Non-Athermal Potting of Optics
Kirk A. Miller
Sensors and Electronics Systems,
Raytheon Systems Company, 2501 W University, McKinney, TX 75070
ABSTRACT
Historicafly lenses and windows on many military production programs, are potted (elastomerically bonded), often
with less than theoretical athermal potting gaps. Actual gaps may vary by factors of 2 to 4 less than ideal for
athermal potting. Yet these military systems have passed rigorous qualification tests and seem to work
adequately. Are problems lurking in the extremes of cold or hot for which MTF measurements are not typically
done? Are typical stress levels high enough to impact optical performance? This manuscript will address these
issues.
Finite element models will be used to examine a number of representative germanium optics to show to what
extent these elements are affected by non-optimal potting gaps. Cases include a convex-concave lens, a concaveconcave lens, and a convex-convex lens. All the optical elements will be potted in aluminum housings.
Keywords: Potting, mounting stress, lens, athermal, elastomeric mount
1. INTRODUCTION
Tactical military optical systems for targeting, surveillance, and situational awareness typically use potted (elastomeric)
mounts for optical elements. Systems designed by the author are typically infrared systems in the long wave and mid wave
regimes. Optical materials are therefore infrared transmitting materials. Housings are usually made of aluminum. The
potting materials are silicone rubber compounds. This method is typical for all tactical uses such as: man portable, wheeled
vehicle, tracked vehicle, fixed wing, or rotary wing. Due to platform dynamics, stiffly mounted lenses reduce image jitter.
Finite element models examine a number ofrepresentative germanium optics to show the effect of non-athermal potting gaps.
Cases are a concave-concave lens , a convex-concave lens, and a convex-convex lens. All optical elements are potted in
aluminum housings. FEM analysis results include maximum stress at high temperature, l resonant mode frequencies, and
typical mode shapes. Potting gaps range from 1 X athermal to .065 X athennal.
2. LENS CONFIGURATIONS
Typical gap calculations only consider the diameter of the optical element. Lens configuration affects the stiffness of the lens
with respect to its mount, and the stresses created. Figures 2-1, 2-2, and 2-3 show physical configuration and sizes.
Germanium
606 1T6
Alumum
Silicone Rubber
Silastic-E
Figure 2-1: Convex-Concave Lens configuration.
Part of the SPIE Conference on Optomechanical Design and Engineeriflg
506
Denver, Colorado • July 1999
SPIE Vol. 3786 • 0277-786X/99/$1 0.00
P8.000
.050
.1.
.100
.050
P 8.000
Figure 2-2: Concave-Concave lens configuration
P 4 .000
P 4.000
Figure 2-3: Convex-Convex lens configuration
3. ATHERMAL POTTING GAP CALCULATION
From Bayar (1981)'.
Do(am..ag)
te
(1)
2(ae-am)
t = .077 inch = radial thickness of potting
D6 = 2.05 1 inch = lens diameter
am = 23E-6 C = mount coefficient of expansion
ag
= 6.OE-6
C = lens coefficient of expansion
ae = 248E-6 C' = potting coefficient of expansion
Note that the above equation derivation does not include the effects of gap length, Poisson's ratio, or Young's modulus. The
simplified assumptions to derive the above equation do not really yield a "stress free" lens mount.
507
4. FINITE ELEMENT MODEL (FEM) ANALYSIS METhODOLOGY
A Raytheon internal FEM code is utilized. Linear and isotropic material properties are used. Axisymetric 2D models
approximate the stress distribution and first dynamic mode of the lens systems. Plate element models predict higher order
mode shapes of the lens system. Inherently, axisymetric 2D models cannot illustrate non-axisymetric modes. For the stress
calculations, the 2D system is restrained on the lens axis. For the dynamic modes, on-axis restraints are free, the external
perimeter ofthe aluminum ring is constrained.
Typical military systems have operational ambient conditions4 that range from —32 C to +52 C. The systems often have
internal and external heat loads that may increase temperatures of housings and lenses another 18 C. The models yield the
same absolute stress levels whether the temperature delta is positive or negative. For visualization reasons, positive deltas
yield better visual results when the displacements are exaggerated. Each node is given a temperature of +50 C. All material
defmitions have 0 C stress free temperature conditions.
Material properties for each material used in the FEM models are tabulated as follows in Table 4-1. Properties come from a
number of sources. Young's modulus for silicone rubber came from an unpublished measurement, this value is higher than
shown in textbook such as Yoder2.
Property
Young's Modulus
(lb/in"2)
Coefficient of thermal
expansion
(in/in/C)
Poisson's ratio
Density
Germanium
14,970,000
600*
.000023
.0000058
.000248 **
.33
.278
.098
.192
(ilastic..E)
(lb/in"3)
*unpublished measurement
** from
Raytheon process
*** maximum value that FEM code will run
Table 4-1 : Material Properties from od2 (unless otherwise denoted)
508
Silicone Rubber
6061T6
aluminum
10,000,000
-
•49***
.047
5. FEM RESULTS
Max. Stress vs Radial Gap
2000
1800
:j
'
; 1400
1200
Cl, 1000
1800
600
400
200
0
0
0.01
0.02
0.04
0.03
0.05
0.06
0.07
0.08
0.06
0.07
0.08
Radial Gap (inch)
Figure 5-1 : Maximum Von Mises stress in the lens as a function ofradial potting gap
Resonance vs Radial Gap
2500
N
. 2000
0
0 I 500
E
*1
C
0
C I 000
0
0
0
0
500
0
0
0.01
0.02
0.03
0.04
0.05
Radial Gap (in)
Figure 5-2: First lens resonance as a function ofradial pottinggap
509
0
U,
I
Figure 5-3: Convex-Concave configurationstress results
.010" Gap
max 787 psi
.005" Gap
0'niax 1840 psi
( I
LtO
3C+c)
L1C2
t$ 7t'G
t.-S3
.035" Gap
0'max 72 psi
.077"Gap
max 108 Ps I
L
L
'
:
A
'"
t>:
p,
.1
(I Us' pcI)
4lfl.
U,
506 psi
Figure 5-4: Concave-Concave configurationstress results
.0 10" Gap
umax 200 psi
3max
.005" Gap
(htts
:I.
L
123
.077" Gap
0max 66 psi
rnax
.035" Gap
psi
<.*dspsi)
)7OC,C*)
vi
844 pSj
347 psi
Figure 5-5: Convex-Convex configurationstress results
0max
.0 10" Gap
0max=
.005" Gap
t43
L.
1.
V
.077" Gap
0'max 74 psi
.03 5" Gap t'4
max 63 psi
w
U,
329.0 Hz
583.7 Hz
1
Mode: 4
Figure5-6: Mode shapes for the Convex-Concaveconfiguration
Mode:
Window 0
L
Window 3
j
414.4 Hz
5 674.4 Hz
Mode: 6 674.4 Hz
900 rotated Z
Mode:
900 rotated Z
Mode:3 414.4Hz
Mode: 2
2
05:15:13 05/29/99
L
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