coatings - Edmund Optics

ANTIREFLECTION
COATINGS
Antireflection
Application notes
COATINGS
Optical coatings are used to vary the reflectance and transmittance of an optical component. Typical coating examples include: antireflection coatings which reduce the amount of light
reflected at a given wavelength or wavelength range; Filters
which allow only a specific range of wavelengths to transmit; and High Reflective Coatings, used in the production
of mirrors.
Coatings are designed for a specific incident angle of light and for a specific polarization of light such as
S-polarized, P-polarized, or randomly polarized. If a coating is designed for light at a 0° angle of incidence,
but is used with light at a 45° angle of incidence, the coating will not perform at the stated transmission or
reflection specifications. Similarly, coatings are generally designed for randomly polarized light so using
S- or P-polarized light with a coating designed for randomly polarized light will again fail to produce the
stated specifications.
Coatings are created by depositing dielectric and metallic materials such as SiO2, TiO2, or Al, in thin layers that are typically equal in thickness to one-quarter the wavelength of the light used in the application.
These thin films alternate between layers of high index of refraction, and low index of refraction, thereby
inducing the interference effects needed. Most of our optics have some form of coating and we also design
and apply custom coating solutions.
antireflection coatings
Edmund Optics® offers all TECHSPEC® lenses with a single layer dielectric,
antireflection (AR) coating to reduce surface reflections. An AR coating can
VIS 0, 45
vastly improve the efficiency of an optic by increasing transmission, enhanc¹⁄₄ Wave MgF
ing contrast and eliminating ghost images. The simplest AR coating is a ¹⁄₄
VIS-NIR
wave MgF2 centered at 550nm (with an index of refraction of 1.38 at 550nm).
UV-VIS
Telecom-NIR
The MgF2 coating is ideal for broadband use though it gives varied results deUV-AR
pending upon the glass type involved. The VIS 0° (for 0° angle of incidence)
NIR I
NIR II
and VIS 45° (for 45° angle of incidence) provide optimized transmission for
400-700nm, reducing average reflection to 0.4% and 0.75% respectively. The
200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
UV
Visible
Near-IR
VIS 0° AR coating is preferred over MgF2 for visible applications. Other anWavelength (nm)
tireflection coatings include our visible/near-infrared (VIS-NIR) broadband
antireflection coating specially optimized to minimize relections for visible
and near-infrared wavelengths. Special care has been taken between 870 and
900nm to provide less than 0.2% reflection per surface. We also offer specialized broadband AR coatings for popular telecommunications wavelengths (Telecom-NIR), on our drum lenses and micro PCX lenses. Ultraviolet (UV-AR) coatings are applied to our
UV fused silica lenses and windows, as well as a UV-VIS coating for applications using UV and visible wavelengths. Two additional
near-infrared coatings are offered NIR I and NIR II. Custom single-layer, multi-layer, V, and 2V coatings are available for both our
off-the-shelf and large volume custom orders.
AR Coating
2
Antireflective MgF Coating Properties
As light passes through an uncoated glass substrate, approximately 4% will
be reflected at each surface. This results in a total transmission of only 92%
of the incident light through a single optic. Antireflection coatings are especially important if the system contains many transmitting optical elements.
Coating each component will increase the throughput of the system and reduce complications caused by back reflections (ghost images). Many low-light
systems incorporate AR coated optics to allow for an efficient use of the light.
This curve demonstrates the difference between an uncoated and coated single surface BK7 substrate. The coating is a ¹⁄₄λ of MgF2 centered at 550nm.
Substrate: BK7
2
6.0
Reflectance %
5.0
Uncoated (Avg.=4.25%)
4.0
3.0
2.0
1.0
400
Coated (Avg.=1.5%)
450
500
550
600
Wavelength (nm)
650
700
750
101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com
® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED
why do i need an antireflection coating?
Antireflection
Application notes
COATINGS
how does an ar coating work?
The transmission properties of a coating are de­pen­dent upon the wavelength of light being used, the in­dex of re­frac­tion of the substrate, the
index of re­frac­tion of the coat­ing, the thickness of the coating, and the
angle of the in­ci­dent light.
The coating is designed so that the relative phase shift be­tween the
beam reflected at the upper and lower bound­ary of the thin film is 180°.
De­struc­tive in­ter­fer­ence be­tween the two reflected beams occurs, cancelling both beams be­fore they exit the sur­face. The optical thickness
of the coat­ing must be an odd num­ber of quarter wave­lengths (l/4, where
l is the design wave­length or wavelength which is being optimized for
peak per­for­mance), in order to achieve the desired path difference of one
half wavelength between the reflected beams, which leads to their cancellation.
The equation for determining the index of refraction of the thin film
needed for complete cancellation of the two beams is:
nf = (no x ns)Z\x
where:
nf is the index of refraction of thin film
no is the index of refraction of air (or incident material)
ns is the index of refraction of the substrate
Air
no
Thickness, t
Optical Thickness = n f * t
Thin Film
=
+
nf
Substrate
ns
ar coatings for the Ultraviolet
ar coatings for the near-infrared
5
5
4.5
4.5
4
UV-AR on
Fused Silica
3
Percent Reflection
Percent Reflection
4
3.5
UV-VIS on
Fused Silica
2.5
2
1.5
Telecom-NIR on LaSFN9
NIR II on LaKN22
NIR I on LaSFN9
3.5
3
2.5
2
1.5
1
0.5
1
0
0.5
200
250
300
350
400
500
450
550
0
600
Wavelength (nm)
700
800
900
1000
1100
1200
1300
1400
1500
1600
Wavelength (nm)
ar coatings for the visible
Percent Reflection
4
3
2
VIS 0° on BK7
MgF2 on BK7
MgF2 on SF5
1
VIS-NIR on BK7
0
200
400
600
800
1000
1200
1400
1600
Wavelength (nm)
Note: All graphs and values are typical and for reference only.
101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com
® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED
5
Antireflection COATINGS
AntiReflection Coatings
Catalog optics are available off-the-shelf with a variety of Antireflection coating options. Coating is applied to both sides of the
optical component. Custom coatings are also available.
Coating Type
Wavelength Range
Specifications
Z\v Wave MgF2 @ 550nm
VIS 0°
VIS 45°
VIS
425-675nm
425-675nm
UV-AR
250-450nm
UV-VIS
250-700nm
VIS-NIR
400-1000nm
NIR I
600-1050nm
NIR II
750-1550nm
Telecom-NIR
1200-1600nm
Rave ≤ 1.75% 400 – 700nm (BK7)
Rave ≤ 0.4% 425 – 675nm
Rave ≤ 0.75% 425 – 675nm
Rabs ≤ 1.0% 250 – 425nm
Rave ≤ 0.75% 250 – 425nm
Rave ≤ 0.5% 370 – 420nm
Rabs ≤ 1.0% 350 – 450­nm
Rave ≤ 1.5% 250 – 700nm
Rabs ≤ 0.25% @ 880nm
Rave ≤ 1.25% 400 – 870nm
Rave ≤ 1.25% 890 – 1000nm
Rave ≤ 0.5% 600 – 1050nm
Rabs < 1.5% 750 – 800nm
Rabs < 1.0% 800 – 1550nm
Rave < 0.7% 750 – 1550nm
Rabs ≤ 0.25% 1295 – 1325nm
Rabs ≤ 0.25% 1535 – 1565nm
Rave ≤ 0.25% 1200 – 1600nm
Available off-the-shelf with AR Coating options
Singlets
Windows
Achromatic Lenses
UV and IR Lenses
800.363.1992
www.edmundoptics.com
101 East Gloucester Pike | Barrington, NJ USA 08007-1380 | 1-800-363-1992 | www.edmundoptics.com
® COPYRIGHT 2008 EDMUND OPTICS, INC. ALL RIGHTS RESERVED
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