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Thread: Filament bulbs - variation of light output with a.c.

  1. #1 Filament bulbs - variation of light output with a.c. 
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    A normal filament bulb running on a.c. has a variable voltage supplied to it (just because of the a.c., I mean).

    Is this variation visible as a variation in light output at the same frequency as the supply voltage? Or is the frequency high enough such that the filament does not cool down enough during the low-voltage part of the supply to make a detectable difference to the light output?

    When I talk about detectable, I mean with a light sensor etc.

    Thanks


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  3. #2  
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    If you connect a photoelectric sensor to an audio amp, you can hear the 50Hz buzz.


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  4. #3  
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    Quote Originally Posted by caKus View Post
    If you connect a photoelectric sensor to an audio amp, you can hear the 50Hz buzz.
    So that's a 'yes' then?
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  5. #4  
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    Yes, it is a yes. But I correct my previous post : you will hear a buzz at 100Hz rather than 50.
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  6. #5  
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    Quote Originally Posted by Trilobite View Post
    So that's a 'yes' then?
    The ability to hear it depends a lot on the thermal properties of the particular bulb in question. I tried in middle school to instrument the thermal variation of an AC-driven 100W light bulb, but with my poor skills, all I could verify was that AC hum is everywhere.

    Fluorescent lights are another story. Their flicker is readily discernible, even with a middle-school student's badly-designed and assembled circuit.
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  7. #6  
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    We in U.S. likely will soon enough only know filament-containing incandescent lamps as a dim memory, anyway. Since we are assured the amount of mercury in Coiled Fluorescent Lamps is so small as to pose no risk, we still are reminded not to break them, etc.

    I happen to like mercury, am not particularly afraid of it, but also like incandescent lamps. Besides, my head has plenty of mercury amalgamated with silver in it, there for 60 years, most of it, so............

    Now, how about the fact that incandescent filaments are often coiled coils, thus introducing inductive (slight, admittedly) effects into the question of heat, and therefore light, being produced after the presence of a given voltage level, current lagging voltage, current being the generator of heat, as I (squared) R. In theory, a straight heated filament of similar resistance and basic physical size should produce slightly more light output than the coiled filament....

    This is getting ridiculously close to splitting hairs, but in this day of miniaturization, well, ......I'm going off to have a cold, home-made beer! jocular
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  8. #7  
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    Quote Originally Posted by jocular View Post
    We in U.S. likely will soon enough only know filament-containing incandescent lamps as a dim memory, anyway.
    Pun intended, I hope.

    Since we are assured the amount of mercury in Coiled Fluorescent Lamps is so small as to pose no risk, we still are reminded not to break them, etc.
    There is a cumulative risk, though. So repeated exposure to small amounts could become significant.

    Besides, my head has plenty of mercury amalgamated with silver in it, there for 60 years, most of it, so............
    Mercury amalgams are very stable. Elemental mercury (vapour) can be absorbed by the body. Other mercury compounds are extremely toxic (e.g. Karen Wetterhahn - Wikipedia, the free encyclopedia)

    Now, how about the fact that incandescent filaments are often coiled coils, thus introducing inductive (slight, admittedly) effects into the question of heat, and therefore light, being produced after the presence of a given voltage level, current lagging voltage, current being the generator of heat, as I (squared) R. In theory, a straight heated filament of similar resistance and basic physical size should produce slightly more light output than the coiled filament....
    Interesting point. You might also need to take into account the non-linear resistivity (quite temperature dependent) and how this might be affected by the ability of the two forms to lose heat...
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    I read somewhere that the compact fluorescent bulbs can give off a lot of UV if you get too close to them. More so than a straight tubular fluorescent because they tend to have more gaps in the fluorescent coating because of the curlicue shape.
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  10. #9  
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    Quote Originally Posted by Harold14370 View Post
    I read somewhere that the compact fluorescent bulbs can give off a lot of UV if you get too close to them. More so than a straight tubular fluorescent because they tend to have more gaps in the fluorescent coating because of the curlicue shape.
    UV passes much more freely through quartz than most glasses. This is why mercury and sodium vapor lamps typically used for large-area illumination, having quartz tubes within the glass envelope, carry warning labeling to never operate if outside glass bulb is broken. We purposely carefully broke the outer shell of one, and used the quartz tube as a sunlamp; burned our butts, pretty good!

    Thus, if the CFLs are of typical plain glass, they likely transmit some UV, but would unleash more if of quartz. No need for quartz; it is used in the big vapor lamps because of it's higher operating temp. limits. I wonder, regarding your remark about gaps in internal coating: since it is the coating which emits all the radiation, would not the portion of coating facing the glass be responsible for emitting the UV, not the gaps? Unless the arc itself, within, produces the bulk of UV, which is possible. Fluorescent tube sunlamps have coatings which specifically emit more UV than visible. I believe in the case of low but detectable UV intensity, insufficient to cause erythema of the skin, still pose a hazard to the eyes, longterm.

    A glass patented way back in the 1930s, I believe it was, called "Vitaglas" by the manufacturer, transmitted a higher degree of UV than plain glass, and was touted for use in windows, especially nursery windows, to allow UV to enter rooms because of it's germicidal properties. Thus, suntan the babies, while sanitizing their air supply! I was unable to find much regarding Vitaglas when searching some time ago, but distinctly recall it's being mentioned in my high school Physics text 9 (in 1957!). jocular
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  11. #10  
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    Here's the article.
    CFL Lamps Might Cause Skin Damage | UV Radiation CFLs | LiveScience
    Energy-efficient compact fluorescent light bulbs, or CFLs, are a popular choice for homeowners and businesses looking for ways to reduce their electricity bills. But researchers at Stony Brook University in New York have discovered CFLs have a darker side, too: The lamps emit surprisingly high levels of ultraviolet (UV) radiation, which can damage skin cells and, at high exposure levels, cause cancer, according to CBS Miami.
    Researchers also believe they know the cause of the UV damage: tiny cracks in the coating inside the CFL bulbs allowed UV radiation to leak out, CBS Miami reports.
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  12. #11  
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    Typical poor science reporting though...what study for example.

    UV is too broad a term as we often use it anyhow. UVA doesn't have enough energy to do any significant damage. It's the UVB and shorter wavelengths where the real dangers lie. This time of year in Western Western state most people are starting to suffer from low vitamin D, is CF bulbs give off the less harmful UVA...that's a good thing. (swings my CF SAD light over my head).
    Meteorologist/Naturalist & Retired Soldier
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  13. #12  
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    Vitamin D deficiency is said by the medical profession to be widspread among American adults. At the same time, the same group claims 10 minutes exposure to sunlight daily produces adequate Vitamin D formation within the skin.

    What I'd like to know is: Is it produced only in the area of exposed skin? Thus, is the amount of skin exposed a determining factor in amount of Vitamin D formed? Some typically receive sunlight only to the hands, face and neck. Can that possibly produce enough?

    Maybe the nudists have something on the ball, after all! jocular
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