Re: UV brighteners
Re: UV brighteners
- Subject: Re: UV brighteners
- From: Joel <email@hidden>
- Date: Fri, 9 Feb 2001 08:17:06 -0600
email@hidden wrote:
In a message dated 2/8/01 2:17:11 PM, email@hidden writes:
What effect does a UV brightener have on a spectral measurements for
profiling?
They make the paper read blue, especially to a scanner, and
sometimes require
adjustment (filter or software) to get the whitepoint to appear
white, so the
profiling software doesn't build a yellow profile to compensate.
When is it appropriate to use a UV cutoff filter, and when isn't
it?
> C. David Tobie wrote:
> That depends on the software you are running...
I am using Gretags ProfileMaker Pro.
Hi tn,
Here's part of Chris Pearson at ColorSavvy's response to one of my
endless CM2s inquiries. Might help. Gotta go...
The job of a reflective color measurement instrument is to break down the
reflected light into component parts. X-Rite instruments do this by
shining white light on the sample being measured and then passing the
reflected light through a series of filters that span the visible
spectrum. Behind those filters are photo detectors that record the amount
of reflected and filtered light.
Gretag instruments also shine white light onto the sample. The reflected
light is then further reflected off of a "grating" that acts like a
prism. The resulting rainbow is then imaged onto a CCD element like a
scanner has, and each pixel of the CCD records the reflection at that
part of the spectrum.
These instruments use a white light source that also has a significant
amount of radiation outside of the visible spectrum. The illumination
from these devices has both infrared and ultra-violet radiation. Almost
every instrument has a filter that blocks the infrared, that's because
photo detectors, photo diodes and CCDs react strongly to infrared. (While
the eye can't see it). As to the ultra-violet portion, photo detectors
and the like are not very sensitive in that part of the spectrum. BUT,
the materials we measure react to ultraviolet radiation by reflecting it
in another (longer wavelength) part of the spectrum. So while we cannot
"see" ultra-violet, if an object fluoresces, it reflects that unseen
radiation as light somewhere in the visible spectrum. Thus, fluorescent
materials seem to glow, because we can't see what's "lighting" them.
So how about the ColorMouse? The ColorMouse goes about the task of
breaking up reflected light into components by shining only those
components on the sample. Rather than filters or gratings to see how an
object reflects "red" light, the ColorMouse illuminates with "red" light
and collects the reflected radiation on a photo detector. In the case of
the CM2S, there are 16 different LEDs that span the visible spectrum.
These LEDs have very little infrared, and no ultra-violet. (Just now
there are ultra-violet LEDs coming to the market). Thus, even if a
material does fluoresce, it will not appear to do so with the ColorMouse
because there is no "excitation" illumination. So the ColorMouse does not
need a UV filter.
Now there are some that might say that the ColorMouse does not accurately
measure materials that fluoresce. After all, when a printed sample is
viewed in a light booth or in the sunlight, there is plenty of
ultra-violet to cause fluorescence, and that affects how we perceive
color. But, unless the color measurement instrument has the exact same
amount and wavelengths of ultra-violet that our viewing environment does,
it is likely to exaggerate the effects of fluorescence (as in an
unfiltered instrument) or understate those effects (as in a ColorMouse or
a UV filtered instrument).
Thus there is no completely correct answer. Fluorescence is a color
measurement nightmare, but it really helps color "come alive" to use a
crude unscientific term. Even laundry detergent has so called "bluing"
agents or flourescent materials added so that your shirts come out
"whiter than white."
I hope that this is somewhat helpful. This topic has been mentioned on
the ColorSync mailing list. I encourage you to pose your question on the
list as there are several knowledgeable folks who will certainly be able
to "illuminate" this topic further.
Savvy Regards,
Chris
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joel johnstone
designtype
Winnipeg Manitoba Canada
email: work: email@hidden
color geek in residence, reality notwithstanding
[We are Colorsync - you will be assimilated]