Re: Optical Brighters while using UV Filter
Re: Optical Brighters while using UV Filter
- Subject: Re: Optical Brighters while using UV Filter
- From: Graeme Gill <email@hidden>
- Date: Mon, 26 Jul 2004 11:49:19 +1000
- Organization: Color Technology Solutions
Roger Breton wrote:
What do you mean by 'significant'? Do you mean to say that all there is
below 400nm is mostly UV energy? This does go against my poorly held beliefs
that, in nature, only sources like daylight could contain significant levels
of UV energy.
It's all relative. An A type source has about 10 times the light level at
the longest visible wavelength than the shortest, and integrating by
the visual sensitivity curve, the difference is even more dramatic,
so by this measurement the A type source has "low levels of UV".
But when used as an instrument illumination source, the source distribution
is divided out, and the relative level of stimulation wavelength UV to
the response wavelengths in the blue region are much less dramatic,
eg. less than 2:1. It is easy to see that there is a blue FWA "lump" in
a whitened media when using an A type instrument illuminant, that goes
away when a UV filter is fitted.
and it is more than enough to excite an Optical Brighter
More than enough? Hmmh. How much is enough?
Enough for the FWA response to contribute to the apparent
blue reflectance.
(actual
Fluorescent Whitening Agent - FWA, since some OB's can be non-fluorescent
dyes)
Is there such thing as Optical Brightners that don't fluoresce? Why bother
with those in the first place?
Yes there is. Many papers use some dyes to reduce reflectance in the green
and red wavelengths to make the paper look "whiter". This is often used
in combination with FWA. Generally cheaper papers do this.
You mean 'significant' as opposed to expecting it not be there in the first
place?
I mean it contributes noticeably to the resulting apparent paper color.
OK. I'll try that. But I'm curious how fitting the instrument with a 'D65'
filter can boost the UV input by two compared to the no filter?
Because it changes the illumination spectrum in such a way that the UV
to non-UV wavelength energy is higher. This means that the light
resulting from the UV simulation will have a higher level than the
illumination used to measure the reflectance in the blue wavelengths,
explaining why the FWA has greater influence.
And don't forget our friend, the sun. If I'm not mistaking, the sun's UV
output is mostly filtered (for now) by the earth's atmosphere. According to
the sun spectra I looked at on some web pages and in some books I have, I
tend to remember that by the time it reaches us that there is very little UV
energy left in its light?
No not at all (although it depends what latitude you are at, and how much pollution
is in your atmosphere). D65 is meant to approximate daylight, and it has a much higher
relative level of UV that an A source. It's the high level of UV in sunlight (particularly
short wavelength UV) that burns your skin so easily. Spend more than 15 minuets out here
in the middle of summer without protection, and you will be burnt.
A few years ago there was an issue with the popular quartz halogen downlights
being used in desk lamps, causing sunburn & eye damage. They were in close proximity to
peoples faces, and being made of quartz rather than glass, all the UV energy passes through
without loss. Such a light source is a higher white point version of an incandescent A light
source. The solution was simply to make sure that such lamps have a conventional glass cover
over them.
Graeme Gill.
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