For mirrors, coatings have replaced solid castings of polished metal in all but a few
specialized applications. Mirror coatings perform with better reflectivity than solid metal
mirrors, are lighter in weight and cost less to produce.

In the last 15 years, uses for optical coatings have expanded beyond their original
applications as antireflectors for lenses and reflectors for mirrors. For example, some
coatings are used as transmissive electrodes to activate electro-optic materials and
highly durable coatings can improve the resistance of sensitive optical components to
harsh environments as well. Mirror coatings reflect light, and antireflective coatings
transmit light by reducing reflection.
It is easy to forget that optical coatings are components because they always work with
lenses, prisms, windows or solid mirror substrates, whose imaging properties occupy
most of the systems-design effort.

Applications engineers usually classify coatings as reflective or
antireflective and broadband or narrowband. Broadband coatings handle many colors, i.e.,
a broad range of wavelengths, whereas narrowband coatings are designed for one color,
i.e., a narrow range of wavelengths.

A high-performance coating is not just one coating but several thin films deposited on top
of each other. Any one of the layers might exhibit modest performance. Working together,
however, a reflective stack of layers can achieve very high reflectivity (99.9%) and an
antireflective stack can achieve very low reflectivity (0.1%).

Each layer is very thin, typically one-quarter to one-half the wavelength of light, or about
10 to 20 millionths of an inch. Design and manufacture of these multi-layer coatings is
complex and difficult, but computers and vacuum deposition techniques make them cost
effective.

Coatings are designed to work under specific conditions of illumination, tilt and
environment because their performance changes with wavelength of light, polarization
of light, intensity of light, angle of incidence, humidity and temperature.

The simpler, single-layer coatings, like MgF2 or aluminum, exhibit more modest
performance and more latitude in application when compared to the high-efficiency,
multi-layer coatings.

Choice of a coating is most influenced by the reflectivity or transmission required at
certain wavelengths, but altogether there are seven issues involved in the design and
manufacture of a high-quality coating. These issues are:

1. Wavelength

All coatings exhibit different reflectivity or transmission at different wavelengths.
They may be classified as either broadband or narrowband.

Broadband coatings handle large regions of the spectrum. For example, a broadband
antireflective coating for visible light will reduce the reflective loss at the surface of a glass
element for wavelengths between 400nm (violet light) and 700nm (red light). A broadband
mirror coating, such as aluminum, can effectively reflect light as short as 350nmin the
ultraviolet and as long as 10,000nm in the infrared.

Narrowband coatings are designed to work in just one narrow region of the spectrum.
V-coats are narrowband antireflective coatings that reduce reflections at a glass surface
over a small range of wavelengths. For example, a high-efficiency V-coat designed for
helium-neon laser light of 632.8nm might reduce reflective loss to just 0.1%. Its reflectivity
might rise to more than that of uncoated glass at 500nm, just 133nm toward the blue.
A narrowband mirror for helium-neon lasers might reflect 99.5% of red 632.8nm radiation
but only 80% of blue-green light at 500nm. By classifying all coatings, reflective (mirrors)
or antireflective (transmitters), as either narrowband or broadband, the wide variety of
coatings can be organized in a simple, logical format:

A.Broadband
        1.Metallic
        2.Dielectric
             a.Single layer
             b.Multi-layer
        3.Hybrid (metal and dielectric)
B.Narrowband (multi-layer, dielectric)

The broadband category includes every kind of coating structure: metallic, dielectric and
hybrid. Narrowband coatings are limited to multi-layer dielectric structures because they
always achieve their performance with complex optical interference effects between the layers.

The reflectivity required of a coating completes its fundamental specification. Reflectivity
usually defines the behavior of both reflective and antireflective coatings. For reflective
mirror coatings, high reflectivity is desired. The opposite is true for antireflective coatings;
low reflectivity characterizes "high-efficiency" performance.

The performance of single-layer coatings is less efficient than that of multi-layer coatings,
but single layers are the most forgiving and the least expensive because of their simplicity.

Broadband multi-layer coatings reflect or transmit over a broad range of wavelengths with
performance exceeding that of single-layer coatings. When compared at specific
wavelengths,a broadband multi-layer coating can outperform a broadband single-layer
coating by a factor of ten. For example, a magnesium fluoride (MgF2) single-layer
antireflective coating might exhibit reflectivity of about 2% at 550nm, whereas a multi-layer
coating designed for the same central wavelength of 550nm might exhibit 0.2% reflectivity.

Narrowband multi-layer coatings can be designed to outperform broadband coatings at
specific wavelengths. Narrowband antireflective coatings are often called V-coats. The
terminology originates in the appearance of graphs that plot reflectivity against wavelength.
A V-coat can perform with 0.1% reflectivity at a specific wavelength, but its reflectivity
rises quickly for shorter and longer wavelengths. The graph of its performance looks like
the letter "V."

3. Polarization of Light

We all observe the effects of polarization every day. Most materials around us - asphalt
roads, window glass, water and vegetation - reflect one kind of polarization better than
the other; we say that they cause glare. Glare often results from the efficient reflection
of horizontal, s-polarized light. Polaroid sunglasses absorb the horizontally polarized
light and thereby reduce glare from horizontal surfaces.

Light is composed of many colors or wavelengths, and each ray of specific wavelength
can be analytically decomposed into one or two linear polarizations. Polarization, which
refers to the direction of the electric field vector in a ray of light, is a concept that grows
out of the electromagnetic wave theory of light.

A "polarized" ray of light is one that maintains a constant state of polarization over time;
"unpolarized" light is polarized at any given instant in time but is always changing.

Engineers loosely refer to polarization as either "vertical" or "horizontal." The terms are
used with reference to the plane of a surface rather than to our usual sense of up and
down or sideways. In other words, when intersecting a vertical surface, a horizontal
polarization can be straight up and down if the ray of light approaches from one side.

The precise terms for polarization are s-polarization and p-polarization. The first,
s-polarization, is derived from the German word senkrecht, meaning perpendicular,
since the electric field vector of s-polarized light is perpendicular to its plane of incidence.
The second term, p-polarization, is taken from the German word parallel.
This polarization is parallel to, or within, the plane of  incidence (Figure 1).

A rule of thumb states that s-polarizations are reflected more efficiently than
p-polarizations.This rule leads to unofficial termsfor the two polarizations: skipping and
plunging. Skipping-, or s-polarization, "skips" off a surface with more intensity than
plunging-, or p-polarization, which "plunges" through the surface. Coatings can be
designed in which this rule does not hold, but they are special cases.

4. Angle of Incidence

Angle of incidence is defined as the angle of light impinging upon a surface as measured
from the normal to that surface. Light with an angle of incidence of 0 degree comes
straight down onto the surface; when light approaches at large angles of incidence,
it skims over the surface.

Optical coatings are designed for peak performance at a specified angle of incidence.
If the angle of incidence is changed during operation, for example by tilting the optical
component, then performance will usually degrade. Narrowband coatings are most
sensitive to this specification; a 15 degree tilt can alter reflectivity at its nominal
wavelength by a factor of five and shift the wavelength of peak performance by about
10nm. Broadband coatings exhibit slightly more tolerance to tilt, but a 30 degree change
in angle of incidence will dramatically alter their performance.

A coating's sensitivity to angle of incidence presents a challenge to the design of fast
systems (small f-numbers, substantial light-gathering power) and wide-angle systems.
In a fast optical system, the converging or diverging cones of light intersect surfaces at
many different angles. The rays at the center of a cone may approach a surface at the
angle of incidence to which the coating was designed, but the outer rays may intersect at
considerably larger or smaller angles. Likewise, a wide-angle system will contain rays
whose angles of incidence cover a broad range.

Designers restrict the most sensitive coatings to planar surfaces in collimated beams.
By definition, the rays in a collimated beam travel parallel to each other. Therefore, they
all intercept a planar surface at the same angle of incidence.

5. Substrate

The exact same coating will perform differently when deposited upon different substrate
materials. This means that the exact formula for the structure and material of a coating,
especially a multi-layer coating, will be tailored to the substrate. The origin of this
variability lies in the fact that different substrates have different optical characteristics.
When tailored properly, nearly identical performance can be measured for the same
class of coatings applied to different substrates.  

6. Intensity or Power of Light

Some coatings are "soft" while others are "hard." For imaging applications where the
intensity of light is rather low, soft coatings withstand the radiant flux; however, for
high-power laser applications, such as welding or surgery, soft coatings would be
destroyed by the radiant flux. Hard coatings have been designed for these high-power
applications.

Basic thin-film design philosophy is the same for soft and hard coatings; both are
designed as stacks of thin layers. Their differenceslie in the details of their prescriptions,
such as material composition, and techniques of application.

Specifications that define the "softness" or "hardness" of a coating are written in terms
of the threshold intensity that will damage the coating. For example, a typical hard
infrared coating is rated for pulsed mode at 1 gigawatt/cm²  A 20-nanosecond,
Q-switched laser pulse with a peak power density of less than one gigawatt/cm² 
should not damage the coating.

Coatings are rated for their damage threshold under pulsed and continuous irradiation. Thresholds for damage are higher for pulsed modes of operation.  

7. Environmental Conditions

Optical coatings must be handled with care. The harder coatings, which are resistant
to laser damage, tend to resist scratching and abrasion, but even they are softer than
many glasses. The softer coatings will be marred by careless or vigorous rubbing.

Today's coatings are much more durable than those used before 1940. Early coatings
would stain easily because their porous microscopic structure trapped finger oils.
They could not be cleaned once they had been touched.

Varying humidity or temperature can alter the performance of a coating.
Those containing water-absorbing layers exhibit sensitivity to changes in relative
humidity because absorbed water changes the layer's refractive index. Temperature
also affects refractive index and even thickness.

In the vast majority of cases, a coating's sensitivity to the environment is small enough
to be ignored. In critical or unusual applications, more sensitive coatings are placed on
components that can be protected from the environment. For example, a multi-element
lens system might feature hard, durable coatings on its outer elements but softer, more
sensitive coatings on its internal elements.

On the following pages you will find detailed graphs and information regarding our
standard antireflective, reflective, polarizing , high-reflection and beamsplitting coatings.
Should none of our standard coatings meet your requirements, we will be happy to
custom design a coating for your specifications.