Measuring the Color of White

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier wrote:
> OK, so I got this Ott Light, a portable desk lamp with a daylight
> balanced fluorescent bulb, in order to check my prints and compare them
> to the monitor.
>
> But the light seems a little green to my sore eyes, so I am wondering
> how I can measure the color temp of it to be sure of what I am looking
> at. I'm trying to come up with a scheme of shooting a piece of white
> paper with my camera and trying to see what white balance it takes to
> correct it to 6500K. But I'm not sure yet how to do that. On my
> particular camera, the Oly E20, it doesn't tell you what the actual
> color temp is when you white balance it. I do have the option of setting
> specific white balances manually, though. So suppose I set 6500, then
> shoot the color of the light, then bring it into my photo program, then
> measure the RGB values of it? Would that make sense?
>
> Any other ideas on this?

If you were to achieve the above, it would lead you to....?

--
John McWilliams

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier writes:

> But the light seems a little green to my sore eyes, so I am wondering
> how I can measure the color temp of it to be sure of what I am looking
> at. I'm trying to come up with a scheme of shooting a piece of white
> paper with my camera and trying to see what white balance it takes to
> correct it to 6500K. But I'm not sure yet how to do that. On my
> particular camera, the Oly E20, it doesn't tell you what the actual
> color temp is when you white balance it. I do have the option of setting
> specific white balances manually, though. So suppose I set 6500, then
> shoot the color of the light, then bring it into my photo program, then
> measure the RGB values of it? Would that make sense?

Set your camera to 6500 K, shoot a piece of blank paper in light that
truly is 6500 K (direct sunlight is roughly this), then pull it up in
your photo program and balance the colors in the image until the color
of the paper shows as a neutral white (RGB values all the same). Save
these adjustments for subsequent use. That will give you a correction
to apply to get neutral colors whenever you use the 6500 K setting in
6500 K light.

However, I find it easier to just look for something in an image that is
supposed to be a neutral gray, then adjust the colors in the image until
it _is_ a neutral gray. Neutral gray means that red, green, and blue
values are all identical. Don't trust the way it looks on your screen,
since that can vary by monitor and display settings.

--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly.

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier <geickmei@tampabay.rr.com> wrote:
> OK, so I got this Ott Light, a portable desk lamp with a daylight
> balanced fluorescent bulb, in order to check my prints and compare them
> to the monitor.

> But the light seems a little green to my sore eyes, so I am wondering
> how I can measure the color temp of it to be sure of what I am looking
> at.

This doesn't make sense with a fluorescent. What you really need to
know is the colour rendering index, or CRI, and to measure that you
need a spectrophotometer.

According to a web site I'm looking at, the Ott-Lites come in two
kinds, the Task bulb and the True Color bulb. The former has a CRI of
83, which is useless for your purpose. The latter has a CRI of about
93, which is more like what you need for photos.

So which kind of bulb do you have?

Andrew.

Archived from groups: rec.photo.digital (More info?)

If you are going to use it to "check your prints" ...

LOOSE the florescent!

Archived from groups: rec.photo.digital (More info?)

RSD99 wrote:

> If you are going to use it to "check your prints" ...
>
> LOOSE the florescent!

What do you mean, "loose" it? Like let it loose?

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier wrote:
>
>
> andrew29@littlepinkcloud.invalid wrote:
>
>> Gary Eickmeier <geickmei@tampabay.rr.com> wrote:
>>
>>> OK, so I got this Ott Light, a portable desk lamp with a daylight
>>> balanced fluorescent bulb, in order to check my prints and compare
>>> them to the monitor.
>>
>>
>>
>>> But the light seems a little green to my sore eyes, so I am wondering
>>> how I can measure the color temp of it to be sure of what I am
>>> looking at.
>>
>>
>>
>> This doesn't make sense with a fluorescent. What you really need to
>> know is the colour rendering index, or CRI, and to measure that you
>> need a spectrophotometer.
>>
>> According to a web site I'm looking at, the Ott-Lites come in two
>> kinds, the Task bulb and the True Color bulb. The former has a CRI of
>> 83, which is useless for your purpose. The latter has a CRI of about
>> 93, which is more like what you need for photos.
>>
>> So which kind of bulb do you have?
>
>
> Good question! The lamp is at
>
> http://staples.com/Catalog/Browse/ [...] able+Lamps
>
>
> but it doesn't tell which bulb it is.
>
> Gary Eickmeier

You can probably tell if you have the CRI 83 or the CRI 93 by taking a
picture of a Kodak Gray card illuminated ONLY by the Ott Light.
Look at the image in Photoshop.
If the RGB values are pretty close to each other (+/- 5 units), you have
the CRI 93. If the G value is way out of line with R&B, you have the CRI 83
Bob Williams

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier <geickmei@tampabay.rr.com> writes:
>OK, so I got this Ott Light, a portable desk lamp with a daylight
>balanced fluorescent bulb, in order to check my prints and compare them
>to the monitor.
>
>But the light seems a little green to my sore eyes, so I am wondering
>how I can measure the color temp of it to be sure of what I am looking
>at.

There is no good answer to this question. Incandescent sources give off
light with a continuous spectrum, but the actual shape of the spectrum
can be biased towards blue or red depending on the source temperature.
Most light sources that are not incandescent (including fluorescent
tubes) have spiky spectra that don't look anything like an incandescent
source.

In the first place, most colours do not have a "colour temperature". If
you look at any version of the CIE colour diagram, which shows all
possible colours while ignoring brightness, you'll see that it is a 2D
space. The set of colours that can be produced by an incandescent
object (e.g. the sun, an incandescent lamp filament) forms a somewhat
curved line on this chart. The position along the line can be specified
by a single measurement (a colour temperature) because the line is 1D.

Colour temperature meters work by measuring the ratio of red to blue
light, or some equivalent comparison, *assuming* that the light is
actually from a incandescent source.

So *if* you have an incandescent source, its actual colour is somewhere
along that line, and a colour temperature value is all that's needed to
fully describe the apparent colour you get, and the shape of the light
source spectrum. If you have an incandescent source, a colour
temperature meter will tell you what the colour temperature is.

But for a light source with a weird spectrum, its apparent colour is
most likely not equivalent to *any* point on the incandescent source
line, and its colour temperature is not defined.

Even if you found a fluorescent lamp that exactly matched the colour of
(for example) 5500K incandescent lamp, it *still* probably wouldn't
look the same for viewing prints. Matching the colour of the light
sources just means that a piece of matte white stuff (that reflects all
wavelengths equally) illuminated by those two sources will look the
same.

But the apparent colour of something like a print is determined by the
effect of the cyan, magenta, and yellow dyes in the print on the
incoming light. And *this* is determined by wavelength-by-wavelength
multiplication of the light in the lamp spectrum by the absorption of
the dye. In general, you're going to get different results if the two
light sources have different spectra *even if the sources look exactly
the same when illuminating a white object*. To some extent, this is
measured by the Colour Rendering Index (CRI) of the fluorescent lamp.
This tells you how much colours will be distorted by the fact that the
fluorescent lamp's spectrum is different from an incandescent source of
the same apparent colour.

The Ott fluorescent tubes are designed to have a higher CRI than most
fluorescents, but they're still not equivalent to a good incandescent
source. If you want to see what prints will look like under sunlight or
incandescent lamps, use incandescents for viewing.

Dave

Archived from groups: rec.photo.digital (More info?)

As in "Do Not Use" ...



"Gary Eickmeier" <geickmei@tampabay.rr.com> wrote in message
news:fyJCd.216326$6w6.145390@tornado.tampabay.rr.com...
>
>
> RSD99 wrote:
>
> > If you are going to use it to "check your prints" ...
> >
> > LOOSE the florescent!
>
> What do you mean, "loose" it? Like let it loose?
>
> Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

"RSD99" <rsdwla.NOSPAM@gte.net> wrote in message
news:ZCVCd.16702$Y57.12969@trnddc08...
> "Dave Martindale" posted:
> "...
> There is no good answer to this question.
> ..."
>
> Wrong. It is a VERY well researched subject. That is, if you are
> talking
> about what illumination to use to "check ... prints" for proper
> color and
> appearance - which I believe is the OP's actual question.

I think the OP wanted to know how to measure the color temperature of
his lightsource. For that he would need a spectrophoto meter, a bit
too expensive for a single measurement I would guess. The Gretag
MacBeth EyeOne Photo would be very useful for that purpose.

Of course pleasant looking (daylight) color also depends on the
illumination level. At lower luminance levels a shift (of up to 65nm
for nightvision) in the human eye's peak sensitivity takes place
(Purkinje effect), requiring a lower Kelvin temperature to match the
intended color balance.

Bart

Archived from groups: rec.photo.digital (More info?)

For a more precise explanation of what colour temperature actually
means, I'll quote a bit from Hunt's "The Reproduction of Colour" (5th
ed, p214):

If the relative spectral power distribution of a source is
exactly the same as that of a Planckian radiator, then the
temperature of the latter is referred to as the _distribution
temperature_ of the source. Most sources, however, do not
duplicate the relative power distribution of a Planckian
radiator exactly, but many have the same chromaticity as that of
a Planckian radiator; in this case the temperature of the latter
is referred to as the _colour temperature_. It is common with
other sources of whitish light to quote their _correlated colour
temperature_: this is defined as the temperature of the
Planckian radiator that produces light most closely matching the
particular source. These correlated colour temperatures then
provide a useful indication of the relative bluishness or
yellowishness of the sources.

In other words, if a source has a distribution temperature, it really
has a spectrum like a black body, and objects viewed under it will look
the same as they do under any other blackbody source with the same
distribution temperature.

If a source has a colour temperature, then it *looks* the same as some
true blackbody source, but the spectrum generally won't match, and the
colour rendering won't be the same if the spectrum doesn't match.

If a source only has a "correlated colour temperature", it doesn't look
exactly like any blackbody source in colour, but it's close to one.
Again, the spectrum may be very different from a blackbody.

Dave

Archived from groups: rec.photo.digital (More info?)

Dave Martindale wrote:

> There is no good answer to this question. Incandescent sources give off
> light with a continuous spectrum, but the actual shape of the spectrum
> can be biased towards blue or red depending on the source temperature.
> Most light sources that are not incandescent (including fluorescent
> tubes) have spiky spectra that don't look anything like an incandescent
> source.
>
> In the first place, most colours do not have a "colour temperature". If
> you look at any version of the CIE colour diagram, which shows all
> possible colours while ignoring brightness, you'll see that it is a 2D
> space. The set of colours that can be produced by an incandescent
> object (e.g. the sun, an incandescent lamp filament) forms a somewhat
> curved line on this chart. The position along the line can be specified
> by a single measurement (a colour temperature) because the line is 1D.
>
> Colour temperature meters work by measuring the ratio of red to blue
> light, or some equivalent comparison, *assuming* that the light is
> actually from a incandescent source.
>
> So *if* you have an incandescent source, its actual colour is somewhere
> along that line, and a colour temperature value is all that's needed to
> fully describe the apparent colour you get, and the shape of the light
> source spectrum. If you have an incandescent source, a colour
> temperature meter will tell you what the colour temperature is.
>
> But for a light source with a weird spectrum, its apparent colour is
> most likely not equivalent to *any* point on the incandescent source
> line, and its colour temperature is not defined.
>
> Even if you found a fluorescent lamp that exactly matched the colour of
> (for example) 5500K incandescent lamp, it *still* probably wouldn't
> look the same for viewing prints. Matching the colour of the light
> sources just means that a piece of matte white stuff (that reflects all
> wavelengths equally) illuminated by those two sources will look the
> same.
>
> But the apparent colour of something like a print is determined by the
> effect of the cyan, magenta, and yellow dyes in the print on the
> incoming light. And *this* is determined by wavelength-by-wavelength
> multiplication of the light in the lamp spectrum by the absorption of
> the dye. In general, you're going to get different results if the two
> light sources have different spectra *even if the sources look exactly
> the same when illuminating a white object*. To some extent, this is
> measured by the Colour Rendering Index (CRI) of the fluorescent lamp.
> This tells you how much colours will be distorted by the fact that the
> fluorescent lamp's spectrum is different from an incandescent source of
> the same apparent colour.
>
> The Ott fluorescent tubes are designed to have a higher CRI than most
> fluorescents, but they're still not equivalent to a good incandescent
> source. If you want to see what prints will look like under sunlight or
> incandescent lamps, use incandescents for viewing.

I can see I have a lot to learn. I almost understand what you are saying
about the spectrum of the light not being along the red/blue line. But
saying just use incandescents for viewing is wrong, because we all know
that incandescents (most) are on the yellow side, compared to daylight.

Or, if I have to look for an incandescent for the reasons you say, then
is there an incandescent that is daylight white balance? Or, better yet,
some ready-made light like the Ott task lamp?

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Dave Martindale wrote:

> If a source only has a "correlated colour temperature", it doesn't
look
> exactly like any blackbody source in colour, but it's close to one.

Pedantic nitpicking aside, the CCT is what is invariably meant when
someone is talking about the "colour temperature" of illuminant X, as
there aren't many sources that near-perfect black bodies (the 2.7K
cosmic background?), nor do many spectrums have the exact same
chromaticity of a perfect black body.

The Robertson's CCT algorithm is well known. Heck, there is even code:

http://groups.google.ca/groups?hl= [...] google.com

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier <geickmei@tampabay.rr.com> writes:

>I can see I have a lot to learn. I almost understand what you are saying
>about the spectrum of the light not being along the red/blue line. But
>saying just use incandescents for viewing is wrong, because we all know
>that incandescents (most) are on the yellow side, compared to daylight.

Perhaps the ideal lamp for viewing prints would be a carbon or xenon
arc. They give pretty continuous spectra, yet have a colour
temperature close to daylight. Not exactly practical for home use.

A halogen bulb incandescent desk lamp is a nice continuous-spectrum
source, and also higher in colour temperature (less yellow) than most
incandescents. That's what I tend to use when colour is important. No,
it's not the same colour as sunlight, but your eye will accomodate away
some of the difference.

>Or, if I have to look for an incandescent for the reasons you say, then
>is there an incandescent that is daylight white balance? Or, better yet,
>some ready-made light like the Ott task lamp?

Probably the best you could do with an incandescent lamp alone would be
a halogen lamp plus an appropriate colour conversion filter.
Fluorescent lamps with a high colour rendering index might also be good
enough; someone else posted that there are two grades of Ott tubes and
one has a high CRI.

Another way of looking at it: what light will your prints normally be
viewed under? Maybe that's the appropriate light to evaluate them, even
if it's nothing like daylight. (For that matter, remember that overcast
daylight is very different than noon sunlight!)

Dave

Archived from groups: rec.photo.digital (More info?)

Dave Martindale wrote:

> Another way of looking at it: what light will your prints normally be
> viewed under? Maybe that's the appropriate light to evaluate them, even
> if it's nothing like daylight. (For that matter, remember that overcast
> daylight is very different than noon sunlight!)

That's not the point. The idea is, I want the correct illumination for
my prints, to be able to compare the printer output to the monitor. A
lot of ink is spilled on monitor calibration, but it is meaningless
without the correct lighting to view your prints with.

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier wrote:
>
> John McWilliams wrote:
>
> > I'd concentrate on using the best WB in the field that you can, and not
> > try to pre-balance for your printer, together with calibrating your
> > monitor.... if that's what you're trying to do; it's not clear to me
> > where you are going with this.
> >
> I'm trying to measure the color of white with my camera.

thats the wrong tool.

you need a spectrophotometer.
what you need to know is the spectral distribution of energy,
a graph of brightness vs wavelength.


> Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Bart van der Wolf wrote:

> Metamerism and viewing conditions (average luminance level) will cause
> colorshifts if the image is viewed under different lighting. I
> understand what you're trying to achieve, but it can only be done by
> profiling input device (camera) and output device (monitor, printer).
> You'll need a colormanaged workflow to convert from one colorspace to
> the other.
>
> If the goal were to match the viewing lightsource to the final viewing
> conditions, you'll need the EyeOne Photo (Spectrophotometer) which can
> also measure ambient light. The difference in color temperature can be
> approximately compensated by applying a simulated CC filter correction
> (e.g. with http://www.mediachance.com/digicam/filtersim.htm) but it is
> of limited accuracy compared to a fully colormanaged workflow (Photoshop
> CS also has "photo filters" for color shifts).

OK, I'll try this one more time, and that's it.

I'm learning about color management. To begin the process of color
management, you have to calibrate your monitor. From what I have read so
far, when you do this, the white that is displayed on the monitor will
be close to normal daylight, somewhere between 5500 and 6500K.

To compare the monitor images to your prints, you would have to view the
prints in light that is somewhere near daylight as well. Ordinarily, we
use incandescents indoors, around 3700K or less. That means that a
comparison between your prints in such light and your monitor which is
daylight balanced would not be valid, so you wouldn't be able to tell
whether your printer calibration were correct yet or not.

Therefore, I was searching for a desk lamp that approximated daylight,
strictly for the purpose of checking the color of my prints as they came
out of the printer. Someone told me the Ott would turn the trick. I
looked at them at the store, and their main point was that they were
daylight balanced, so I bought one.

When I got home, however, and looked at my prints compared to my
monitor, they looked greenish. So I suspected the color balance of the
Ott light was not daylight, and I wondered how I might check that. I
don't own a color spectrometer of some sort, but I so own a digital
camera. So I am trying to think up a method of using the camera to
measure the white balance of the light. I thought of the method that you
read about in the newsgroup, but I haven't tried it yet.

So the ultimate type of light I will be viewing my prints in is not the
point. The idea is to find a near daylight desk lamp that I can use to
check my color under controlled conditions - conditions that match my
monitor as closely as possible (as I understand it).

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Crownfield wrote:

> did you try taking the print outside?
> that usually approximates daylight pretty well.

Yes, but it would be difficult to view my monitor outdoors as well.
Besides, moving my editing room to the driveway would not work in
inclement weather.
>
> you may well find that much of your viewing
> is under tungsten lighting.

True. Sigh.

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Crownfield wrote:
> Gary Eickmeier wrote:
>
>>John McWilliams wrote:
>>
>>
>>>I'd concentrate on using the best WB in the field that you can, and not
>>>try to pre-balance for your printer, together with calibrating your
>>>monitor.... if that's what you're trying to do; it's not clear to me
>>>where you are going with this.
>>>
>>
>>I'm trying to measure the color of white with my camera.
>
>
> thats the wrong tool.
>
> you need a spectrophotometer.
> what you need to know is the spectral distribution of energy,
> a graph of brightness vs wavelength.
IAE, I am still pretty convinced that even should you achieve that, does
it really get you to the end result you want?

--
John McWilliams

Archived from groups: rec.photo.digital (More info?)

John McWilliams wrote:
>
> Crownfield wrote:
> > Gary Eickmeier wrote:
> >
> >>John McWilliams wrote:
> >>
> >>
> >>>I'd concentrate on using the best WB in the field that you can, and not
> >>>try to pre-balance for your printer, together with calibrating your
> >>>monitor.... if that's what you're trying to do; it's not clear to me
> >>>where you are going with this.
> >>>
> >>
> >>I'm trying to measure the color of white with my camera.
> >
> >
> > thats the wrong tool.
> >
> > you need a spectrophotometer.
> > what you need to know is the spectral distribution of energy,
> > a graph of brightness vs wavelength.
> IAE, I am still pretty convinced that even should you achieve that, does
> it really get you to the end result you want?

the trick to solving a problem is to eliminate variables.

if ew can show that the light is very close to daylight,
then
if the monitor is known,
and the light source is known, or equal to a known,
then the print is the variable left.

>
> --
> John McWilliams

Archived from groups: rec.photo.digital (More info?)

John McWilliams wrote:

> Gary Eickmeier wrote:
>
>>
>> When I got home, however, and looked at my prints compared to my
>> monitor, they looked greenish. So I suspected the color balance of the
>> Ott light was not daylight, and I wondered how I might check that.
>
>
> By looking at the very same prints in actual daylight.

You guys are starting to rile me...

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

Gary Eickmeier wrote:
>
> John McWilliams wrote:
>
> > Gary Eickmeier wrote:
> >
> >>
> >> When I got home, however, and looked at my prints compared to my
> >> monitor, they looked greenish. So I suspected the color balance of the
> >> Ott light was not daylight, and I wondered how I might check that.
> >
> >
> > By looking at the very same prints in actual daylight.
>
> You guys are starting to rile me...

you walked right into that one.

>
> Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

andrew29@littlepinkcloud.invalid wrote:

> Gary, listen. On the Ott-Lite front page there is a link at the very
> top to the "TrueColor(TM)" desk lamps. These are the ones that are a
> daylight approximation. Now, did you buy one of these or not? It's
> impossible to have a sensible discussion without knowing. If you
> didn't buy a TrueColor lamp, there's no mystert about your odd
> results.

It's really tough to find out from anything that is in my package or
anything on the cards that the new bulbs come on, what the color balance
is. So what is the Ott web site? I was operating on the Staples web
site. But I bought it at a store.
>
>
>>When I got home, however, and looked at my prints compared to my
>>monitor, they looked greenish. So I suspected the color balance of
>>the Ott light was not daylight, and I wondered how I might check
>>that. I don't own a color spectrometer of some sort, but I so own a
>>digital camera. So I am trying to think up a method of using the
>>camera to measure the white balance of the light.
>
>
> Forget it. It can't be done. Fluourscent tubes do not have a
> continuous spectrum.

Then why are you advising me on the True Color Ott lights?

Gary Eickmeier

Archived from groups: rec.photo.digital (More info?)

andrew29@littlepinkcloud.invalid wrote:
>
> Gary Eickmeier <geickmei@tampabay.rr.com> wrote:
>
> > OK, I'll try this one more time, and that's it.
>
> > I'm learning about color management. To begin the process of color
> > management, you have to calibrate your monitor. From what I have
> > read so far, when you do this, the white that is displayed on the
> > monitor will be close to normal daylight, somewhere between 5500 and
> > 6500K.
>
> > To compare the monitor images to your prints, you would have to view
> > the prints in light that is somewhere near daylight as
> > well. Ordinarily, we use incandescents indoors, around 3700K or
> > less.
>
> More like 2800, usually.

above 100 watts, they are usually close to 3200. type b.
below that they drop fast. 2800 is probably a good guess.


>
> Andrew.