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Thread: Colour Temperature of Non-Blackbodies

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    Default Colour Temperature of Non-Blackbodies

    While answering another thead on the colour temperature of halogen lamps, I thought about this question that I have no answer to:

    How do you define a colour temperature for a light source that is not a blackbody, e.g., a street lamp or a tungsten bulb?

    Or, don't tell me, street lamps and tungsten bulbs are blackbodies, because I remember from O level physics that they are not.

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    no, they are definately not black bodies.

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    Default Re: Colour Temperature of Non-Blackbodies

    Originally posted by ckhaos
    While answering another thead on the colour temperature of halogen lamps, I thought about this question that I have no answer to:

    How do you define a colour temperature for a light source that is not a blackbody, e.g., a street lamp or a tungsten bulb?

    Or, don't tell me, street lamps and tungsten bulbs are blackbodies, because I remember from O level physics that they are not.
    if my memory does not fail me, a black body is a mass that can let light waves can go in but cannot come out. A typical blackbody will be a box with just a small tiny hole. Light can go in but the hole appears to be black. Why? because the light cannot come out. A black shoe is not a blackbody because you still see the black. The box (example above) can become a non-blackbody by burning the box, then there will be fire, and you see light (wave) emitted by the box. (roughly like that, forgot some)

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    Default Re: Re: Colour Temperature of Non-Blackbodies

    Originally posted by 007

    if my memory does not fail me, a black body is a mass that can let light waves can go in but cannot come out. A typical blackbody will be a box with just a small tiny hole. Light can go in but the hole appears to be black. Why? because the light cannot come out. A black shoe is not a blackbody because you still see the black. The box (example above) can become a non-blackbody by burning the box, then there will be fire, and you see light (wave) emitted by the box. (roughly like that, forgot some)
    wrong!

    but since i'm rushing off to attend lectures, i shall be back later to give my answer.

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    Well, I know what a blackbody is, I am just not sure whether tungsten lamps are blackbodies or not, and more importantly, how does one calculate the colour temperature of a non-blackbody.

    I think 007 is correct in his definition of a blackbody, which I remember from O levels. Simply put, a blackbody is a perfect absorber of radiation.

    Thus, any light that is incident on the hole in the box with a tiny hole (in 007's example) gets trapped inside the box with negligible probability of escaping...

    So, again, for a non-blackbody, how does one calculate its colour temperature?

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    Default Re: Re: Re: Colour Temperature of Non-Blackbodies

    Originally posted by mervlam


    wrong!

    but since i'm rushing off to attend lectures, i shall be back later to give my answer.
    paiseh. returned most to my teacher oredi.

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    A box with a pin hole is blackbody meh?
    What if there's a light source inside the box? Can't you see the light emitted through the hole then if you're seeing from outside?

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    Originally posted by Sin
    A box with a pin hole is blackbody meh?
    What if there's a light source inside the box? Can't you see the light emitted through the hole then if you're seeing from outside?
    just a very small tiny hole leh. and i didn't say that there is a light source inside the box.


    well, if you want to say so, up to you loh. I assume you are very good /excellent in physics. Why don't you answer the question?
    by the way, well , if you see like this:

    if you have a box with a tiny hole. Can light goes in or no? If yes, can you see the light or just a black(small tiny) hole? well, i don't know.

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    Originally posted by 007


    just a very small tiny hole leh. and i didn't say that there is a light source inside the box.


    well, if you want to say so, up to you loh. I assume you are very good /excellent in physics. Why don't you answer the question?
    by the way, well , if you see like this:

    if you have a box with a tiny hole. Can light goes in or no? If yes, can you see the light or just a black(small tiny) hole? well, i don't know.
    No, I'm not good at all with physics. Am I'm not ashamed to say that. Just a logical man trying to understand your logical english.

    Perhaps you'd like to enlighten me about what IS a blackbody then? Obviously you know more than me.

    If the box has a hole, then light can go in and can come out. (Anything with a hole can let things in and out, unless you have a valve.) Why do you not see the light and the hole appears black? Well, logically the hole appears black because there's NO light inside. Absence of light causes darkness. If the only light entering is the thin beam through the small hole, then it reasons that the light is not strong enough to illuminate the inside. Hence the "black" hole (by the way, the hole itself is a misnomer, as hole refers to lack of substance how can a "hole" reflect light?)

    As you can see, I'm not good at physics at all. Just a logical man. Would you like to explain to me what a black body is, please? No jest, I really would like to be educated on this.

    If you're talking about a black body being one that absorbs all radiation, then it reasons to be about the material/make of the box and has nothing to do with the hole?

    And please, I'm just trying to understand this. Let's not turn this into a flame or ego war ok? I got enough fun for one day as it is.

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    Colour of halogen bulbs in black enclosures that have a lamp life of 3000 hours and use 100 watts < mostly have a temperature of 2200 kevin to 2800 kevin. Same applys for those 12 volt type with their own reflector.

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    Originally posted by Sin


    No, I'm not good at all with physics. Am I'm not ashamed to say that. Just a logical man trying to understand your logical english.

    Perhaps you'd like to enlighten me about what IS a blackbody then? Obviously you know more than me.

    If the box has a hole, then light can go in and can come out. (Anything with a hole can let things in and out, unless you have a valve.) Why do you not see the light and the hole appears black? Well, logically the hole appears black because there's NO light inside. Absence of light causes darkness. If the only light entering is the thin beam through the small hole, then it reasons that the light is not strong enough to illuminate the inside. Hence the "black" hole (by the way, the hole itself is a misnomer, as hole refers to lack of substance how can a "hole" reflect light?)

    As you can see, I'm not good at physics at all. Just a logical man. Would you like to explain to me what a black body is, please? No jest, I really would like to be educated on this.

    If you're talking about a black body being one that absorbs all radiation, then it reasons to be about the material/make of the box and has nothing to do with the hole?

    And please, I'm just trying to understand this. Let's not turn this into a flame or ego war ok? I got enough fun for one day as it is.
    Hello, we meet again.

    A blackbody can be assumed to be a black box with a small hole. Light can enter the box through this hole. However, light is assumed to be not able to escape this box. This is logical English.

    By a more scientific explanation, a space is viewed to have a gravitational potential field. A "black hole" is so called because in gravitational potential terms, it's a gravitational well, thus the term hole. This "hole" has such high gravitational attraction force that anything, even light (btw, in Nuclear Physics, light is a particle called photon.) is attracted to it. No amount of force can break this gravitational attraction force set up by the "hole".
    Last edited by mervlam; 29th January 2003 at 08:20 PM.

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    btw, if i remember correctly, Black bodies got nothing to do with colour temperature. do correct me if it's wrong.

    Colour temperature actually depends on the wavelength of the light emitted by an excited body. It involves a bit of Nuclear Physics concepts.

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    Originally posted by mervlam


    Hello, we meet again.

    A blackbody can be assumed to be a black box with a small hole. Light can enter the box through this hole. However, light is assumed to be not able to escape this box. This is logical English.

    By a more scientific explanation, a space is viewed to have a gravitational potential field. A "black hole" is so called because in gravitational potential terms, it's a gravitational well, thus the term hole. This "hole" has such high gravitational attraction force that anything, even light (btw, in Nuclear Physics, light is a particle called photon.) is attracted to it. No amount of force can break this gravitational attraction force set up by the "hole".
    Oh hello! Yes, we meet again, hopefully this time we dont "have to go at it" again.

    So the "black box with hole is a black body" thing is actually an analogy ? They're actually likening a black body to a black box with hole for illustrating purpose? Well then, that makes more sense to me now. Though still abit blur about the concept (not too good at physics as you can see)...

    Thanks.

    ps: I don't remember being taught "Blackbody" in physics at O levels. This must be something new in the syllabus.

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    Originally posted by Sin

    ps: I don't remember being taught "Blackbody" in physics at O levels. This must be something new in the syllabus.
    Did you say O levels...!!? Goshhh... I am feeling old. What did I know about black body in O levels??? *Sigh*

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    Originally posted by Andy Ho


    Did you say O levels...!!? Goshhh... I am feeling old. What did I know about black body in O levels??? *Sigh*
    Me too. Gosh, kids nowadays sure are bright aren't they.
    I had enough problems with understanding voltage, power, current, etc back then. Nowadays, they teach nuclear physics...
    *shudder*

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    Default Re: Colour Temperature of Non-Blackbodies

    Originally posted by ckhaos
    While answering another thead on the colour temperature of halogen lamps, I thought about this question that I have no answer to:

    How do you define a colour temperature for a light source that is not a blackbody, e.g., a street lamp or a tungsten bulb?

    Or, don't tell me, street lamps and tungsten bulbs are blackbodies, because I remember from O level physics that they are not.
    This is a really complex issue to attempt to explain without the use of a fair number of graphics, however here goes.

    Photographs are generally taken where the light travels from the subject to the camera, that is the light is reflected light from the subjects surface. The sources of the light can be either natural such as the sun, sky and clouds or man made in the form of flash lighting, and other light sources.

    Artifical lightsources are classified according to the method used to produce the light, that is by burning, heating, electric spark/arc or electric discharge.

    Examples:

    Burning: - candles, fires, oil lamps, flash powder and flash bulbs.

    Heating: - Carbon and tungsten filament lights, eg household lights, studio lamps and tungsten-halogen lamps.

    Electric Spark/Arc: - Carbon Arcs, spark gaps

    Electric Discharge: Electronic flash, metal halide lamps, sodium and mercury vapour lamps.


    Spectral Quality
    The radiation from most sources comprises a mixture of light of various wavelengths. The Spectral Quality or Hue of the light from a source may vary considerably depending on the distribution of energy at each wavelength in the spectrum. Most of the sources of light used in photography give what is known as white light. This is light that to the human eye appears to be white or close to white, it is not visually deficient in any particular band of wavelengths while not implying any definate colour quality. Most white-light sources vary considerably amongst themselves and from daylight. Howvever because of the perceptual phenominon of colour constancy these differences matter little in everyday life, but they are very important in photography, especially when using colour emulsions and materials. Thus is it is desirable to describe the light source in precise terms and one of the most important for the photographer is the exact hue of the light. Since visible light is a particular region of the electromagnetic spectrum and a form of radient energy the colour quality may be described in terms of Spectral Power Distribution (SPD) throughout the spectrum of visible light.

    Spectral Power Distribution Curves
    This is the sticking point area, without a several curve plots it's difficult to explain how little the actual variance is between many lightsources yet the difference in hue can be large. However after analysis of curves the scientists etc can classifiy the curve in to one of three basic categories, these being:

    Continiuous Spectra Energy is present at all wavelengths in the region measured.

    examples: All incandescent-filament lamps.

    Discontinuous Spectra (Line Spectrum) Energy is confined to one or a few narrow bands, at these wavelengths the energy levels are high and at all other wavelengths the energy level is near almost nil.

    examples: typical low pressure mercury and sodium vapour lamps.

    The final type (name forgotten ARGH) comprises broad bands of energy across the spectrum accompanied by a continuous background spectrum of varying energy levels.

    examples: Typical high pressure mercury and sodium vapour pressure lamps, some types of flourescent lamps, metal halide, xenon and argon vapour lamps.


    COLOUR TEMPERATURE

    For the purpose of photography a more common method of describing the light quality of an incandescent light source is by means of its colour temperature. This is defined in terms of a Planckian Radiator, otherwise known as a Full Radiator or a black body. This is a light source that emits radiation in which the SPD depends only on its temperature and not on the material or nature of its source.

    The colour temperature of the light source is the temperature of a full radiator that would emit radiation of substantially the same spectral distrubution in the visible region as the radiation from the light source. Colour temperatures are measured on the thermodynamic scale (ie: Deg Kelvin) with a zero reference of -273.15 deg C.

    A simple way of appreciating the idea of colour temperature is to imagine the progressive change in colour of the light emitted from a piece of metal as its temperature is raised, going from dull black through red, orange and on to white heat. The quality of the light emitted is thus a function of the temperature of the metal. While its true that in theory the concept of colour temperature can only be applied to sources that are full radiators, in practice it is extended to include sources that have an SPD that is similar to a full radiator.

    I could go on for a lot longer .. but by now I'm sure most will have got the general drift.
    The Ang Moh from Hell
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    Originally posted by Andy Ho


    Did you say O levels...!!? Goshhh... I am feeling old. What did I know about black body in O levels??? *Sigh*
    sorry. it's A Level S paper or University Physics First Year stuff

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    yup. Ian sums it up pretty well.

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    Originally posted by mervlam
    yup. Ian sums it up pretty well.
    Yeah it's the basic version, no maths and with quite a few things left out.

    Even 20+ years after doing it it's hard to forget .. unlike most of what I learned at uni.
    The Ang Moh from Hell
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    Originally posted by mervlam


    sorry. it's A Level S paper or University Physics First Year stuff
    Phew.... I know I wasn't paying much attention during my O levels physics lessons... but that was a bit too much! but CKhaos learnt that during his O levels????

    Anyway my understanding of colour temperature is what Ian explain about the heated metal... at least thats' how my lighting lecturer taught!

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