This article is an excerpt from Popular Mechanics:

WINDOW FILM
New high-tech glass films claim energy savings for your home.
BY MERLE HENKENIUS

All window films start with the film, of course, which is always polyester, 2 to 7 mils
thick. Quite often, several thin layers of film are bonded together. One side is coated
with either a pressure-sensitive or water-activated adhesive. The exposed surfaces of
most films are also treated with a hard, scratch-resistant coating.

To filter out ultraviolet radiation, chemical UV blockers (cyclic imino esters) are
incorporated. If the film’s purpose is to provide only UV protection and shatter
resistance, no other materials need to be added.

DYED FILM

From there, three separate technologies are applied to achieve different performance
characteristics. The first is simply a dye, which absorbs heat. Because most films are
applied to the inside surfaces of windows, it’s easy to imagine that the absorbed heat
would disperse indoors. In fact, the heat rejected by the film is stored largely in the
glass, and is drawn away by external air movement. A tiny percentage does bleed
inward, but because the average speed of external air movement is so much
greater–the daily average is 15 mph, versus 1/2 mph indoors–the ratio is 30:1 or
better in favor of outdoor heat dissipation. Because double-glazed windows don’t
allow air movement between panes, interior-dyed films should not be used on thermal
glass.

The other two processes, called deposition technology (vacuum coating/metallizing)
and sputtering technology (advanced metallizing), deposit a layer of metallic particles
on the film, giving it a reflective coating. In each case, a second layer of film protects
the coating. Metallized films reject heat by reflecting it before it can be transferred
through the glass.

DEPOSITED FILM

In deposition technology, the film is drawn through a tank containing metal
ingots–usually aluminum or nickel-chrome, and occasionally copper. A vacuum is
created by reducing the pressure in the tank, which is then flooded with argon gas
and the ingots are heated. The heat causes the metal to give up particles that
migrate to the film’s surface. The density of the metal deposition is controlled by the
speed of the film through the chamber.

While deposition technology works well and is relatively inexpensive, it has its limits.
To be effective, the metallized coating must be fairly thick, as the particles are
comparatively large. What this means at a practical level is that it produces a darker,
more highly mirrored surface. And second, the list of metals that can be deposited
evenly is fairly short, which means fewer product options.

SPUTTERED FILM

Sputtering technology is more complicated. Sputtering is also done in a vacuum
chamber, but the metallizing is achieved at the atomic level. In brief, electromagnetic
fields direct streams of ions from a chemically inert gas (usually argon) toward the
metal. This ion bombardment, which is often described as “atomic billiards,” causes
groups of atoms to dislodge in small bursts and scatter uniformly across the film.

The practical benefits of sputtering are that 25 to 30 different metals can be used and
the metallized coating is much lighter. It’s possible to sputter metal in a layer
one-hundredth the thickness of a human hair. Different metals are chosen to subtract
specific bands of radiation from the solar spectrum. The result is a highly reflective
layer with very little mirror effect, heat absorption or color shift. Because sputtering is
more expensive, these films occupy the high end of the price range. Metallic films
control radiation through reflectivity. Simplified film consists of polyester layers,
metallic coating, adhesive and scratch-resistant coatings.

While the performance characteristics of dyed and metallic films are generally distinct,
there is some overlap. Heat-absorbing dyed films are somewhat reflective, and
metallic films do absorb some heat because of the mass and color of the metals
involved.

HYBRID FILM

To further complicate the issue, many films contain both dyes and reflective metals.
By combining dyes and metals, the negative effects of each can be reduced without
sacrificing performance. A good example is gray dye and titanium coating. If used
alone, dye would darken the film significantly, while the titanium would produce a
highly mirrored surface. When paired, less of each can be used, resulting in a film
that is relatively bright and nonreflective.

This point is significant, if only because it quells the notion that the darkest films reject
the most heat. In most cases, dark films are chosen because they offer greater
privacy.