Synopsis of technical report
“Non image forming optical components” by P. R. Yoder Jr.
Yuming Shen
November 1, 2006
Introduction
There is a group of optical components serving extremely useful purpose in optical
instruments other than forming images. They do not have power since their surfaces are
all flat. In this paper, the author reviewed the geometrical characteristics of these optical
components, discussed representative configurations and typical mounting designs for
these components. The components reviewed in this paper include:

Windows

Filters

Reticles

Beam folding components – including flat mirrors and prisms

Beamsplitters and beamcombiners

Image erectors, image rotators and derotators

Scanning devices
Windows
A window is normally a plane parallel plate of optical glass, fused silica, plastic, or
crystalline material that allows the desired radiation to pass through while keeping the
effect on beam intensity and quality minimum. Some windows are wedged rather than
plane parallel to control spurious surface reflection or to get a specified beam deviation.
Typical applications

As a transparent interface between the internal optical components and the outside
environment.

Constrain the internal environment in some optical applications.
Parameters of concern in the design

Transmission throughout applicable spectral region.

Geometrical dimensions – optical aperture, diameter or width and height,
thickness, wedge angle and special shape.

Optical properties – transmitted wavefront quality, relative aperture, surface
quality, optical material requirements, coating requirements and polarization
characteristics.

Environment conditions – temperature, pressure, vibration, shock, humidity,
corrosion, contamination etc.

Mounting configuration – orientation, mechanical stress, thermal properties of
material and mechanical interface.
Example windows


Catalog products. A large variety of ready-to-use plane parallel plates are
available from optical component vendors, non catalog items can be met by
special orders.
He-Ne laser tube window. Elliptical shaped windows used to seal the low
pressure gas tube, thickness is sufficient to maintain optical figure while
supporting a modest pressure difference between low pressure gas inside the tube
and the ambient. Surface requirements are important here, two surfaces must be
parallel to a few arc-seconds and flat to less than 0.01 waves over used aperture.
Figure 1 shows the construction of this application.
Figure 1: A He-Ne laser tube with external resonator mirrors employs
Brewster’s angle windows to seal low pressure gas-filled tube.

Military telescope entrance window. In this application, the light beam
transmitted through this window is collimated and nearly fills the clear aperture at
all times, the critical specifications are transmitted wave-front error (0.5 λ wave
power, 0.05 λ irregularity for green light over the clear aperture) and wedge angle
(less than 30 arc-sec). The window is antireflection coated on both sides for
mechanical durability and environmental resistance while providing high
transmission at visible wavelengths. The window is mounted in a stainless steel
cell and the cell is bolted to the telescope housing with a rubber O-ring.
Figure 2: Example of a circular aperture window for a military telescope .
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The author also presented a multi-aperture window subassembly used in an airborne E-O
sensor system. The large window is infrared transmitting while the two smaller ones are
BK7 optical glass. The requirements addressed in this application include dimensions,
coatings, rain erosion resistance, transmitted wave error, infrared material (ZnS) selection
and machining, mechanical strength of the materials and refractive index
inhomogeneities.
Optical filters
An optical filter is essentially a special optical window has the ability of blocking
particular wavelengths of radiation while passing other wavelengths with little or no
change. The most important property of an optical filter is the spectral property which
defines what wavelengths will be blocked and what will be passed. A plot of
transmittance vs. wavelength is the best means for defining a filter’s spectra-photometric
characteristics.
The transmission factor Tλ
This is given by
Tλ = [2N / (N2+1)] Ttx/t, where N is the index of refraction, Tt is provided by filter
manufacturer for a specific t of the material at the wavelength in question, x is the
thickness of the filter.
Optical filter categories

Sharp-cut filters. Sharp cut filters are defined as filters that cut off as much as
possible of the wavelength light shorter than a specific wavelength, while
transmitting as much of longer wavelength light as possible, within a wavelength
range of 350nm to 800nm.

Band-pass filters. Transmit only a certain specific spectral region. Figure 3A
shows the transmission curve for a typical band-pass filter.

Compensating filters. Often used in photography to modify the spectral
characteristics of illumination to more closely match the sensitivity curve of the
film.

Neutral density filters. Have uniform transmission over extended spectral ranges.
Function by either absorption or reflection from coatings. Figure 3B shows a
typical transmission curve for a ND filter. Adjustable ND filters are achieved by
techniques such as counter-rotating Polaroid films or polarizing prisms.

Interference filters. Multi-layer thin film devices controlling its spectral
transmission primarily through interference effects. Have two basic types, edge
filters and band-pass filters.
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Figure 3: (A) Representative transmission curve for a green transmitting
band-pass filter. (B) Transmission curve for a ND filter.
Reticles
A reticle is a device which causes a geometric pattern to appear superimposed upon the
field of view of an optical system such as visual telescope or camera. Reticles serve as a
pointing reference or measurement scales.
Reticle patterns
Figure 4 shows some typical reticle patterns commonly used in optical instruments.
Figure 4: Some reticle patterns used for
various purpose in optical instruments
Figure 5: The reticle pattern is projected as if at
infinity superimposed upon the view of the
distance target.
Location of reticles in optical system

At system image plane. (most common)

At the focal plane of collimating lens to provide a projection in optical path. An
example of this application is illustrated in figure 5.
Fabrication methods

Diamond scribing and filling the grooved pattern.

Photo lithography.

Glue silver process.

Black print process.

Direct photography.
Things affecting appearance of reticle pattern

Reticle line width.

Illumination conditions.
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

Background.
Magnification
Beam folding components
This category includes mirrors and prisms, the principle uses of mirrors and prisms in
optical systems are:

To bend or deviate light beams

To fold an optical systems into given shape or package size

To provide proper image orientation

To displace the optical axis laterally

To provide optical path length adjustment

To split or combine beam by intensity sharing

To disperse light spectrally (prisms)

To modify the aberration balance of the system (prisms)
Most mirrors are aluminum or silver coated with protective coating on the top of
metal coating. The substrates for mirrors consist of crown glass, fused silica (low
CTE, such as ULE or Zerodur), and metal (aluminum, beryllium, copper).
Design of mechanical mounting depends on a lot of issues, such as inherent rigidity
of the optic, tolerable movement and distortion of the reflecting surface, location and
orientations.
For small to medium sized mirrors and prisms, an easy and feasible way for mounting
is to bond glass to metal using adhesives, this results a reduced interface complexity
and compact packaging. The most common adhesive is epoxy. Typical adhesive layer
thickness is from 0.075 to 0.125 mm.
Reflecting prisms are usually designed as entering and exit faces are both
perpendicular to the optical axis of the transmitted beam. For collimated beam
normally incident on prisms, there is no aberration introduced. Aberrations are
introduced if the beam is divergent or convergent.
Beam splitters and beam combiners
Beam splitters and beam combiners are essentially the same except for direction of
light propagation. There are two varieties, the plate and cube. Figure 6 shows these
varieties.
Figure 6: Plate and prism varieties of beam splitter and beam combiners
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Image erectors, rotators and de-rotators
These components are achieved by systems of mirror and/or prisms. They provide
proper orientation of the image produced in an optical system.
Example of image erector: Porro prism
Example of image rotator: Dove prism
Example of image de-rotator: Pechan prism
Scanning devices
Scanning devices are frequently used in electro-optical systems for beam steering, to
track targets and to stabilize the line of sight against vibration. Non-imaging-forming
optical components are commonly used in scanning devices.
Types of scanning motion
Rotational – achieved by a beam passing through an optical wedge or reflecting from
a slightly tilted mirror.
Oscillatory – nodding mirrors.
Un-directional – translating a prism or refraction through a plane parallel plate that
rotates about an axis perpendicular to the optical axis.
Combined motion – combination of two scanning devices operating at the same or
different frequencies.
Limitations
Scanning speed is limited by the reversible optical surface deformations. High
stiffness materials serve well (such as beryllium).
Diffractive optics and ultrasonic acoustic-optical deflectors as scanning devices are
increasing.
Conclusion
The author explored many key forms of non-imaging-forming optical components
and their typical applications. This synopsis merely details some parts while briefly
introduces other parts.
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