An immersed thermistor bolometer for the detection of ultraviolet, visible, and infrared radiation is described. Two types of immersed bolometers are discussed. The immersion of thermistor flakes in a lens, or half immersed by optical contact on a lens, is examined. Lens materials are evaluated for optimum immersion including fused aluminum oxide, beryllium oxide, and germanium. The application of the bolometer to instruments in which the entrance pupil of the immersion optics has a high aspect ratio is considered

Dreyfus, M. G.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON. D.C. 20546
(S&SA) 3 P
CSCL '
B79-33U19
Unclas
_00/35__38887_
_____ •^^ •M-wa.-j.-rc— &— recfinTcal Information Division
FROM:
Attn: Miss Winnie M. Morgan
GP/Office of Assistant General
Counsel for Patent Matters
SUBJECT: Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP and Code
KSI, the attached NASA-owned U.S. Patent is being forwarded for
abstracting and announcement in NASA STAR.
The following information is provided:
U.S. Patent No. :
Government or
Corporate Employee
Supplementary Corporate
Source (if applicable)
NASA Patent Case No.
NOTE - If this patent covers an invention made by a corporate employee
of a NASA Contractor, the following is applicable:
YES NO r~i
Pursuant to Section 305(a) of the National Aeronautics and Space Act,
the name of the Administrator of NASA appears on the first page of the
patent; however, the name of the actual inventor (author) appears at
the heading of column No. 1 of the Specification, following the words
".i.with respect to an invention of ..."
Bonnie L. Henderson
Enclosure
r
I >
https://ntrs.nasa.gov/search.jsp?R=19790025278 2020-03-20T19:33:59+00:00Z
•
" * " •
Jan. 21, 1964
 M. G. DREYFUS 3,119,086
WEDGE IMMERSED THERMISTOR BOLOMETERS
Filed Aug. 29. 1961
FIG. 2
INVENTOR.
MARC G.DREYFUS
BY
ATTORNEY
United States Patent Office 3,119,086Patented Jan. 21, 1964
3,119,086
Vr'EDGF. IMMERSED THEIIMISTOR BOLOMETERS
Marc G. Drejfus, Stamford, Conn., assignor to Barnes
Engineering Company, Stamford, Conn., a corporatioa
of Delaware
Filed Aus. 29, 196!, Ser. No. 134,619
7 Claims. (Cl. 338—18)
This invention relates to a wedge immersed thermistor
bolometer for the detection of optical radiations, i.e.,
ultraviolet- visible and particularly infrared.
Immersed thermistor bolometers in which the ther-
mistor flakes are immersed in a ler:s or half immersed, by
optical contact on a lens, have achieved great success in
instruments particularly for the use in the infrared. Two
types of immersed bolometers are described and claimed
in patents to Wormser No. 2,983,883 and De Waard No.
2,994,053. The effect of immersion, to greatly increase
the sensitivity of a thermistor bolometer, results from
the greater concentration of the incoming energy over a
smaller area. This produces a more intense response.
While bolometers have been immersed by contact with
various lens materials such as fused aluminum oxide,
beryllium oxide and the like, the much higher refractive
indices of the semiconductor materials such as germanium
and silicon (and in particular germanium, which has a
refractive index of four), yield the greatest increase in
bo'ometer speed by immersion. As a result, where :he
wavelengths of the radiation to be detected are longer
than 1.8/1, the semiconductors and in particular germa-
nium are often used as the lens materials. The present
invention \vi!l be described in connection with bolometers
in which the active detector clement is optically contacted,
or immersed, onto the surface of a semiconductor such
as germanium. It should be understood that the inven-
tion is not limited to particular lens materials and may
involve operation with any optical .radiation including the
visible or any transparent lens material. As the greatest
advantages in immersed bo!ometers are obtained with the
very high refractive index material used in infrared in-
struments these form the preferred embodiment of the
present invention which, however, is not limited thereto.
The invention is essentially an optical one and its ad-
vantages are obtained regardless of what lens material
is used in the immersed bolometer.
Some general considerations are needed to appreciate
the specialized type of problem which is solved by the
present invention. In ordinary practice the field of view
. of the lens in which the detector is immersed, or the
aperture of that Sens, is symmetrical such as round or
square. In a number of specialized instruments in which
immersed detectors are to be used, either the field stop
or the entrance pupil of the immersion optics, or both,
depart markedly from round or square, that is to say the
stop aspect ratio departs markedly from unity. The pres-
ent invention deals with the problems presented by this
type of instrument. It permits marked improvement in
sensitivity of response, and in some cases aiso improved
mechanics! structure.
In determining aspect ratio the direction of the long
axis of the stop or pupil must be considered; that is to
say the ratio is always based on the direction of the sides.
For example, if a field stop with a horizontal angular sub-
tense five times the vertical is combined with an entrance
pupil in which the vertical is five times the horizontal
the product of these two aspect ratios is unity and the
present invention cannot be used to improve response.
V/hen, however, the product of the ratios is not unity
then the present invention can be used advantageously.
The magnitude of the improvement is proportional to
the square root of the product of. the aspect ratios.
10
15
20
40
45
00
55
CO
C5
70
While as is pointed out above the present invention
broadly is capable of improving instruments when the
product of the aspect ratios of field stop and entrance
pupil departs from unity, the most important single field
is where the entrance pupil has a comparatively high as-
pect ratio. A type of instrument in which the problem
•become.': very acute is presented by spectrometers where
different wavelength bands are represented by long and
comparatively narrow slits in the spectral focal plane and
where the energy through these slits is to be focused on as
small a detector as possible. In addition to improved
sensitivity of response, the -present invention often per-
mits improved mechanical structure in this case, since
there is not room to mount a series of symmetrical im-
mersed bolometers behind the slits of the spectrometer
when those slits are close together.
According to the present invention, in such a case, the
entrance pupil of the wedge lens roughly conforms to the
slit's length and width and is in the form of a small slice
through the symmetrical immersion lens. In this case,
the lens front surf nee is partially masked by the spectrom-
eter slit and might equivalent!/ be considered as
trimmed or cut to yield pieces having full lens dimension
in one direction corresponding to the long dimension of
the spectrometer slits, and having the oilier dimension
smaller corersponding to the slit width. Energy incident
along the long meridian of the s!it is refracted by the
immersion lens surface and directed toward the thermis-
tor flake with the maximum convergence consistent with
the critical angle of the immersion interface. The image
size at the immersion interface is inversely proportional
to the convergence of the image-forming rays. Therefore
the detector flake size is minimized and its response is
maximized by the immersion !ens refraction for the fan
of rays incident along the long meridian of the slit.
V/hen the lens focal length is such as to converge the
two rays from the ends of the slit at an optimally steep
angle onto the detector, the rays frcm opposite points
from the two sides of the slit are not converged as steeply.
The detector flake could be made smaller and more sensi-
tive if the convergence in that azimuth were mscie greater.
According to the present invention, the equivalent of this
increased convergence in one azimuth is obtained by
making the sides of the immersion !ens slice at an angle
to form a funnel or wedge. Practical construction may
depart slightly from theoretical perfection but the differ-
ence is small and the improvement achieved is in propor-
tion to a very large fraction of the square root of the
aspect ratio product. The width of the edge of the wedge
cannot be decreased indefinitely because of a limitation
imposed by the critical angle at the immersion interface.
Therefore, the edge of the wedge has to be of finite width,
the limits to which ate set in each case by the nature
of the materials of which the wedge and immersion inter-
face are formed.
It should be noted that the present invention, in its most
important commercial aspect, is applied to instruments in
which the entrance pupil of the immersion optics has a
high aspect ratio. In this case it effects marked improve-
ment in noise equivalent power of the system with consid-
erable concomitant savings in weight and in size. For
certain in;!rumental use this improvement is vital.
The invention will be described in more detail in con-
junction with an i:nmorsicn bolometer for infrared spec-
trophotometric use an.I in connection with the drawirgs
in which:
FIG. 1 is an isometric view of a wedge immersed
bolometer;
FIG. 2 is a horizontal cross-section; and
FIG. 3 is an enlarged detail of the edge of the wedge"
with attached detectors.
3,110,080
Ths wedge lens 1 is of germanium with a spherical front
surface 2 having an aspect ratio of about 5:1. A narrow
edge of the wedge 3 extends into a holder made up of two
germanium wedges 6 spaced by thin pieces of plastic 7,
which may be in the form of buttons or strips the whole
being cemented together with a suitable epoxy cement 8.
The active thermistor flake 4 with attached leads 5 is
immersed on the thin edge of the wedge with the custom-
ary insulating -layer of arsenic modified selenium 9. As
is customary the leads are connected to the thermistor
through an area of thin deposiied gold. This defines an
active area of flake 10 which is the dimension that deter-
miner. responsivity. The gold film is so thin ihat it cannot
be clearly shown in a drawing and the active area is,
therefore, shown by two boundary lines. Theoretically the
wedge would operate even more effectively if the holders
6 were absent. However, this is a mechanically imprac-
tical structure, since support is needed for the flake leads.
Also it is difficult to polish a narrow edge effectively unless
it is held between larger surfaces. Therefore, the spaced
germanium holder performs both of these necessary func-
tions.
In FIG. 2 the cement layers 8 and the plastic spacers
7 are greatly exaggerated in thickness. In order to show
the ihin edge of the wedce 1 with the very thin insulating
layer 9 and flake 4, the parts are shown exploded in FIG.
3. Ideally the polished sides of the germanium wedge
should contact air as this would produce practically per-
fact internal reflection. However, the very thin layer of
cement doss not degrade performance significantly though
front rigorous theoretical considerations the internal re-
flection is slightly frustrated and introduces losses due to
the fact that when internally reflected the radiation wave
penetrates of the order of about a tenth of a wavelength
into the medium of lower refractive index. Some loss may
occur by absorption in this short path but with suitable
cements the absorption is very small and with a typical
cement amounts to a very few percent.
The supports represented by the wedges 6 may be of
almost any suitable constructional material. Germanium
is shown in the drawings for the supports. This minimizes
problems due to differential thermal expansion of the
supports and the centra! wedge I . The aspect ratio illus-
trated is 5: 1 and effects an improvement in sensitivity, due
to flake size reduction, approximating the square root of
the ratio which approximately doubles the sensitivity that
would have been obtained if the wedge shape had not
been used to compensate for the departure of the aspect
ratio product from uniiy. One of the factors which re-
duces the gain somewhat from the theoretical is a geo-
metrical property of the front surface of the wedge lens.
With any aspect ratio there is an area on the wedge lens
surface of the general shape of an ellipse over which the
theoretical gain is obtained; however, in the corners of
the lens surface there are small areas where the efficiency
is less than 100%. For example, the elliptical area ap-
proximates SO percent of the total area «tnd the fall-off of
efficiency beyond this area depends on the aspect ratio.
The fall-off is more with a. higher aspect ratio than with
a lower one.
A series of wedge immersed bolometers can be used
behind an infrared spectrometer, each lens receiving only
a narrow band of infrared radiation of a particular wave-
length. When the lenses are packed very close together,
as is sometimes necessary where the different wavelengths
20
9:3
40
45
so
55
are near each other, there is a slight possibility that radia-
tion from one band might get onto an adjacent immersed
bolometer. This can be avoided by projecting baffles be-
tween the lens slices.
The construction of the wedge immersed bolometer of
the present invention requires close tolerances. However,
the craftsmanship required' in manufacture is available in
high precision instrument construction.
In the drawings there has been shown a hyper-immersed
bolometer, that is (o say one in which the radiation detec-
tors are on a surface beyond the center of curvature of
the face of the immersion lens. It is possible, however,
to use the present invention in conjunction with bolom-
eters which are cither hypoimmersed or immersed', that is
to say with detector elements located between the center
of curvature of the front surface of the lens and the sur-
face or at the center of curvature itself.
I claim:
1. In an instrument having a field stop and an entrance
pupil the product of the aspect ratios of field stop and
entrance pupil departing significantly from unity, the im-
provement which comprises in optical alignment to receive
radiation passing through the field stop,
(a) a wedge shaped immersion lens the front surface
of which has an aspect ratio along o'.hogonal merid-
in.is differing from unity, the long meridian being
parallel to the long dimension of the field stop, the
lens having wedge surfaces tapering from the smaller
meridian to a narrowed rear surface,
(b) the wedge surfaces being bounded by material of
lower refractive index than the lens material, and
(c) at least one radiation detec!or immersed on the
tapered edge.
2. A wedgs immersed bolometer according to claim 1
in which the radiation detector is a thermistor.
3. A wedge immersed bolometer according to claim 2
in which the wedge material is germanium.
4. A wedge immersed bolometer according to claim 3
in which the tapered edge of the wedge is mounted in a
tapered support sepcri-.tcd from the wedge by material of
much lower refractive index than germanium, the separa-
tion being at least about one-tenlh wavelength of the
longest radiation for which the bolometer is to be used,
said material of lower refractive index having at least
moderate transmission for radiations of the wavelengths
for whijL the bolometer is to be used.
5. A wedge immersed bolometer according to claim 4
in which the support members are of germanium sepa-
rated from the germanium wedge by the material of lower
refractive index.
6. A wedge immersed bolometer according to claim 1
in which the aspect ratio of the orthogonal meridians is
of the order of 5:1.
7. A wedge immersed bolometer according to claim 3
in which the aspect ratio of the orthogonal meridians is
of the order of 5:1.
References Cited in the file of this patent
UNITED STATES PATENTS
2,668,867 Ekslein Feb. 9, 1954
2,788,381 Baldwin Apr. 9, 1957
2,964,636 Gary Dec 13. 1960
2,983,888 Wormser \fay 9, i>v,l
2,994,053 De Waard July 25, 1961
3,062,964 Lubin Nov. 6, 1962


Wedge immersed thermistor bolometers

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server