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From: don on 10 May 2010 22:53
When choosing a Flourescent Bulb for a Task Light what is the best for
reading.......my choices are 2700 Kelvin, 3000, 3500, or 4100.....



From: Don Klipstein on 10 May 2010 23:37
In article <hsagru$2fo$1(a)reader1.panix.com>, don wrote:

>When choosing a Flourescent Bulb for a Task Light what is the best for
>reading.......my choices are 2700 Kelvin, 3000, 3500, or 4100.....

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".

--
- Don Klipstein (don(a)misty.com)
From: Bob Myers on 11 May 2010 16:32
On 5/10/2010 9:37 PM, Don Klipstein wrote:
> In article<hsagru$2fo$1(a)reader1.panix.com>, don wrote:
>
>
>> When choosing a Flourescent Bulb for a Task Light what is the best for
>> reading.......my choices are 2700 Kelvin, 3000, 3500, or 4100.....
>>
>
> 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".
>

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.


From: Don Klipstein on 11 May 2010 21:34
In article <hsce4c$qpe$1(a)usenet01.boi.hp.com>, Bob Myers wrote in part:

>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)

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.

> and is fairly close to the "equal energy" white (a light
>source that's flat in terms of energy across the visible spectrum).

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.

>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).

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).

--
Don Klipstein, best as I can say now, Best Regards to everyone/anyone, ..
(don(a)misty.com)
From: Bob Myers on 13 May 2010 16:01
On 5/11/2010 7:34 PM, Don Klipstein wrote:
> In article<hsce4c$qpe$1(a)usenet01.boi.hp.com>, Bob Myers wrote in part:
>
>
>> 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)
>>
> 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.
>

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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