Fluorescence
Fluorescence
- Subject: Fluorescence
- From: Robin Myers <email@hidden>
- Date: Thu, 15 Feb 2001 21:47:50 -0800
- Organization: Robin Myers Imaging
I would like to clear up a misconception that some people have about
fluorescence. It is not only caused by ultraviolet radiation. Fluorescence can
also be initiated by light in the visible spectrum.
For papers and fabrics, fluorescent brighteners are part of the method used to
make them look white. As part of their manufacturing process, the substrate
materials are bleached to make them white, or as white as possible (the
natural
color for most papers and fabrics is yellowish), then they are treated with a
fluorescent brightener to counteract any residual yellowness. In the case of
fabrics, as they are laundered during their lifetime, they tend to return to
their natural yellowness, so whites are rebleached and the fluorescent
brighteners reapplied to maintain their whiteness. For a long time these
fluorescent materials were known as "bluing" agents.
Many white papers encountered in color management contain fluorescent
brighteners that convert invisible ultraviolet light into visible blue light.
This is an absorption of higher energy light and a re-emission of light at a
lower energy. Since this is a process of higher energy light being converted
to lower energy light, it can occur with higher energy visible light being
converted to lower energy wavelengths of visible light. If anyone is
interested, I can email them a graph showing an orange color measured with a
GretagMacbeth Spectrolino both with and without a UV blocking filter applied.
The results clearly show that the material continues to fluoresce when the UV
light is completely blocked, thus proving that the fluorescence is caused by
visible light.
This means that the ultraviolet blocking filters will only effect the
materials that fluoresce with ultraviolet light. It also means that any
spectrophotometer can cause a fluorescence during its measurement if the
material being measured will fluoresce with visible light, including the CM2s.
So, why worry about visible fluorescence? Just put on an ultraviolet blocking
filter and measure away. At one point in time, one very popular printer
manufacturer produced almost all their magenta colorants as a fluorescent
version. It made color matching almost impossible. Most manufacturers do not
use fluorescent colorants, but you should be ever vigilant. Never assume your
instrument will not cause fluorescence or that you are not measuring
fluorescent colorants until you run a test. And if you are measuring paper
with a fluorescent brightener, you should use an ultraviolet blocking filter
for your measurements.
Robin Myers