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Sun's surface temperature & halogen bulbs

O

onix

Member
Super-white halogen bulbs are advertised to be so white that they are bluish-white. The bulbs have a rated temperature from 3800K-4175K, and some go even higher. From physics we know that blue stars are hotter than white, white hotter than yellow, yellow hotter than red. If our sun's surface temperature is 5000K+ and it appears yellow, why do the much cooler halogen bulbs appear white, or even blue?

Here's one source causing confusion:

Motorcycle headlights

These links are useful - still not clear how halogen bulb temperatures are determined:

http://acept.la.asu.edu/PiN/rdg/color/tungsten.gif
http://www.jb.man.ac.uk/distance/life/sample/java/spectype/specplot.htm
 
The bulbs and the sun are a much different case. Light from the sun is traveling through space and then the earths atmosphere, so it is refracted, reflected, and filtered. The sun appears different colors all the time because of the angle it is in relation to you and so the color temperature changes. I believe this is because the sun emits black body radiation, so it is not just in the range of 5000k.

When I stare into the sky, the sun appears white to me for the brief moment I look at it. I would imagine that in space it might appear even more white than yellow for sure.
 
I cant really help here, but I can tell you that the sun is yellow, and the temperature is related to the wavelength of the peak intensity in the 'black-body' spectrum by Wien's displacement law. Google is your friend.

I would imagine that halogen bulbs are not black-body, unlike normal incandescent bulbs. This is probably the source of the difference.
 
Originally posted by: rezinn
The bulbs and the sun are a much different case. Light from the sun is traveling through space and then the earths atmosphere, so it is refracted, reflected, and filtered. The sun appears different colors all the time because of the angle it is in relation to you and so the color temperature changes.
Click to expand...

This is basically right. The Sun's output peaks around a wavelength of 500nm (as you can calculate from its temperature using Wien's Displacement Law mentioned above) and most of the high frequency colors are lost due to Rayleigh scattering in the Earth's atmosphere (which also makes the sky blue.) The closer the Sun is to the horizon, the more atmosphere its light must travel through to reach your eyes, resulting in more of the higher frequencies being scattered away and causing the Sun to appear more red as it sets.

 
Anything sufficiently bright will wash out our perception of color. Even very very bright red will appear white. I don't think the sun's color being scientifically defined as yellow is dictated by how it appears after light passes through our atmosphere. The peak color of 550nm (being bluish-green) in outer space sounds about right. While that is the surface temperature the internal temperature of the sun is like a million Kelvin, but we don't see that.

Anyways, I think we are on the right track to answering the question.

Why don't halogen bulbs emit a blackbody spectrum?

Isn't heat the dominating mechanism of the bulb's illumination? Is that the same as the sun's?

Is there some generation/recombination I'm not aware of?
 
Contrary to the statement in your question: Compared to the sun, the halogen lamp will appear more yellow. (But please don't look directly at the sun. It will harm your eyes.)

Since blue light is dispersed by the atmosphere, the sun will appear increasingly yellow, the more atmosphere the sunrays have to pass through. But I kinda doubt midday sun will have any problem "out-bluing" the halogen lamp.
It may even be that the 5000k color temp of the sun, actually refers to midday sun, and that the sun itself (without atmosphere) is actually even "bluer". I don't know that. But I would suspect so, since the 5000K figure is used as a reference to balance colors correctly in photography and film.

The sun is classed as a "yellow" sun. It's an astronomical classification. That doesn't mean it's yellow at all. The sun is, in physical sense, white. Which actually means that the light contains radiation from the entire spectrum. A candle light is also "white". As are all lightsources that emits light from heat. If you compare such white lightsources side by side, one may appear yellow, another blue. That is because the energy is differently weighted in the spectrum. Our brain seeks some 'ambient' color temperture for reference, in order to judge colors. (Amazing trick actually, that lets us correctly recognize a color, even if it's reflected spectrum fluctuates wildly, in different lighting.)

So, the sun appears yellow, is because it's more "yellow" than the ambient whitepoint, which is also contributed to by the light from the blue sky. Which in turn is actually blue light that the sun has shed.

Similarily, the halogen light appears blue, indoors, because it's bluer than the ambient whitepoint, which largely comes from "yellow" artificial light.

Experiment of truth: If you illuminate an object with the halogen lamp, and take a picture with a camera, no flash please. The developed picture will turn out quite yellowish. That is because you have daylight film in the camera. The film expects the much bluer light from the sun. Your halogen lamp is "yellow" in comparision.
The reverse is also possible. One used to be able to buy film for 4200K, I think. If you take outside pictures with this, they will come out rather blue. Proving of course that the sun really is bluer than your 4175K halogen lamp.

Film doesn't have any brain, that compensates for the type of illumination. But photo printing machines have, ...sort of.
For this to work, you need to use diapositive film. Negative color film will not give such obvious results. In the printing process, the colors are automatically "balanced" by the printer. (That's why prints so often turn out crappy, BTW 🙂 )

Final words of this, don't get color temperature and color mixed up. The sun's color is white. Nothing else. Same as a light bulb.
 
Halogen bulbs (unlike incandescent ones) does not generate light from their temperature. They generate light from transitions of nucleus from a high energy state to a lower energy state. The light that is emited depends very much of the particular transitions that take place.
 
Odd.. I alwasy though the "K" in referance to light bulbs was refering the the Kandel power not the temp. "K" in temp stand for the measurement in Kelvin.

If the light bulbs were realy 5000K, as in temp, it would make them 8540.334 Fahrenheit and 4726.85 Celsius.

While the bulbs are indeed hot they aren't that hot.

Unless i missed something there is a big difference here.
 
Originally posted by: Calin
Halogen bulbs (unlike incandescent ones) does not generate light from their temperature. They generate light from transitions of nucleus from a high energy state to a lower energy state. The light that is emited depends very much of the particular transitions that take place.
Click to expand...

I think you're thinking of Fluorescent bulbs. Halogen bulbs are simply a specific example of an Incandescent bulb, where the filament is surrounded by a halogen gas and a quartz envelope. The light still comes from glowing Tungsten filament.

See:
Halogen Bulbs


 
Originally posted by: FrankSchwab
Originally posted by: Calin
Halogen bulbs (unlike incandescent ones) does not generate light from their temperature. They generate light from transitions of nucleus from a high energy state to a lower energy state. The light that is emited depends very much of the particular transitions that take place.
Click to expand...

I think you're thinking of Fluorescent bulbs. Halogen bulbs are simply a specific example of an Incandescent bulb, where the filament is surrounded by a halogen gas and a quartz envelope. The light still comes from glowing Tungsten filament.
Click to expand...

You're right that halogens are a form of incandescent bulb, but Calin's describing how incandescence works (i.e., why the filament glows), though incandescence involves atomic transitions, not nuclear transitions (which would emit fatal gamma rays instead of visible light.) The filament in an incandescent light resists the current flow, causing energy to be released as both atomic motion (heat) and atomic transitions to higher energy levels. Light is produced (i.e., the filament glows) when the atoms transition from the higher energy levels to lower ones.

Fluorescent bulbs work through a two-stage process. Electricity is used to excite atoms in a gas like neon or argon, which then emit ultraviolet radiation. Ultraviolet radiation isn't visible, which is why the inside of the bulb is coated with a fluorescent paint. The coating absorbs the ultraviolet light and emits lower frequency visible light in response. The color of a fluorescent light depends on the coating used. Older and cheaper flourescents use a coating which emit mainly yellow and blue light, but more expensive newer flourescents emit more evenly across the spectrum to produce a white light.
 
Originally posted by: JoeBleed
Odd.. I alwasy though the "K" in referance to light bulbs was refering the the Kandel power not the temp. "K" in temp stand for the measurement in Kelvin.

If the light bulbs were realy 5000K, as in temp, it would make them 8540.334 Fahrenheit and 4726.85 Celsius.

While the bulbs are indeed hot they aren't that hot.

Unless i missed something there is a big difference here.
Click to expand...

Light temperature can be adjusted by filtering. Which of course results in a loss of light.
But I don't think there has been any discussion here, involving 5000K bulbs.
 
Originally posted by: cquark
Originally posted by: FrankSchwab
Originally posted by: Calin
Halogen bulbs (unlike incandescent ones) does not generate light from their temperature. They generate light from transitions of nucleus from a high energy state to a lower energy state. The light that is emited depends very much of the particular transitions that take place.
Click to expand...

I think you're thinking of Fluorescent bulbs. Halogen bulbs are simply a specific example of an Incandescent bulb, where the filament is surrounded by a halogen gas and a quartz envelope. The light still comes from glowing Tungsten filament.
Click to expand...

You're right that halogens are a form of incandescent bulb, but Calin's describing how incandescence works (i.e., why the filament glows), though incandescence involves atomic transitions, not nuclear transitions (which would emit fatal gamma rays instead of visible light.) The filament in an incandescent light resists the current flow, causing energy to be released as both atomic motion (heat) and atomic transitions to higher energy levels. Light is produced (i.e., the filament glows) when the atoms transition from the higher energy levels to lower ones.

Fluorescent bulbs work through a two-stage process. Electricity is used to excite atoms in a gas like neon or argon, which then emit ultraviolet radiation. Ultraviolet radiation isn't visible, which is why the inside of the bulb is coated with a fluorescent paint. The coating absorbs the ultraviolet light and emits lower frequency visible light in response. The color of a fluorescent light depends on the coating used. Older and cheaper flourescents use a coating which emit mainly yellow and blue light, but more expensive newer flourescents emit more evenly across the spectrum to produce a white light.
Click to expand...

Yes, atomic and not nuclear transitions. The electrons in the atom, to be more precise.
Sorry for the mistake
 
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