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BACKGROUND OF THE INVENTION
This invention relates to a thermal line scanning instrument, and more
particularly to such an instrument which generates a single thermal line
scan and displays the thermal profile thereof superimposed on a visual
view of the target.
Infrared thermography has been employed extensively for remote temperature
sensing and is being utilized in many applications for non-destructive
testing of materials and processes, etc., and for diagnostic purposes as
well as many other applications. Use of an infrared camera in many of
these applications provides a thermal image of the entire target area
where, in fact, only a small area or spot of the entire target area may be
of interest. It is believed that an infrared camera is used in such
applications in order to orient the camera on the subject or target and to
identify the objects whose temperature is desired to be examined. For
those applications where the IR camera is used primarily to physically
locate a small area of interest on the target surface, the expense, bulk,
complexity, or other disadvantages in such an application may inhibit the
use of the infrared approach.
One approach to the problem is shown and described in U.S. Pat. No.
3,641,348 entitled "Thermal Imaging System with Thermal Image Superimposed
on a Target Scene," which is assigned to the assignee of the present
application. In this approach, the field of view is scanned by a Nipkow
scanner and applied to an infrared detector which modulates a light source
in accordance with the intensity of the radiation applied from the field
of view which is scanned. The intensity modulated light source is imaged
through the same reticle and superimposed on the sight of a viewing
telescope. Looking through the telescope a view of the target scene is
presented with a red tinge in the regions in which the target is warm or
overheated. This system requires a rotating reticle, and also provides for
the scanning of the entire target area. Quantitative data with respect to
the temperature displayed is not easily interpreted by viewing the target
scene in which the temperature is provided in the form of a red tinge.
Although the hotter areas would have a brighter tinge than the cooler
areas, the differences therebetween would be difficult to interpret.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel infrared line
scanning instrument which is compact, simpler and less expensive than
instruments now available for performing the same or similar functions.
A further object of this invention is to provide a thermal line scanning
instrument which is capable of presenting the thermal distribution over a
restricted region without requiring a two-dimensional high-resolution
infrared camera.
Another object of this invention is to provide a novel thermal line
scanning instrument which provides quantitative thermal information on a
target scene which may be viewed directly on the target scene by an
observer, and readily interpreted.
In carrying out this invention in one illustrative embodiment thereof, a
scanning mirror which is transparent in the visible region of the
electromagnetic radiation spectrum and reflective in the infrared region
scans a line in the field of view of the instrument. The observer views
the field of a view of the instrument through the scanning mirror, while
an infrared detector is provided for receiving the infrared radiation
reflected from the scanning mirror. The output of the infrared detector is
quantized and applied to a multi-element display means which activates
individual elements in the display in accordance with the intensity of the
infrared radiation received from the field of view. Radiation from the
display means is applied to the back side of the scanning mirror for
providing a thermal profile of the scanned line which is superimposed on
the field of view of the instrument.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an optical layout of the thermal line scanner in accordance
with the present invention.
FIG. 2 shows a block diagram of an illustrative embodiment of the
electronics for the thermal line scanner shown in FIG. 1.
FIG. 3 illustrates the thermal display superimposed on a target view for
the thermal line scanner instrument shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The thermal line scanner embodied in this invention is shown generally, and
indicated by the reference numeral 10 in FIG. 1. The thermal line scanner
instrument 10 includes an oscillating scanning flat mirror 12 which is
transparent in the blue to yellow visible region of the spectrum and has a
reflective gold coating for reflecting red and infrared radiation from
0.65 to 35 micrometers. The scanning mirror 12 is driven by a galvanometer
motor 14 which may be, for example, a General Scanning, Inc. G-330
galvanometer motor. Since the scanning mirror 12 is partially transparent
in the visible region, the operator may view the target directly or
through the reflex sight 42 of a camera 40, through the flat scanning
mirror 12.
Infrared radiation 15 from the field of view is reflected from the scanning
mirror to a primary spherical mirror 16, and from there to a secondary
flat mirror 18, and applied therefrom to an infrared detector 20. Any
suitable infrared detector may be utilized, for example a 0.4 .times. 0.7
mm DTGS pyroelectric detector is preferable for the current application,
although other types of detectors, for example indium antimonide or
others, may be utilized. The infrared detector 20 produces an output which
varies with the intensity of the radiation applied from the field of view
of the instrument. After suitable processing, the signals from the
infrared detector 20 are used to drive a multiple element display means
which is preferably in the form of a multiple element, light-emitting
diode (LED) array 22. The LED array 22 may consist of gallium arsenide
phosphide LED's emitting at 0.65 micrometers. As an illustrative example,
an 18-element LED array 22 may be utilized and may be type OPE 518 made by
Optron, Inc. The diodes in the array 22 are selectively lit in accordance
with the amplitude of the signal detected by the infrared detector 20.
Radiation 25 from the display 22 is reflected by folding mirrors 26 and
28, and through a collimating lens 30 to the back side of the scanning
mirror 12. The array or display 22 is thereby seen through the viewer 42
and appears to the viewer to be scanning across the field superimposed on
the target in the field of view of the instrument 10. The lowest (first)
and highest (18th) elements in the array or display 22 are always
illuminated, thus defining the dynamic range limits of an A-scope type
display. The lowest LED also defines the location of the scan line on the
target. As the array 22 is scanned across the field, one of the
intervening sixteen LED's which is equivalent to the instantaneous analog
signal level seen by the infrared detector 20 in that position is
illuminated. The viewer accordingly sees an A-scope type display of
temperature versus position on one horizontal line in the field of view of
the instrument 10 which is clearly identified in the viewer. The thermal
or video signal is defined as a 1 part in 18 resolution. The appearance of
the display is a dotted wave form which is illustrated in FIG. 3.
As is illustrated in FIG. 1, a camera, for example a Miranda single-lens
reflex camera 40, is positioned having a film plane 38 onto which the
field of view as well as the superimposed display 22 are imaged so that a
permanent recording can be made of the thermal profile on the target scene
if so desired. The viewer may still view the field of view through the
reflex viewfinder 42 of the camera. A fixed aperture stop 32 is provided
for the camera which admits the visible and the LED energy 34 through the
lens 36 onto the film plane 38 for recording.
It will be apparent from the above that the LED array or display 22 scans
across the field of view in the same direction as the detector 20 from the
back side of the scanning mirror 12. This provides perfect synchronization
between the IR scan of the field of the instrument 10 by the detector 20
and the LED display 22.
Referring now to FIG. 2, the electronic circuitry for the instrument 10 is
considered conventional, and therefore is shown and described in block
form. The power supply 44 includes a battery pack 46 which may be charged
by a battery charger 48. Battery pack 46 may consist of six 6-volt
rechargeable gel electrolyte batteries which in turn drive a regulator and
DC to DC converter 50 for producing a 5-volt reference for the digital
circuitry which is employed. It will be apparent that other power supplies
may be utilized, and in fact an AC supply may be utilized with suitable
accompanying circuitry for providing the voltages necessary. A 15-Herz
triangular wave generator 52 drives the scanning mirror galvanometer motor
14 through a scanner amplifier 54. The scanning mirror 12 as shown in FIG.
1 may be a General Scanning, Inc. model G-215 with a 1 inch .times. 1 inch
mirror. The scanner motor driving the scanning mirror 12 produces a
15-line per second scan rate. The pyroelectric detector 20 is coupled to a
preamplifier 58 which provides a treble boosted frequency response to 1200
Hz, thus making the system response flat from the pyroelectric detector 20
electrical time constant of frequency from 0.05 to 1200 Hz, which
maintains the optical resolution along the scan line. The detector signals
from the preamp 58 are applied to a clamp circuit 60 which clamps the
detector signal to the most negative part, accordingly the lowest
temperature on the scan line, to produce a unidirectional display
representing temperature deviations from the coldest point in the scan.
This circuit also includes an adjustable equivalent temperature offset
(ETO) control which allows any part of a large signal to be examined with
high gain by adjusting the clamp level. The output of the clamp is
provided to an analog-to-digital converter 62 and from there to a decoder
driver 64. The analog-to-digital converter 62 and decoder-driver 64
function to quantize the amplitude of the detector signal so that each
element or diode in the display or array 22 lights when the signal
amplitude falls within the respective limits of that particular element. A
delay 56 is coupled between the wave generator 52 and the decoder-driver
64 to provide retrace blanking between scanned lines. In the 18-element
LED array 22, the lowest, or first, LED is aligned optically conjugate
with the detector 20 and always activated, thus indicating the horizontal
line in the field whose temperature profile is being displayed. This will
be seen in FIG. 3, and is identified by line 65 which corresponds to the
lighting of the first LED in the array 22, with the detector signal 20
being clamped on the coldest spot on a target being viewed. The visible
view of the subject is seen through the viewer 42 of the camera. As will
be seen in FIG. 3, the LED display is superimposed on the target scene,
giving a clear thermal profile of the scanned line.
It will be apparent that the display may include more or fewer diodes, as
required. The number of diodes utilized will determine the number of
temperature intervals in a linear range which are quantized, but are not
necessarily required to be equal. In any event, the first and last diodes
would always be lit to indicate the range limits of the instrument. The
LED diodes are operated as on-off devices with the temperature being
indicated by the specific LED being activated in the array. Alternative
displays are possible. A small cathode ray tube 24 as shown in dotted form
on FIG. 2 could be used in place of the diodes, which would give a
continuous rather than an amplitude-quantized trace. However, this
requires a high voltage supply, and eliminates the quantization which may
be more desirable than a continuous trace for certain applications.
Instead of recording the scene on a film, as illustrated, the camera could
be replaced by a vidicon and the display presented on a television
monitor.
An alternative mode of operating the LED display would be to cause all of
the diodes to light in a chain up to the one indicating the amplitude of
the signal on the detector. This mode would tend to illuminate the entire
area between the base line and the temperature profile, but may produce
some scene obscuration, which may be objectionable.
The instrument described has a size of 9.25 inches .times. 3.75 inches
.times. 6 inches and weighs 11.75 lb. including the batteries and a
camera. By eliminating the batteries, 3.3 lbs. would be subtracted from
the overall weight. The instrument 10 operates at a frame rate of 15 per
second, with a scan field of 25.degree. .times. 0.43.degree., and an
angular resolution of 0.25.degree. .times. 0.43.degree. high. The
compactness and the light weight characteristics of the instrument make it
quite suitable for hand-held operation, and provide a portability feature
which is difficult to achieve in other types of scanning radiometers or IR
cameras. It will be apparent that the instrument may be mounted on a
tripod or other suitable support if desired.
Since other modifications and changes, varied to fit particular operating
requirements and environments, will be apparent to those skilled in the
art, the invention is not considered limited to the examples chosen for
purposes of illustration, and covers all changes and modifications which
do not constitute departures from the true spirit and scope of this
invention.
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