The possible uses of pulsed carbon dioxide lasers for analysis of plasmas and flows need appropriate infrared image converters. Emphasis was placed on liquid crystal detectors and their operational modes. Performance characterstics and selection criteria, such as high sensitivity, short reaction time, and high spatial resolution are discussed

Hugenschmidt, M.

Vollrath, K.

English

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NASA TECHNICAL MEMORANDUM NASA TM-77999
INFRARED DIAGNOSIS USING LIQUID CRYSTAL DETECTORS
Hugenschmidt, M., Vollrath, K.
NASA-TM-77999 19860012344
Translation of "Infrarotdiagnostik mit Hilfe von
Flussigkristalldetektoren,,,Bundesministerium der
Verteidigung, Forschungsbericht aus der Wehrtechnik
BMVg-FBWT 73-16, 28 pp.
IviAR! ._.198G
LANGLEYRESEARCHCENTER
LIBRARY,NASA
HAMPTON, VIRGINIA
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 20546 FEBRUARY 1986
https://ntrs.nasa.gov/search.jsp?R=19860012344 2020-03-20T14:57:46+00:00Z
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Hugenschmidt, M., Vollrath, K.
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Translation of "Infrarotdiagnostik mit Hilfe von I
Fl_ssigkristalldetektoren," Bundesministerium der
Verteidigung, Forschungsbericht aus der Wehrtechnik
BMVg-FBWT 73-16, 28 pp.
The possible uses of pulsed carbon dioxide lasers for
analysis of plasmas and flows need appropriate infrared image
converters. Emphasis was placed on liquid crystal detectors
and their operational modes. Performance characteristics
and selection _riteria, such as high sensitivity, short
reaction time, and high spatial resolution.
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INFRARED DIAGNOSIS USING LIQUID CRYSTAL DETECTORS
Hugenschmidt, M., Vollrath, K.
Institut Franco-Allemand de Recherches, St. Louis
/III*
Abstract
The possibility of applying pulsed CO2 lasers, particularly
using the TEA principle, for plasma or flow diagnosis, requires
appropriate infrared image converters. In addition to visualiz-
ing radiation by thermal extinction of UV-activated fluorescent
layers, liquid crystal detectors are very important here.
The structure and performance of various detectors are
described, with special emphasis on their suitability as to high
sensitivity, fast response and maximum spatial resolution.
*Numbers in the margin indicate pagination in the foreign
text.
1
Contents /0
Page
Introduction 3
TEA Laser 3
Liquid Converters 4
Sensitivity, Response Time and Resolution 5
Application of Optical Infrared Methods 7
Conclusions 8
Figures i0
2
Introduction /I
To visualize infrared radiation, a number of partially
reversible physical processes exist, including Czerny evaporo-
graphy, thermal melting or sublimation, effects of chemical
reactions on photochromic layers, extinction of fluorescence and
the use of cholesterol liquid crystals. The last procedure is
discussed in detail below. These methods are interesting since
infrared light sources are used in plasma physics. Moreover,
there are numerous fields of application in medicine, geography
and non-destructive materials testing, as well as quite generally
in the field of short-time physics. In particular, short-time
photography and cinematography are concerned with opening a
broader range of the spectrum besides the previous emission light
sources in the X-ray range, UV, visible and near infrared.
The present paper examines the properties and behavior of
cholesterol liquids, primarily with regard to their suitability
for recording rapidly changing processes, using a TEA laser as a
short-time infrared light source.
TEA Laser
A transverselyexcited CO2 laser was used at atmospheric
pressure, as first developedby Beaulieuand similarlyused by
B_chl to generate and heat hydrogen and deuteriumplasmas [i,2].
The laser is almost 4 m long and at a maximum pulse repeti-
tion frequency of 40 Hz (for slowly flowing gas) it emits pulses
with an energy of 0.3 to 0.5 J and a half-amplitude duration of
0.2 to 0.3 _s.
Figure 1 shows a diagram of the produced laser and an
oscillogram of the course of the output pulse over time, measured
with a Ge-Au detector. This involves superimposing i0 individual
3
pulses. The reproducibility is excellent.
/2
Figure 2 shows the specific structure of the resonator with
a plane-convex Ge lens (focal length f = 20 cm) as a catcher
reflector, as well as the transverse intensity distribution
before the focal plane at A-B and C-D.
Developed Polaroid film was used for recording. The power
density is sufficient to provide a good picture of transverse
distribution. The spatial resolution achieved by this simple
method, as also measured by D_rr, Decker and R6hr, is 20 lines
per mm. But the sensitivity is no longer adequate for a greatly
expanded beam, such as is required to illuminate larger surface
areas [3].
Liquid Converters
In liquid crystals, a distinction is made between smectic
and nematic phases, which possess thread-shaped molecules in
essentially parallel orientation. Cholesterol liquids, however,
consist of flat asymmetric molecules which are parallel in each
layer but rotated at a certain angle from one layer to the next.
Such layers present a high optical activity. The effect used in
the present paper arises from the fact that this periodic
layering and its temperature dependency produces a strong
dependency on direction and frequency, particularly of the back-
scattered light. Measurements are performed with a mixture of
cholesterol oleyl carbonate and nonoate (85:15). This solution
is sprayed homogeneously, with a layer thickness of a few _m,
onto thin absorbent films which themselves are only a few i0 _m
thick. A drawback is that in the open atmosphere one observes an
aging process attributable primarily to contaminants and humid-
ity. Therefore, each layer must be replaced every few days.
This drawback disappears when one uses liquid crystals from NCR,
which are delivered in micro-encapsulated form.
4
Films of Rhodialine, Vynan and homemade films of polyvinyl
alcohol with colloidal graphite were studied as supports for
liquid crystals. The intensity absorption coefficients of these
materials were between 1.3 x 102 and 7.5 x 102 cm-I. If one /3
requires the greatest possible absorption, e.g. greater than 98%,
this yields minimum necessary layer thicknesses of 50 to 300 _m.
Most measurements were performed with Vynan films 40 _m thick
from the "La Cellophane" company.
Figure 3, as a first example, shows two photos: A recording
of transverse intensity distribution with and without a diaphragm
in the far field of the expanded beam.
Sensitivity, Response Time and Resolution
For short-time photography applications, the essential
parameters are sensitivity, response over time and spatial
resolution.
Assuming a layer with a color change interval of 1 to 2°C
(from red to blue), an energy density of 4 mJ/cm2 is necessary
for a temperature increase of 0.3 to 0.5°C that is sufficient for
good color contrast, given a total layer thickness of 40 _m and a
mean specific heat of 0.34 cal/°C g.
To reduce the influence of thermal conductivity, the
temperature distribution characterized by color changes should be
photographed as soon as possible after impingement of the CO2
laser.
Figure 4 shows the measurement setup and some results. It
indicates a recognizable color change after only 5 ms. Subse-
quently the contrast increases, and at the relatively late time
of 50 ms a thermal diffusion of the initial distribution is
5
already noted. The superimposition of two interference schlieren
systems was chosen as an example: Concentric circles provoked by
the uncoated catcher lens, and parallel strips due to transmis-
sion through a Ge plate with a small wedge error. /4
Figure 5 shows another example: As shown schematically in
the lower half of the figure, these are shadow photographs of an
electrical spark discharge, recorded 1 _s after arc-through. As
the photographs show, at this early point the plasma is still
opaque to the radiation of the CO2 laser, so that both the spark
channel and the areas shaded by the electrodes present a reflec-
tion maximum in the red range. These sample photos also show
that the employed crystals require a time of 5 to i0 ms to
develop adequate schlieren contrast. Within this time, heat
transport mechanisms can already play a significant role.
This influence was studied numerically from a simple model,
shown in Fig. 6. A one-dimensional treatment is possible, since
the layer depth is negligible compared to the lateral dimensions.
Taking the initial and boundary conditions into account, for a
cos2 initial temperature distribution one gets a solution shown
in Fig. 6.
^ is the schlieren separation. For the thermal conductivity
a2, with the data for Vynan, one gets a value of 1.13 x 10-3
cm2/s.
Two examples of the decay of the temperature distribution
are shown in Fig. 7. For ^ = 0.i mm, i.e. a resolution of i0
lines per mm, a sharp decay of schlieren contrast occurs within a
few seconds.
Resulting from a number of further calculations, Fig. 8
shows the decay of schlieren contrast over time. These calcula-
tions show that because of the relatively slow liquid crystal
6
response time of 5 to i0 ms, with the present setup the upper
limit of achievableresolutionis set by the steep drop in the
contrast factor K. This value is 5 to l0 lines per mm.
Application of Optical Infrared Methods
As the measurements and calculations described above show,
such liquid crystal image converters permit the application of a
number of different optical procedures. This will be shown from
a few examples.
Figure 9 contains three photos of electrical spark dis-
charges. The recording times are 1 to 2 _s after arc-through.
The capacity of the discharge circuit is 0.02 _F, the charge
voltage is 7.5 kV and the electrode gap is 2.5 mm. The photo-
graphs show the shaded areas of the spherical electrodes and the
discharge channel, which has already achieved a diameter of 1.2
to 1.4 mm.
To reduce the maximum current, the discharge circuit was
attenuated with an additional l0 n resistor. It is interesting
that in these photographs the outer edge zone of the plasma is
still opaque for i0 _m because of the greater geometric layer
depth, while the inner area is already becoming transparent.
Further photographs at a somewhat later point are shown in
Fig. i0. With the same electrical data, here the channel
diameter in the first picture on the left is already 2.1 mm. The
two photos on the right were obtained using a schlieren tech-
nique.
For this the experimental setup was slightly modified by
applying a schlieren edge in the shared focal plane of the beam
expansion system.
7
Besides these examples,which only supply qualitative
informationabout refractiveindex gradients,Fig. ii also
contains two photos: interferogramsrecorded using an infrared
Michelson interferometer.
Such interferograms allow direct quantitative statements
about refractive index distribution and thus about further
variables of state, especially electron density. ___
Conclusions
The described measurements and calculations show that liquid
crystal layers are suitable for recording quickly changing phase
objects, using CO2 laser pulses.
The maximum spatial resolution of the studied setup,
determined by the finite rise time, is i0 lines per mm. By
contrast, for holographic applications resolutions of 150 to 200
lines per mm would be needed.
On the other hand, liquid crystal detectors allow excellent
optical recording by conventional methods, such as:
-- the schlieren and shadow method or
-- by interferometry.
REFERENCES
[1] J.A. Beaulieu, "High peak power gas lasers," Proceedinqs of
the IEEE 59 (1971), 667.
[2] K. B_chl, "Production of Plasma with a CO2-TEA Laser from
Solid Hydrogen Targets," Max-Planck Institut f_r Plasma-
physik, Garching, IPP IV/16 (1971).
[3] A. D_rr, G. Decker, H. R6hr, "Side on interferometry at 10.6
_m of theta pinch plasmas," Z. Physik 248 (1971), p. 121.
/7
Fig. i. Diagram of TEA laser, curve of coupled pulses over time.
Key: a. relative intensity b. ges = total
c. Ge-Au detector
000 cI""'" ""%:::G-=::I--I
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-"I I; IC02:N2:He= I:I:5
"T" b tOges= 760 Torr
relotive l
IntensitOt
a
"-'-"q5,u_'cm
Fig. 2. Structure of the resonator and beam expansion system,
and transverse intensity distribution.
Key: a. steel reflector b. Ge lens c. diaphragm
d. transverse intensity distribution in plane ...
C ,,!i
a 3tahlspiegel b Gt-Lins# Blendec
a. lOre _. ZOom
t
d d
Transversale Transversale
Intensitfttsverteilung Intensltatsvertel/ung
inderEbeneA-8 inderEbeneC-O
i0
Fig. 3. Photographs of transverse intensity distribution of TEA
laser pulses a) without and b) with diaphragm in the focal plane
of the confocal beam expansion system. Recording with a liquid
crystal image converter.
Key: c. stop d. liquid crystal layer
a b
c
BlendeI
11
Fig. 4. Measurement of response time of liquid crystal layer.
The parameter is the delay time At for turning on the flash lamp.
(The photographs show the superimposition of two interference
schlieren systems after transmission through a Ge plate.)
Key: a. Ge plate b. liquid crystal layer
c. flash lamp
_t . ?0ms a 4t . 50ms
Ge-Platte
12
/13
Fig. 5. Time response, shown from the example of shadow photo-
graphs of an electric spark discharge.
Channel stage: 1 ps after initiation
Parameter: Delay time At for triggering the flash lamp
Key: a. liquid crystal layer b. flash lamp
m
at. f rns At. 5ms at = fores Jtt =50m$
13
!j_:
Fig. 6. Solution of heat conduction equation for a thin film in
the case of a cos2 initial temperature distribution.
Heat conduction in a thin film
0
General heat c°nducti°n equati°n I _%- a2 Ia2 = thermal conductivity _ _ - F(x,t)
Solution for the one-dimensional _K(x't)"_(x)'TK(t)
case of a homogeneous equation 1 -KZa2tIfor a periodicinitial _(x,t)- _ _cosKxetemperature distribution K
cos2 initial temperature distribution
_x
Initial condition _(x,0)-_0¢os2_
A = schlieren separation _ I - 0Boundary condition _x x-0
ee 2_x -'_ -a_t
e(x,t) = --y- i +cos -'K- e
Data for Vynan: Thermal conductivity _ . 5.10-4
Specific heat - 0.34 [¢a17c
Specific gravity 7" 1,3 [--_
x 1
14
...... f
Fig. 7 Spatial temperat . . '
based on a cos_ znitial __dlstrlbutio _ _ / i
t_"Purature dis_J_frerent times, _
Parameter: Schlieren Separation . -_uulon. i
A
1 1ms
0,,# lOres
A "O,,2mrn
X
Q5 I
15
/19
Fig. 8. Decay of schlieren contrast over time. Parameter:
schlierenseparationA.
_max - _min
_K=_max * _min
\ \, \
o.,\ \\\ \
• \ \ \
\' \\\
_Smm
10-I, 10o 10_ 10zt_ns]----
Fig. 9. Infrared shadow photographs of electric spark dis-
charges.
C = 0.02 _F, U = 7 kV, electrode separation d = 2.5 mm
Time stages of discharge ca. 1 to 2 #s after initiation.
16
Fig. i0. a) Infrared shadow photograph and b,c) schlieren
photographs of late stages of electric spark discharges, 4 to 15
_s after initiation.
C = 0.02 _F, U = 7 kV, electrode separation d = 2.5 mm
II_.°. _<_ ;:_, _._4-:IX,Z_.=.. _';4_
a) b) ¢)
Fig. ll. Interferograms of the late stage of an electric spark
discharge, 20 _s after initiation.
Resolution of flash lamp: a) 15 ms, b) 50 ms, both after
impingement of CO2 laser on liquid crystal layer.
I
a) b)
17
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Infrared diagnosis using liquid crystal detectors

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