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Relationship of heat output to wattage to light emitted

Discussion in 'Lighting and Electrics' started by derekleffew, Dec 13, 2007.

  1. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Sorry if I appeared to close the "Preheating Conventionals" thread abruptly or prematurely. Please continue the (fascinating:rolleyes:) discussion here.
     
  2. gafftapegreenia

    gafftapegreenia CBMod CB Mods

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    I'd say this has been a rather 'heated' debate, but overall I've found it quite 'illuminating'.
     
  3. fredthe

    fredthe Active Member

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    I have to jump in here, and correct what seems to be a misconception in how incandescent lights work.
    In the previous thread, one poster stated
    This is, unfortunatly, incorrect. The resistance of the filament is not what (directly) creates light. The only function of the (low) resistance of the filament is to create heat.

    It is the fact that the filament is hot that causes it to emit light. This is the very definition of incandescence. If I were to take a filament, and heat it to the same temperature using a blowtorch, it would emit the exact same amount of light as if it were heated by electricity.

    Wikipedia (citing The Standard Handbook for Electrical Engineers gives the folllowing table (for a non-halogen bulb):

    For a supply voltage V:
    Light output is approximately proportional to V ^ 3.4
    Power consumption is approximately proportional to V ^ 1.6
    Lifetime is approximately inversely proportional to V ^ 16
    Color temperature is approximately proportional to V ^ 0.42

    So, there is a trade off among light output, power consumption, lifetime, and color temperature.

    Halogen lamps are designed to work at a higher temperature, giving more light output at a higher color temperature for a given power consumption and lifetime, as compared to an incandescent lamp.

    Wikipedia (yes, I know the dangers of using it) gives an overview of incandescent light output and efficiency here.
     
    icewolf08 likes this.
  4. icewolf08

    icewolf08 CBMod CB Mods

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    Thanks, fredthe. That is what I was looking for in someone telling me I was wrong. And if I had thought about it, I probably knew that. I guess the question that still remains is: Do different lamps of the same wattage produce different amounts of heat?
     
  5. gafftapegreenia

    gafftapegreenia CBMod CB Mods

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    The blowtorch thing is correct, in fact Popular Science a few months back did just that.
     
  6. jmabray

    jmabray Active Member

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    Yes, but not always, and probably by not a significant amount.

    If one filament is longer than another, then it has more capacity to produce heat simply because there is more mass heated. Take for instance a 1000W cyc light vs a 1000 watt ellipsoidal lamp.

    Does that make sense?
     
  7. avkid

    avkid Not a New User Fight Leukemia

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    That is quite easy to test, line up several fixtures with different type, but same wattage lamps and take readings at regular intervals with an IR Thermometer.
    http://www.stanleysupplyservices.com/product-group.aspx?id=10291
     
  8. icewolf08

    icewolf08 CBMod CB Mods

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    Makes sense. So if compared to the black body radiator theory where a body glowing at a "warmer" color temp is at a lower radiant temp compared to a "cooler" color temp, then, though the difference may be small, lamps of the same wattage but different color temps should have different radiant temps. Correct?
     
  9. icewolf08

    icewolf08 CBMod CB Mods

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    I have an IR thermometer it can measure up to 518˚F so if that is enough range then I have lamps I can test.
     
  10. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    Thanks Fred. I do want to point out there is a bit of "Chicken and Egg theory" going on in Incandescence. I'm also just realizing that this dual nature of heat being produced is probably the source of the confusion in the previous thread.

    The resistance to electricity causes heat, which excites the atoms, releasing electromagnetic radiation. Most of that EM released however is infrared not visible light... so a second layer of heat is released. You heat the filament to create EM which creates more heat and light. This is where you get the variation between different types of lamps. Different filament designs and manufacturing processes will incandesce different levels of IR and visible light. It will be very hard to determine (without a physics degree and a lot of math) how much of the heat produced by a lamp is from the filament being heated and how much is from the Infrared released. Both types of heat are coming from the filament but two different principles of physics are involved to create that heat.
     
  11. JD

    JD Well-Known Member

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    Some basics on the two types of heat, as well as other energy:

    Absolute zero: Atoms/ molecules are not moving at all.

    Add some energy-

    Now they are moving, but the only way they can transfer it is by direct contact. (first type of heat)

    Add a bunch more-

    Now they are really moving, but not quite incandescent, so they still only transfer by direct contact.

    Add a bunch more-

    The birth of Incandescent Output! The molecules are now being bounced around so much that electrons are jumping from lower shells to higher shells. When this happens, the atoms actually take in and store some energy like a capacitor, only to release it as a photon* when the electron drops back down to it's lower shell. These photons are too low a frequency to be seen as light, but can be absorbed by an object at a distance, causing it's atoms/molecules to be excited and move around.--- Radiant Heat!

    Add some more-

    Now things are really buzzing and the photons that are being released are at higher frequencies. Visible light!

    Add some more-

    Oops! Light is gone again! It is now ultra violet. These photons are so energetic that when they slam into another object, it may cause the second objects electrons to bounce to higher shells. When they return to their lower shell, they emit secondary photons. This is what is happening in phosphorous.

    Add some more-

    Danger Will Robinson! The photons are so over excited that they may knock the electrons right off of the molecules of the second object! This is known as Ionizing radiation.

    So, what's it all mean to us? We want to excite things just enough to produce visible light. Too low, and its wasted as radiant heat. Too high and it's ultra violet. Unfortunately, we can't get tungsten hot enough before it melts, so we settle for some light and lots of heat. The opposite is true on arc lamps, which are more likely to waste output as UV. With regards to tungsten, we end up with a very counterintuitive output. At a "low" temperature, almost all of it's output is infrared. (think IR heat lamp) At a "high" temperature (think 3400k) a whole lot more of it's output is visible light! So, crazy as it sounds, the hotter the filament, the cooler the light is as more energy is leaving in the form of visible light photons. (For a given wattage.)

    So, here is the loophole: We consider "heat" to be photons of a certain bandwidth in the IR spectrum. We consider "light" to be photons of a certain bandwidth between Red and Blue, the visible spectrum. Depending on the ratio between the two, we rate the "efficiency" of the light source to produce visible light. The amount of energy per given watt of work is the same, it's all a question of the spectrum of it's output.

    <ADD tangent>
    Now, here's a wildcard: LEDs work by causing a crystal latticework in there structure to resonate at a frequency that produces photons of a specific frequency. (Think Quartz crystal or tuning fork.) Although the output is monochromatic, it is incredibly efficient! In fact, the inefficiencies that produce heat are more in the support structures and impurities in the silicon (or other) crystal structure. So, although we will never achieve perfection, it will be interesting to see large scale lights that produce almost no heat!
    </ADD tangent>

    * plenty of disagreement as to what a "photon" really is, but most think it is a form of focused wave that exhibits characteristics of both energy and particles.
     
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