Copyright © 2000 Tailored Lighting Inc.
Last modified: 19 Apr 2004 20:04:02 -0000
The goal of this tutorial is to clarify issues that are relevant to a
discussion about ultraviolet radiation. The following topics will be discussed:
Understanding the difference between UVB and UVA radiation
UV specification background information for Museums
What is being done particularly for Museums?
Understanding the difference between UVB and UVA radiation
It is well known that light has a Wave-Particle Duality. That is, light
exhibits both particle and wave properties at the same time. In 1923,
Compton verified that a photon hitting an electron is like a billiard ball
collision. There is a momentum transfer from the photon to the electron
which manifests itself as a wavelength shift. Compton used X-ray photons of
known wavelength and found that the scattered photons had longer wavelengths.
The shift to a longer wavelength provided proof that the initial momentum
of the photon was partially transferred to the electron.
In 1905 Einstein proposed that light consists of "light quanta" that have
energy E=hc/x where h is Planck's constant (4.14x10^-15 ev*sec) , c is the
speed of light (3.0X10^8m/sec), and x is wavelength. Using Einstein's
equation: 300nm radiation contains 4.14 ev (electron volts) per photon where
as 380nm contains 3.27 ev/photon.
Let us use Compton's billiard ball and perhaps a soft ball of the same size
and shape to illustrate an important point. If a window can generally
withstand a force of 30 pounds per square inch (psi) and a billiard ball can
generate up to 35 psi as opposed to a soft ball which can only generate a
maximum of 29.0 psi, it doesn't take an Einstein to realize that while a
singe billiard ball will break the window, it may take hundreds of tries
with a softball before finding the weak spot in the window. Likewise
with UVA (soft ball) radiation, you can expose certain materials
to it with little or no damage ever occurring. In addition,according to the
Lighting Handbook Reference and Application , Copyright1993,
"It has been found that ultraviolet energy of wavelengths shorter than 300nm
(UVB) may cause rapid fading and other forms of product deterioration in some cases."
UVB is not only more harmful to materials but also is potentially more
harmful to the human eye. A condition known as Photokeratitis is a painful
inflammation of the outermost layer of the cornea. The action spectrum
(highest absorption) peaks at 270-280 nm, falling off to negligible values at
320 nm. In addition, recent studies have also shown "statistical
significance" in the relationship between cortial lens opacities (cataracts)
and lifelong UVB exposure.
UVB radiation ("B" for "Bad") is clearly the most harmful radiation both to
man and materials. Fortunately, SoLux, has the least amount of UVB radiation
of any light source on the market, 2.4 Microwatts/Lumen.
UV specification background information for Museums
The 75 microwatt per lumen standard was derived from the work of G. Thompson,
The Museum Environment (London: Butterworths, 1986). The 75 microwatt per
lumen standard is based on the output from a standard incandescent lamp.
Subsequent work, by D.R. Saunders, UV filters for Museum Light Sources
(National Gallery Technical Bulletin) suggests total levels of 10 microwatts
per lumen. In a recent article by Kevan R. Shaw, Lighting Dimensions
(September 1996), Mr. Shaw states what he believes should be the considered
for Lux and UV limits. "Max level sensitive (materials) Lux 100, microwatt
per lumen 10, Max level normal Lux 200, microwatt per lumen 40."
What is being done particularly for Museums?
Since the above values are simply guidelines and not standards, many museums to
be on the safe side with their priceless artifacts do not discriminate between UVA
and UVB radiation they simply want zero UV. New 3500K and 4100K SoLux lamps
have been designed with the museum market in mind with the lowest possible UVB
and UVA output.
Learn more about choosing the proper daylight color temperature for museum lighting by reading
Daylight: Is it in the eye of the beholder?
The new SoLux daylight color temperatures (3500K and 4100K) have been available
for over two years now and have quickly become the most specified light sources
in the museum world.
Author: Kevin P. McGuire