Tech Briefs Magazine - May 2022 - PIT-21

coating would have a flat, uniform thickness
(Figure 2). However, since the area
of a curved surface is larger than that of
a flat surface with the same diameter, the
conservation of mass demands that the
coating become thinner towards the
edges of the surface. The thickness in
the center will be the same as the thickness
on a flat surface. The steeper the
curved surfaces of a lens, the more pronounced
the deposition effect will be.
The thinner the coating, the more the
spectral performance is shifted to longer
wavelengths, leading to different spectral
performance at the lens edges versus
the center.
The AOI effect occurs in the end application,
and is demonstrated in Figure
3, where collimated rays incident
on a lens experience larger angles of
incidence toward the edges compared
to the center of the lens. Again, this effect
is more pronounced for lenses with
steeper surfaces. Clearly, both the
deposition effect and AOI effect impact
performance as a function of diameter,
as rays at the edges of the lens will be
incident at both higher angles and
onto thinner regions of the coating. To
simplify this discussion, we will ignore
the convolution of these two effects
and evaluate them separately. The larger
incident angles at the edges of the
lens also cause a shift in spectral performance
to longer wavelengths.
How these Effects Change Lens
Performance
It was stated earlier that the deposition
and AOI effects shift spectral performance
to longer wavelengths, but
they can also lead to unwanted reflections,
which reduces lens throughput
in the desired wavelength range. As an
example, these effects will be analyzed
for a plano-convex asphere with a 50
mm diameter, 30 mm focal length, simple
MgF2
(magnesium fluoride) coating
on the curved surface, and an
N-SF6 substrate. The angle of incidence
of the rays striking and leaving the lens
can be calculated through optical design
software like Zemax OpticStudio if
the lens prescription is known. Figure 4
shows increases in unwanted reflections
as a function of the lens' semi-diameter.
In the plots, " S1 " is the aspheric
surface while " S2 " is the planar
surface. Average reflections are shown
for the wavelength range of 425 - 625
nm. For semi-diameters around 0 - 12.5
mm, which corresponds to angles of inPhotonics
& Imaging Technology, May 2022
cidence of approximately 0 - 30┬░, there
are minimal impacts from both the
deposition and AOI effects. However,
at larger semi-diameters, and therefore
steeper angles, average reflectance increases
significantly, which leads to
losses in lens throughput. Both the
deposition and AOI effects impact performance
quite noticeably, with the exception
of the non-existent deposition
effect on the flat side of the lens ( " R_
avg_S2 " on the left of Figure 4). Note
that beyond 12.5 mm the deposition
Incident Vapor Stream
Thickness
on Flat
Thickness
on Asphere
effect causes reflection to increase even
more sharply than the AOI effect.
Different coating designs are impacted
by the deposition and AOI effects
differently. Figure 5 compares how different
types of coatings from Edmund
Optics®
are affected. An interesting
finding is that some coatings that perform
better at low angles of incidence
are more sensitive to angle and have
sharp increases in unwanted reflections
at higher semi-diameters. For example,
the coating titled " Std BBAR " , repreIncident
Light AOI On Flat
AOI ON Asphere
Figure 2. The deposition effect describes how
coatings are thinner toward the edges of curved
surfaces and thicker in the center.
Deposition Effect on 30mm FL Asphere
2.5
2
1.5
1
0.5
5
10
15
Semi-Diameter (mm)
20
25
R_avg_S2
R_avg_S1
2
1.5
1
0.5
5
10
15
Semi-Diameter (mm)
20
25
R_avg_S2
R_avg_S1
Figure 3. The AOI effect describes how collimated
light rays striking the edges of a curved lens will
encounter steeper angles of incidence compared
to rays towards the center.
AOI Effect on 30mm FL Asphere
Figure 4. Increases in average reflectance after semi-diameters of 12.5mm indicate that the deposition
and AOI effects are leading to losses and reduced light throughput.
Comparison of Different Coating Types
5
6
7
1
2
3
4
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
% of CA (25mm Diameter)
Figure 5. Some coatings that have low reflectivities when only a small percentage of the lens' clear
aperture (CA) is used are more sensitive to the steepness of the lens' surfaces, resulting in much worse
throughput towards the edge of the lens. Because of this, options like MgF2 with a more uniform reflectivity
may be better options. (Terminology: QW: quarter wavelength, Std: standard, BBAR: broadband
anti-reflection, RedExt: red extended, and deg: degree)
21
Fresnel
QW MgF
Std BBar
RedExt 0 deg Opt BBAR
RedExt 35 deg Opt BBAR
RedExt Highest Angles Opt BBAR
PIT Application Briefs 0522_1.indd 21
Cov
ToC
4/19/22 1:39 PM
Reflectivity at Radial Point (%)
Avg. Reflectance (%)
Avg. Reflectance (%)
http://info.hotims.com/82321-809

Tech Briefs Magazine - May 2022

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