavelength end of the UV range, very close to visible light. Most light sources aren’t purely monochromatic, so most UVA lamps produce some visible light as a by-product. It can be blocked, but there’s usually no reason to do so.
Like the others say. Moreover, you are correct to speculate that it would cost considerably more to pick and choose exactly which wavelengths are let out by special coatings, though it would be possible.
You are probably not going to find any infrared light “bulbs” that emit exclusively in the infrared, because the “bulb” construction (a thin glass envelope partly evacuated) is designed to let a filament glow very brightly. This lets the filament emit plenty of infrared and, inadvertantly, visible light. You could make an infrared radiator, like the ceramic type mentioned, without needing the bulb envelope. So the fact that an IR source looks like a bulb is an immediate indication that it will have such a hot radiator inside that you will also be able to see it.
>The light you see from IR and UV lights is the bleed from non-visible spectrum into the visible.
This could be taken several ways, some right and some wrong. The eye can see into what is officially called IR or UV, though with less and less sensitivity. Very bright light sources of purely IR or UV wavelengths can be visible this way - I use 785 nm laser radiation that is most definitely categorized as infrared, but I can see it, because it’s on the order of a hundred million watts per square meter. This is one sense of this “bleeding”, I think. Or, as others point out, light sources are usually not so monochromatic that they don’t emit radiation of very different wavelength from their intended. This is another sense of this “bleeding” statement, I think, one which is correct for several of the examples people give. A third interpretation of this statement would be that it is not possible to build a source so that none of its radiation is visible, and this would not be true. Carbon dioxide lasers that emit radiation of about 10.6 um wavelength in the infrared are utterly invisible to the eye, for instance.
>An interesting feature of these curves is that they don’t cross each other- a blackbody at 3000 Kelvin will radiate more energy at every wavelength than one at 2000 Kelvin, even at the peak wavelength of the 2000 Kelvin blackbody. So if you want to generate more infrared radiation, the obvious solution is to raise the temperature of the filament.
Anne raises this interesting point. A very simple law about blackbody radiation (the Stefan - Boltzmann law, if I have the names straight) says that the total radiated energy is proportional to the absolute temperature raised to the fourth power. But a less well known consequence of the way blackbodies radiate is that, if we are comparing blackbody curves only in the region well to the longer wavelength side (usually these are graphed so this would be the right side of the curve), the total radiation over all wavelengths longer than some fixed value is only proportional to the absolute temperature raised to the first power. As a consequence, the weak radiation that blackbodies emit around room temperature in the longer wavelengths can practically only be increased by a factor of about ten, because there aren’t any construction materials to make blackbodies out of that are stable at temperatures above ten times room temperature. The best you can practically do is with a carbon arc, which emits most of its energy at much shorter wavelengths, but which is also the strongest blackbody emitter you can find (on Earth) for very long wavelengths.
That’s…sorta what I thought. I figured that it would be difficult to use a filiment to generate this kind of light without having the side effect for most products of producing a visible portion near these wavelengths. Funny thing is (and this is also the sales pitch of IR lighting for nighttime heat) is that some reptiles, like crocodilians, have REALLY GOOD night vision for the same spectrum of humans and mammals but do not apparently see this red range. Period.
I have a spectacled caiman who gets agrevated at shadows and lighting from outside the window and people who enter the room in pitch black. Turn on the light and her pupils dialte as would any human being or other mammal. Turn on the IR light and they go back to the way they were. The whole room can be bathed in red and she won’t know it. For some animals the advertising is your “pets” (well, she dosen’t snuggle) can sleep soundly while YOU can still keep a close eye on THEM. Which when you own a small crocodile, no matter how cute this is a good idea to know where fingers are in relation to her jaws when having to do night maintenance.
But yes, that also makes sense from the standpoint of knowing when the device is even on. And yes they can burn the tar out of you. In fact some have busted if moisture hits them while hot, especially since Alex likes to thrash around when really knicked off at …anything.
I had heard about the ceramic heaters but didn’t know the source of the heat was IR. Interesting.
They might also be more likely to keep their eyes open and look at the bulb if there were no visible light. That’s not good, long-term, as ultraviolet radiation is a major contributor to cataract formation.
And if it’s hot enough, it emits visible light, as well. IR is not heat. Everyday objects that we’re commonly exposed to emit IR as the main component of their blackbody spectrum only because that’s where the spectrum peaks for that temperature range. Any EM radiation is capable of heating an object which absorbs it. Saying that IR is heat is at least very misleading, if not just plain wrong.
