Glare (1):

In the context of matching for both Optical and ordinary RF
applications encountered in the Ham Shack; it is meant to be described
as the unwanted reflection of energy (or power, or what-have-you).

Glare (2):

In the context of the physiological response to observing unwanted
reflections; it is meant to be described by applications that give
rise to this annoyance. This can sometimes be confused with the first
meaning of Glare that is suitable close, but can come into conflict
with that meaning.

Glare, and Glare reduction:

When the topic of Glare is expressed in regard to the second usage, it
is always wavelength specific, and application specific. In fact this
is true of both meanings, but the commercial marketplace overwhelms by
example in the second. With that in mind, we have to consider WHO is
being served by Glare reduction. This is often the naive public who
would otherwise be encumbered by Glare where their attention is drawn
to it as it masks what is trying to be presented to them.

So, Glare masks the perception by reflections of light from, most
usually, a pane of glass, plexiglass, or acrylic (and so on down the
line) that covers or windows an exhibit. Usually we are speaking of
covered art work, or display cases exhibiting samples or products
(this being the most common form of Glare reduction application).

The quickest and most absolute method of Glare reduction in a showcase
window on the street looking into a storefront is to simply tilt the
window at the top, toward the viewer. This renders all light source
reflections (typically higher than the head of the viewer) down. Ten
or twenty degrees of tilt is often enough to give the illusion of
there being no glass at all. Note my usage of "illusion" as Glare is
all about perception in this second usage. This form of Glare
reduction is wide-band and exhibits very little sensitivity to
wavelength. The same Glare reduction of tilting glass covered artwork
is also suitable for the same reason. However, I would point out that
in such cases, the display staff make every effort that the lights be
spotted on the artwork, otherwise if your face is illuminated, the
artwork suddenly becomes a mirror.

Where both meanings of Glare converge for the same purposes, we find a
more specific remedy. Specific because it employs tuned interfaces to
reduce (not eliminate) reflections.

Here is where the convergence demands two different wavelength
specifications, and importantly WHY. As I have offered elsewhere,
there are two types of vision, Photopic and Scotopic (with a third,
Mesopic, between the two which is the process of accommodation of
migrating from one to the other - "acquiring" night vision is a
commonplace description).

Anti-Glare materials that employ thin-film technologies seek to
accommodate the major sources of Glare that fall into these two wide
band regions of 550nM and 510nM. They are also application specific
in that there are two forms of common illumination involved that
compounds this to a four way solution. Those sources of illumination
are Metal Halide and High Pressure Sodium lamps. There are others,
certainly, but these specie exhibit characteristics that are
extrapolated to similar sources.

Metal Halide lamps exhibit a comb of resonances, as does the High
Pressure Sodium, that makes Glare elimination impossible. However, it
does not make it impractical as among those major lines of emission,
the optical engineer can select the biggest contributor such as 510nM
for the night vision component of the eye's response. However, this
doesn't make much sense because it also reduces the eye's ability to
perceive what is being illuminated by that wavelength too. Not much
night vision advantage in that, is there? However, when considering
this is placed over artwork commonly, artwork exhibits a Lambertian
distribution of reflected light, not a specular form of reflection.
Hence the specular form exhibited by a pane of glass is reduced and
the Lambertian reflection emerges with a greater contrast (another
benefit of choosing anti-Glare products).

High Pressure Sodium, on the other hand, has a very strong line at the
610nM wavelength. This, too, would be a natural thin-film wavelength
selection. In regards to night vision, such a reflection (that is,
without Glare reduction) would be nearly invisible anyway. As such,
its use would be oriented towards those WHO have a light adapted eye,
that are in a darkened viewing situation.

However, as described, these are very poor solutions to the problem
they attempt to answer. This is because these sources exhibit a comb
of such wavelengths in their emissions. It follows that one
interference layer may impact one line, but certainly not all (there
being easily a dozen lines of emission for either source in either
light adapted vision). Hence we have the multi-layer methods that my
quote offered from the marketplace.

It should come as no surprise that anti-Glare products are wholly
useless under common tungsten lighting whose bandwidth is broad and
continuous. There is one exception, and it is commonly found for
computer screen reflection reduction. I must note that such reduction
has absolutely nothing in common with wavelength interference.

This class of reflection reduction employs a wide band light
reduction. Principally it seeks to reduce all ambient light from
striking the display's surface and then reducing the reflections even
further. This is a double whammy, where the display suffers it only
once. However, you can control the brightness of the display to
replace that loss, and in this sense you have increased your
signal+noise/noise ratio.

Finally there is one source of reflection elimination that is common
to both the optical engineer and the rf engineer (and apparently
wholly unknown by binary engineers). I offered above one display
window trick of tilting the top of the window towards the viewer.
When this is done at a particular angle (and the ground below is not
illuminated by the reflection - another feature of display window
design), then there will be no perception of the window at all. Many
people have knocked there heads against such windows trying to get a
closer look at what was being displayed. Lawyers trumped designers by
begging this was a possible source of litigation for those so injured.
Some of these windows may yet survive in the older parts of your town
where they cater to high-end shoppers. You may note that there are
rails placed in the way to keep the curious from getting to close and
bruising their noggins.

Now, I offered that there is a particular angle. For both the optical
engineer and the rf engineer, it is called the "Brewster Angle." This
is also known as the angle of maximum absorption and minimum
reflection. When light or RF (it doesn't matter which) of vertical
polarization strikes an interface at this angle, all of that energy
passes through the interface with minimum reflection. When you
perform a vertical antenna far field measurement, you may note that
the lobe "sucks in" at the very low radiation angles. This angle that
exhibits poor propagation results is a consequence of the air/earth
boundary that is identical in all respects to an optical interface
between two materials of differing indices. The optical engineer
calls it the index of refraction/reflection (and can be examined
through Snell's Laws); the rf engineer finds the same property in the
ratio between the characteristic Z of air and the characteristic Z of
the earth.

An instance to illustrate the rf scenario. With Air the Z is roughly
400 Ohms, for the Ocean it is roughly 10 Ohms. That ratio, in this
case 40, results in a Brewster Angle of about 1½°. If we were to
consider something more remote, and dryer like the ground in my
neighborhood where the characteristic Z is closer to 100 Ohms (luckily
I can see a vast body of seawater from my window); that ratio is 4
which results in a Brewster Angle of about 15°. Try as I might in
directions other than towards Puget Sound, I will never launch any
significant signals at angles lower than this 15°.

As a closing comment about the Brewster Angle, nearly every Laser uses
this in their output window.

Next in this series: T HE FAILURE OF POOR CONCEPTS IN DISCUSSING THIN
LAYER REFLECTIONS otherwise called by me as the WHEREFORE.

73's
Richard Clark, KB7QHC