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Best Light for reading?

Discussion in 'Electronic Basics' started by don, May 11, 2010.

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  1. don

    don Guest

    When choosing a Flourescent Bulb for a Task Light what is the best for
    reading.......my choices are 2700 Kelvin, 3000, 3500, or 4100.....
     
  2. My personal preference is for the light to be whiter. If the light will
    be used only where the illuminance of what you are looking at exceeds
    500-600 lux or so ("in my words half of slightly lowish side
    'office-bright / classroom-bright'), I would use 4100K. That would be
    typically achieved within 25 inches of a 42 watt CFL or within 20 inches
    of a 26 watt one, or somewhat greater distances from a CFL in a fixture
    that has a reflector or at least a white surface behind the CFL.

    If you will be illuminating your reading material less intensely than
    that or if you will also use this light for some of your general home
    lighting, my preference is to go 1 step lower to 3500 K. Higher color
    temperatures often have a "stark" or "dreary gray" effect with the lower
    illuminances common in homes.

    There are linear fluorescents and CFLs of even higher color temperature
    ratings of 5000 and 6500 K. I find those to be even worse at "stark" /
    "dreary gray" effect than 4100. However, I have noticed that 5000-6500 K
    tends to achieve in most home use "an icy clean stark or dreary gray",
    while 4100 K deployed insufficiently to look "nice and bright" appears
    to me to be a "dusty-dirty stark or dreary gray".
     
  3. Bob Myers

    Bob Myers Guest

    I'd have to agree but also to add a bit of additional explanation: as
    noted,
    the "XXXX Kelvin" specification refers to the "color temperature" of the
    light source, which is roughly the temperature of a supposed "black body"
    radiator (an object which produces light solely through incandescence -
    think of the way a horseshoe glows red when taken from the forge).
    "Best" for reading, especially, is a matter of personal preference, as long
    as the source in question provides *sufficient* light for your needs
    (such that
    you're comfortable with it). The real concern with the various color temps
    (or "colors of white") has to do with how well they do at reproducing color
    accurately when you're dealing with reflected images such as the printed
    page. Incandescent lights, for instance, produce a MUCH "redder" light
    than, say, direct sunlight, so colors will look a good deal different in the
    two situations.

    A "6500K" light source is supposed to be roughly equivalent to sunlight
    illumination (there are other conditions that go along with that, but that
    will do for now) and is fairly close to the "equal energy" white (a light
    source that's flat in terms of energy across the visible spectrum).
    Anything lower than that is basically biasing the white more toward the
    yellow, and eventually red, end of the spectrum, and will appear
    "warmer" (more yellow/orange/red) to the eye. Much above 6500K,
    and the light source is turning distinctly blue, and so appears "colder."
    Your eye adapts to anything that's reasonably "white," though, if it's the
    only (or primary) source in the field of view, so after a while you don't
    really notice that anything is wrong (unless you have some other source
    to compare colors to).

    Bob M.
     
  4. I do beg to differ, since sun's surface temperature and color temp.
    of direct sunlight outside Earth's atmosphere is more like 5800 K.

    Overcast days appear to me to be less bluish than 6500K lamps - I have
    liking to consider "typical overcast conditions" to be 6000K even.

    5500 is supposed to be some sort of typical of direct sunlight plus "to
    relevant extent" light reflected towards illuminmation of
    sunlight-iluminated photography subjects by "blue sky" and clouds.

    I do sense some mention of to some extent using a "UV/Haze" filter to
    attenuate UV to an extent such that spectral response of "daylight color
    slide film" after modification by such a filter to result in a spectral
    response closer to that of human vision.
    I do find "equal energy/power per unit wavelength version-of-white"
    to have "correlated color temperature" of 5455 K or by some accounts
    closer to 5400, maybe around 5420 or possibly as low as in the upper
    5300's.
    What about 4100 or for that matter even if that needs 4300-4400 K
    ("mildly overheated" "cool white" fluorescent lamp") appearing to be
    white? And "direct midday sunlight" in Philadelphia and nearby suburbs
    appears to me to achieve color temperature mostly 4400-4800 K, but
    "sometimes gets as high as 5100" as I see things here. I consider 5200 K
    to be an extreme of Washington DC on a favorably clear-air day close to
    "high noon"and close to "summer solstice", and 5400 K to be high side of
    "direct sunlight" from sun-at-zenith at lower altitude/elevation with
    low-side existence of whatever causes haze.
    As a result, I expect illumination onto a planar surface that direct
    sunlight is perpendicularly illuminating without obstructions to "light
    from ther sky" to average at 5500 K, maybe closer to 5775-5800 K in
    more-ideal situations of lack of cloud presence with sun at least
    30-45-or-whatever degrees above horizon, or-similar...
    And "idealized 5500K daylight" still appears to me to be
    best-photographed in when the film is "color slide film" or "color movie
    film" uf a UV-attenuating filter is deployed in order to make the "roughly
    below-440-nm-spectral-response" of the film plus photographinh optics
    including filters more like that of human vision. That part does require
    attenuation of wavelengths near 400 nm and in the upper and mid-upper
    300's of nm (including 350-360 and 390-390's).
     
  5. Bob Myers

    Bob Myers Guest

    Yes, but therein lieth the problem - all of these identifications of various
    "color temperatures" (more correctly, "standard illuminants" like the CIE
    D65, etc., standards) as "daylight white" or whatever do come with a
    list of qualifications a mile long - things like "daylight white, with
    the scattering
    components removed, except on Thursdays in months with no "R" in the name,
    as measured at noon with a background of new-fallen snow." Stuff like that.
    Still, if you read a reference to "daylight white," it is VERY often the
    6500K
    point or D65 illuminant they're talking about. For more on the gory
    details, see:

    http://www.hunterlab.com/appnotes/an05_05.pdf

    and

    http://en.wikipedia.org/wiki/Standard_illuminant

    Note that actually, standard illuminants with CCTs ranging from about 5000K
    to 6774K are all referred to, with various qualifying statements, as
    "daylight
    white." The entire CIE "D" series was supposed to be "daylight" - but D65
    (CCT of 6504K) is most commonly the one referred to as simply "daylight
    white." It may or may not appear bluish to various observers, depending on
    - well, a whole lot of things.

    Bob M.
     
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