Glossary
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Holographic Grating - A type of grating in which
a sinusoidal "blaze" pattern is generated on the
surface by covering the surface with photoresist and
exposing it to an interference pattern.
Homogeneity - A characterization of the refractive
index variations in a material, which lead to
a deformation of the transmitted wavefront and
polarizing transmission effects.
IEC Laser Class - A laser class standard
administered by the Commission Electrotechnique
Internationale, which publishes international standards
for electronic components. It is an alternative
to the CDRH laser class standard used throughout
North America.
Index of Refraction (n) - The ratio of the speed
of light in a vacuum (c) to the speed of light in a
refractive material (v). The index of refraction of
a given medium varies with wavelength because
the speed of light in that medium varies with
wavelength.
Infrared (IR) Spectrum - The wavelength
interval of the electromagnetic spectrum that corresponds
to wavelengths just longer than visible light.
IR radiation is often divided into three regions:
0,750-3μm, 3-30μm, and 30-1000μm defined as
near-infrared (NIR), mid-wave infrared (MWIR),
and far-infrared (FIR), respectively.
Interference Fringe - Alternating light and dark
bands in an intensity pattern resulting from light
waves that are in or out of phase with each other.
The phase difference between the waves remains
constant along each individual fringe. When using
an optical flat, straight, parallel, and evenly spaced
interference fringes indicate that the work surface
flatness is equal to or higher than that of the
reference surface.
Interferometry - A family of techniques that
utilize the interference of waves of light for precise
determinations of wavelength, flatness, or other
attributes.
Ion-Beam Sputtering (IBS) - A physical vapor
deposition technique used to increase the density
of thin-film coatings, which creates "hard-coated"
filters. The thin film is deposited onto the filter
substrate by sputtering, or ejecting, material from a
target using ion bombardment.
Irradiance - The incident optical power per unit
area, denoted by E and measured in W/m². Also
known as radiant flux density.
Irregularity - A type of surface accuracy specification
that describes how the shape of a surface
deviates from the shape of a reference surface.
Knoop Hardness - A measure of material hardness.
A higher Knoop hardness value corresponds to
a stronger material.
Laser - An acronym for light amplification by
stimulated emission of radiation. The basic structure
is based on an active medium contained between
two reflectors. The reflectors contain light and
repeatedly oscillate it through the active medium,
allowing energy to build up with each pass. Laser
radiation escapes due to a partially reflective
mirror in the assembly.
Laser-Induced Damage Threshold (LIDT) -
The maximum amount of laser power per area
that a surface can withstand before it is damaged.
LIDT is typically measured in mJ/cm² for pulsed
lasers and W/cm² for continuous wave lasers. LIDT
is very important for mirrors or any other optical
component used in conjunction with laser products.
Linear Polarizer - A type of filter used to limit
the polarization of light to a specific plane. It does
this by absorbing components that are oscillating
in planes outside of the specified polarization
transmission axis.
Longitudinal Mode Spacing - The spacing
between multiple frequencies of oscillation in a
laser that results from a combination of the gain
curve for the laser and the resonance characteristics
of its cavity. Since the mode spacing is defined
as the speed of light divided by twice the cavity
length, short cavities can be used in applications
that require a minimal frequency spread.
Longpass Filter - A type of filter in which wavelengths
longer than a specific cut-on wavelength
are transmitted while wavelengths below the cut-on
wavelength are blocked.
M² Factor - Also known as the beam quality factor,
this is a comparison of the true shape of a laser
beam to that of an ideal Gaussian beam. A value
of 1 corresponds to an ideal diffraction-limited
Gaussian beam.
Magneto-Rheological Finishing (MRF) -
A deterministic process using a precise interferometrically
documented sub-aperture tool to correct
for waveform errors by selectively removing
material under a controlled and predictable process.
It provides high performance finishing in less time
than standard polishing techniques because of its
precise control of the removal location and a high
removal rate.
Nd:YAG and Nd:YLF - Common laser types
where the lasing medium is a YAG or YLF crystal
with Nd ions embedded as the lasing species.
Fundamental wavelengths are typically 1064 nm
and 1047 nm or 1053 nm for YAG and YLF crystals,
respectively.
Nebular™ Technology: Nano-structured antireflective
surfaces developed by Edmund Optics
featuring high throughput and high broadband
transmission.
Non-Polarizing Beamsplitter - A type of
beamsplitter that splits the incident light into separate
beam paths by a specific percentage without
altering the polarization state of the transmitted or
reflected light.
Notch Filter - A type of filter designed to block a
pre-selected bandwidth while transmitting all other
wavelengths within the designed range of the filter.
Numerical Aperture (NA) - The sine of the
vertex angle of the largest cone of meridional rays
that can enter or exit an optical system. In other
words, it indicates the amount of light entering or
exiting a system and is inversely related to the f/#.
Optical Density (OD) - Describes the amount of
energy that can pass through an optical material.
OD directly relates to the transmittance of the
material, which is the ability of light to propagate
through a given medium. A large OD indicates that
more incident light is being blocked.
Parallelism - The specification for the deviation in
alignment of an optical surface relative to another.
It causes angular deviation in optical or mechanical
systems.
Plano-Convex (PCX) Lens - A singlet lens with
a positive focal length consisting of one planar
(flat) surface and one outwardly curved (convex)
surface. PCX lenses are optimized for infinite/finite
conjugate imaging and are ideal for autocollimators,
light detection, laser collimation, and
infinity-corrected objectives.
Peak to Valley (PV) - An absolute value specifying
the distance between the largest and smallest
values of a data set.
Plate Beamsplitter - A type of beamsplitter
fabricated from glass plates with a partially
reflective metallic or dichroic coating. The thin glass
substrate allows for small beam deviation and a
very low absorption.
Polarization - A property arising from the wavenature
of light. Polarized light consists of waves
which oscillate in a defined and predictable manner.
Polarizing Beamsplitter - A type of beamsplitter
that splits unpolarized light into s- and
p-polarization states.
Polarizing Efficiency - A common measure
of the polarizing characteristics of a polarizer.
Efficiency of 100% is ideal though not feasible due
to manufacturing limitations. Typical polarizing
efficiency is 95-99%.
P-Polarization - The linear polarization state
parallel to the plane of incidence.
Radius of Curvature - The directed distance
from the vertex of a surface to the center of
curvature.
Rayleigh Range - The distance from the beam
waist where the cross-sectional area of the beam
is doubled.
Reflection - The change in direction of an incident
wavefront at a boundary between different media
where the wavefront returns into the first medium
without transmitting into the second medium.
Refraction - The bending of light rays that occurs
at the interface between two media with different
indices of refraction. A higher index of refraction
results in a lower refracted angle.
Resonator Mode - Electric field distributions that
maintain their amplitude profile in a laser resonator
cavity and determine the intensity cross-section
of the laser beam.
Retardance - The degree of angular shift in the
phase of incoming polarized light.
Retarder (Waveplate) - Materials that exhibit
birefringence. Orthogonal components of light that
enter a retarder (or waveplate) will experience a
relative phase shift upon output.
Root Mean Square (RMS): Square root of
the mean square of a dataset, providing a broad
description of a parameter not overly skewed by
outliers.
Shortpass Filter - A type of filter where wavelengths
shorter than a specific cut-off wavelength
are transmitted while wavelengths above that
wavelength are blocked.
Solid State Laser - A laser using a transparent
substance (crystalline or glass) as the active
medium, where the medium is doped to provide the
energy states necessary for lasing. The pumping
mechanism is the radiation from a powerful light
source, such as a flash lamp.
Spatial Filter - An assembly used in conjunction
with laser applications to eliminate spatial noise.
A pinhole is used to select only the central peak of
the intensity pattern from a focused spot. The size
of the pinhole is based on the wavelength, objective
focal length, and input beam diameter.
Spectral Irradiance - Irradiance per unit wavelength
interval at a given wavelength. Expressed in
watts per unit area per unit wavelength interval.
S-Polarization - The linear polarization state
perpendicular to the plane of incidence.
Strehl Ratio - The ratio of the peak focal
spot irradiance of a manufactured optic to the
diffraction-limited peak irradiance used to compare
the real performance of an optic to its theoretical
nominal performance. Strehl ratios from 0,8 to 1
are generally considered “diffraction limited”.
Subsurface Damage - Defects, such as cracks,
residual stresses, contaminants, or voids, below the
surface of an optical component.
Surface Accuracy - Peak-to-valley flatness of a
surface given in terms of waves (a multiple or part
of a reference wavelength).
Surface Flatness - A type of surface accuracy
specification that measures the deviation of a flat
surface from a perfectly flat surface.
Surface Quality - Evaluation of surface imperfections,
such as scratches and pits, or digs, which may
be caused during the manufacturing or handling
process. There are several standards for specifying
surface quality such as U.S. Military Performance
Specification Mil-PRF-13830B and ISO 10110.
Surface Roughness: High spatial frequency
errors on a surface which can lead to scatter.
Temperature Coefficient of Refractive Index
- A measure of the change in refractive index with
respect to temperature, denoted as dn/dT.
Thermal Conductivity - A measure of the ability
of a material to transfer heat via conduction,
denoted as k.
Total Internal Reflection (TIR) - The complete
reflection that occurs within a material because the
angle of incidence of the light striking the boundary
surface is in excess of the critical angle.
Transmission - The amount of radiant energy
that passes through a medium instead of being
reflected, absorbed, or scattered.
Ultraviolet (UV) Spectrum - The interval of the
electromagnetic spectrum with wavelengths shorter
than visible light. UV radiation is often divided into
three smaller regions, 320-400 nm, 280-320 nm,
and 100-280 nm defined as UV-A, UV-B, and UV-C,
respectively.
Unpolarized - Characterization of light that contains
random combinations of x and y components
of the electric field.
Visible Spectrum - The part of the electromagnetic
spectrum that is visible to the human eye
(approximately 400-750 nm).
Wavelength - The peak-to-peak distance covered
by one cycle of an electromagnetic wave. Wavelength
is inversely related to optical frequency.
Wedge Angle - A small angle of incline between
two faces in a wedge element.
YAG - Yttrium aluminium garnet (YAG, Y3Al5O12),
a common laser crystal. The crystal does not lase
by itself but requires doping with specific ions to
become a lasing crystal.
YLF - Yttrium lithium fluoride (LiYF4), another
common laser crystal. Non-lasing in its pure form,
Nd atoms embedded in the crystal typically lase at
1047 nm and 1053 nm.
/LO