Re: What is better for window protection, UV glass or IR glass.

Greetings Larry:

Reference:

1. NASA's Electromagnetic Spectrum web site:
emspectrum.html

The answer to your question is not an easy one. The choice of IR or UV filtering
glass is complicated by considering relatively simple engineering issues versus
complex health issues.

Photons

Infrared IR energy and ultra violet (UV) energy is carried by photons.
A photon is a force carrying particle and is a packet of electromagnetic energy
that has no mass and must travel at the speed of light to exist. Photons exhibit
both wave like behavior and particle like behavior.

Photons Interaction with Atoms and Molecules

Dielectrics (insulators) such as glass have tightly bound electrons and when a
photon hits a surface atom it can be absorbed as heat (absorption), or the atom
can emit a new photon that can proceed on into the material in a direction
similar to the direction of the original photon until it hits another atom and
the process is repeated (we call this transmission), or the atom can emit a new
photon back out of the material (we call this reflection). We have all seen
visible light absorbed, transmitted and reflected from glass. As the transmitted
photons go from atom to atom through the dielectric material, there is a slight
delay at each atom as the photon excites the atom and the atom emits a new
photon. This delay appears to slow down the speed of light in the material and
we call this the index of refraction of the material. Adding special absorbing atoms
to window glass can selectivly absorb IR or UV photons.

The amount of energy (E) in a photon is equal to Planck's Constant (h) times
the frequency (f) of the photon:

E = h * f

Where: E = energy in joules
h = 6.625 * 10 ^ - 34 joule second
f = frequency in Hertz (cycles per second)

Note: one joule for one second = one watt of power. Thus (h) provides a very,
very, very small amount of power for detecting one photon per second or even
1000 photons per second. However, each square yard (square meter) of noon time
sunlight can transmit as much as 1000 watts of heating into a room.

The equation shows that the energy of a photon is linear with
increasing frequency so that there are large energy differences between IR
photons and UV photons.

Radio photons have enough energy to vibrate the nucleus of water molecules
as used in magnetic resonance imaging (MRI). Microwave photons have enough
energy to rapidly vibrate molecules as used in the heating of water molecules
in microwave ovens. Infrared photons vibrate atoms and molecules causing them
to heat up. Visible light photons interact with electrons causing them to change
orbits generating coherent light as used in lasers. UV photons, and higher
frequency X- ray photons, have enough energy to pass through matter while
tearing atoms and molecules apart and they can be harmful to life. The
NASA Electromagnetic Spectrum reference goes into more detail about photon energy.

Window Glass Fabrication
IR energy passing through glass causes heat build up inside a closed room and
the selection becomes an engineering problem. In the winter IR light can reduce
heating costs; however, in the summer it will increase cooling costs. By adding
IR filters to the glass you reverse the issue. Without IR heating you will
require more heating in the winter and less cooling in the summer. So one way
to look at the issue is what costs your facility the most each year, heating or
cooling. I believe Concord, California has a moderate climate similar to my home
in Thousand Oaks, California where annual cooling costs are greater than heating
costs and so IR filtering glass would be best.

UV filtering presents a different problem, a health problem. Long term exposure
to UV radiation from sunlight can cause skin cancer and there are a number of
people that have eye problems that UV radiation can exacerbate. UV exposure is
a problem for people with plastic lens implants in their eyes or that have
diabetic problems or other disorders of the retina in the eye. Many of the people
with lens implants must ware eye glasses and they should have included UV
filtering coatings on their corrective lenses. This adds about $50 to the cost
of purchasing eye glasses. Thus the decision of whether or not to add UV
filtering to class room window glass is a difficult one and it would be helpful
to determine what population of people using the class room have eye disorders
where the UV light would be harmful. The population is probably small but it is
difficult to compare or trade-off with solid engineering data.

I hope this information helps clarify the issues for you in determining whether
to add IR or UV filtering to your class room windows.

Best regards, Your Mad Scientist
Adrian Popa