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BACKGROUND OF THE INVENTION
Current airport aviation practices depend on the use of de-icing fluid to
remove ice and prevent its future build-up for time periods of 5-10
minutes. Verification that wing and other aerodynamic or control surfaces
are ice free is done visually, often under difficult viewing conditions.
Occasionally significant ice build-ups are not noticed, with tragic
results. Responsibility for detecting such ice rests with the aircraft
crew who rely on visual viewing, perhaps supplemented with an ordinary
flashlight. Obviously, a need exists for a system which is capable of
accurately and easily determining the presence of ice on an aircraft wing.
SUMMARY OF THE INVENTION
Metallic surfaces and dielectric surfaces behave differently when
illuminated with light, particularly with respect to their polarization
properties. One of the strongest differences and most easily observable is
the property of metals to reverse the rotational direction of circularly
polarized light. For example, the specular reflection of right handed
(clockwise looking towards the source) circularly polarized light from a
metal surface changes it to left handed (counterclockwise) polarization
and vice versa. This effect is used in the construction of optical
isolators which permit light to initially pass through the isolator but
prevent specularly reflected light from returning through the isolator
back to the source. The optical isolator is a circular polarizer that is
usually implemented from a linear polarizer and a quarter wave retarder
plate that has its fast and slow axes located 45.degree. from the
polarization axis of the polarizer. The polarizer must precede the
retarder in the light path.
When a metallic surface (or surface painted with a metallic paint), such as
the wing of an aircraft, is illuminated with circularly polarized light
(which may be generated by passing unpolarized light through a circular
polarizer) and the reflected energy viewed through the same circular
polarizer, the resulting image is extremely dim since the circular
polarizer is performing as an isolator with respect to the specular
reflection from the metal surface. Other types of surfaces (dielectric,
matte, etc.) viewed through the same circular polarizer maintain their
normal brightness because upon reflection they destroy the circular
polarization. If the circular polarizer is flipped (reversed) so that the
retarder precedes the polarizer, it no longer acts as an isolator for the
illuminating beam and the metallic surface's image will now be viewed of
normal (bright) intensity.
Most non-metallic and painted or matte surfaces illuminated with circularly
polarized light and viewed through the same circular polarizer will
approximately maintain their normal intensity. Such surfaces, as well as a
coat of ice on the metal, whether matte white due to a snow covering or
crystal clear due to even freezing will destroy the circular polarization
of the reflected light and therefore take on the depolarizing property of
a matte painted surface with respect to the optical isolator. A
transparent dielectric over metal depolarizes circularly polarized light
passing through it if it has numerous internal point scatterers or is
birefringent. Ice has this characteristic. Thus, circularly polarized
light reflected from a painted surface, snow, ice, or even transparent ice
over metal will be depolarized and will not be affected by the isolator.
Therefore, the image of a clear metal surface that is ice-free will
alternate between dark and bright when alternately viewed through an
isolator and non-isolator structure, respectively. Apparatus other than
the combination of optical isolators and non-isolators via circular
polarizers can produce the same effect. Any ice or snow covering the metal
surface will cause the image to maintain the same brightness regardless of
whether it is viewed through an isolator or non-isolator structure or
equivalent structures.
The present invention provides various arrangements for inspecting a metal
surface for the presence of ice which compares views of the surface in an
optical isolating and non-isolating manner. Making such comparisons in an
alternating manner results in the metal surface producing a blinking,
on-off, viewing of the reflected light and the ice producing a steady
level of illumination.
In accordance with the invention, various embodiments are provided for
inspecting a metallic surface in which there is a comparison or switching
between an optical isolator structure and non-isolator structure. In one
embodiment, switching is implemented by switching the light illuminating
the metal surface between circularly polarized and non-circularly
polarized light while observing through a circular polarizing filter of
the same hand, i.e., CW or CCW, as required to complete the isolator. In
another embodiment, the light illuminating the surface may be kept
circularly polarized but viewed alternately through a circular polarizer
of the same hand and a non-circular polarizing element having the same
optical attenuation. This is most easily accomplished by viewing through
the same type of circular polarizer flipped over (reflected light enters
the polarizing element first) to keep it from acting as the circular to
linear polarizing element of an isolator while simultaneously maintaining
the slight light attenuation of its elements.
Another embodiment maintains the illumination in a circularly polarized
state and alternately views the scene through right handed and left handed
circular polarizers which will alternately change between the isolating
and non-isolating states. A non-isolating state may also be achieved by
rotating either the receiver or transmitter quarter wave retarder plate
forming a part of the polarizer by 45.degree.. This aligns the slow or
fast axis of the retarder with its polarizer. The effect is that, if done
at the transmitter, linearly polarized light passing through the quarter
wave plate remains linearly polarized and if done at the receiver,
circularly polarized light (which) passes through the retarder plate
first) emerges linearly polarized at 45.degree. to the original
direction--it can then pass through the linear polarizer with just slight
attenuation.
Rotation of either the transmitter or receiver quarter wave retarder by
90.degree. from the position in which it serves to operate as an isolator
also changes the state to non-isolating because the specularly reflected
circularly polarized wave is then exactly aligned with the receiver
polarizer as it emerges in the linearly polarized form from the receivers
quarter wave retarder. Isolating and non-isolating states may also be
achieved by various combinations of crossed and aligned linear polarizers,
respectively.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus
for detecting the presence of a depolarizing dielectric material, such as
ice or snow, on a metal specular reflecting surface.
A further object is to provide a system for detecting ice and/or snow on
the metal (or metallic painted) wing of an aircraft.
An additional object is to provide a system for detecting ice and/or snow
on a metal (or metallic painted) surface which is specularly reflective to
light using circularly or linearly polarized light.
Yet another object is to provide a system for detecting ice or snow on a
metal or metallic painted surface in which optical means are used to
produce an on-off light blinking response for a metal surface and a steady
light response for any part of the surface covered with ice or snow.
Other objects and advantages of the present invention will become more
apparent upon reference to the following specification and annexed
drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is an optical schematic of a circular polarizer with the linear
polarizer facing the illumination source so that the polarizer acts as an
optical isolator.
FIG. 1B is an optical schematic of a circular polarizer with the quarter
wave plate facing the illumination source so that it passes specularly
reflected light, i.e., it is a non-isolator;
FIG. 2 is an optical schematic of two circular polarizers, one in the
transmit path and one in the detection path, that together form an optical
isolator;
FIG. 3A is a schematic view of an ice detection apparatus based on direct
visual observation using two spotlight illuminators, one polarized and one
not;
FIG. 3B is a schematic view of an ice detection apparatus based on direct
visual observation which uses one circularly polarized light source;
FIG. 3C is a detail of the FIG. 3B apparatus for switching the polarizer
between isolating and non-isolating states in the detection path;
FIG. 4A is a schematic diagram of a video based ice detection system
suitable for use with high background illumination levels which employs
two strobed light sources;
FIG. 4B is a schematic diagram of a video based ice detecting system
employing one laser based strobed light source suitable for use with high
background illumination levels;
FIG. 5A is a schematic view of the device used in FIG. 4B to switch the
polarizer from an isolating to a non-isolating state in the detection
path;
FIG. 5B is a plan view of the motor, polarizer and encoder assembly used
with the apparatus of FIG. 5A;
FIG. 5C is a schematic view of the photo interrupter device used in the
encoder assembly of FIG. 5B;
FIG. 6 is an optical schematic diagram of the laser light source of the
system of FIG. 4B;
FIG. 7 is a schematic of another embodiment of the invention which utilizes
synchronous detection; and
FIG. 8A is a schematic view of an embodiment which uses two video cameras
and a beam splitter device;
FIG. 8B is a schematic view of the optical path of FIG. 8A using two
mirrors to replace the beam splitter device of FIG. 8A;
FIG. 9A is a schematic view of a polarization sensitive camera based upon a
variation of color camera technology which is particularly suitable for
use in the receive path; and
FIG. 9B is a section view of details of the polarization sensitive camera.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1A illustrates the operation of a circular polarizer used as an
isolator. Light is emitted from an unpolarized source 13, which preferably
is as close to monochromatic as possible. The light is shown as
unpolarized by the arrows in two orthogonal directions along line 20, the
path the light is following. The unpolarized light passes through a linear
polarizer 11 which has a vertical polarization axis. The light passing
through linear polarizer 11 takes path 21, along this path illustrated as
vertical polarization by the double arrow.
The vertically polarized light at 21 passes through a quarter wave retarder
plate 12. The retarder 12 is a plate made from birefringent material, such
as mica or crystal quartz. Its purpose is to change linearly polarized
light from polarizer 11 into circularly polarized light. Any ray incident
normal to the retarder plate 12 can be thought of as two rays, one
polarized parallel to the parent crystal's optic axis (e-ray) and the
other perpendicular (o-ray). The e-ray and o-ray travel through the plate
12 at different speeds due to the different refractive indices. The plate
12 is said to have a "fast" and a "slow" axis.
The quarter wave retarder plate 12 has its slow and fast axes both at
45.degree. relative to the vertical axis of the linear polarizer 11 so
that the emerging circular polarized light from plate 12 along path 22 is
rotating in a CCW direction as viewed facing the light source from a
reflecting surface 14. A metallic surface, which is a specular reflector,
and a dielectric surface, i.e., ice or snow, behave differently when
illuminated with light, particularly with respect to their polarization
properties. A strong and easily observable difference is the ability of a
metal to reverse the rotational direction of incident circularly polarized
light. The specular reflection of right-handed (CW) circularly polarized
light from a metal surface changes into left-handed (CCW) polarization and
vice versa.
This effect is used in the construction of optical isolators which permit
light to initially pass through the isolator but prevent such light when
specularly reflected from returning through the isolator back to the light
source. When the optical isolator is a circular polarizer it is usually
implemented from a linear polarizer and a quarter wave retarder plate that
has its fast and slow axes located 45.degree. from the polarization axis
of the polarizer.
In FIG. 1A, surface 14, which is a specular surface, reflects the incident
circular polarized light back along path 23. The reflected light continues
to rotate as viewed from the surface 14 in the CCW direction but has now
changed "hand", in terms of right hand and left hand, because it is
rotating in the same direction with its direction of travel changed.
The reflected light on path 23 passes through the quarter wave retarder 12
and emerges no longer circularly polarized but linearly polarized in the
horizontal direction, which is shown along ray path segment 24. Because
the light ray 24 is horizontally polarized it is not passed by the
(vertical) linear polarizer 11. Therefore, none of the specularly
reflected light gets through to path segment 25 to enter the eye 26, which
is shown near the location of the light source 13. Thus, the quarter wave
retarder plate 12 acts as an optical isolator. That is, light from the
source 13 is passed through the circular polarizer and reflected by the
specular surface 14 but cannot pass through the circular polarizer back in
the other direction and so is blocked before it gets to the eye.
FIG. 1B shows the same quarter wave plate and linear polarizer combination
used but the sequence of the elements is reversed. Here, the quarter wave
retarder plate 12 is facing the illumination source 13 and the linear
polarizer 11 is facing the output side towards the reflecting surface 14.
The light rays now emerge from source 13 in an unpolarized form along ray
path 20 and pass through the quarter wave plate 12. However, because the
light is not polarized the quarter wave plate 12 does not change any
polarization properties. The light then passes through the linear
polarizer 11 and becomes vertically polarized along ray path 22.
Surface 14 specularly reflects with the same polarization the vertically
polarized light which travels along ray path 23 back towards the linear
polarizer 11 with the same polarization. The light now enters the quarter
wave retarder plate 12. Because the light entering plate 12 is polarized
in the vertical direction, it emerges from the quarter wave plate
circularly polarized. However, this is of no consequence to the eye 26, so
the eye sees the light that has been reflected from the surface 14. Thus,
in this case with the light first entering the quarter wave plate 12 and
then passing through the linear polarizer 11 and being specularly
reflected back to the eye through the linear polarizer and the quarter
wave plate, there is little loss in the light intensity.
As can be seen in the comparison of FIGS. 1A and 1B, light from the same
source 13 reflected from the specular reflection surface 14 is viewed by
the eye 26 either dim or bright depending upon the location of the quarter
wave retarder plate 12 relative to the linear polarizer 11. That is, FIG.
1A effectively is an optical isolator while FIG. 1B is a non-isolator.
FIG. 2 shows the same implementation of a circular polarizer as in FIG. 1A,
with the receive path and the transmit path each having their own circular
polarizers. Both circular polarizers are in the same order. That is, both
linear polarizers 11a and 11b are on the left, one adjacent to the light
source 13 and the other the eye 26, and both quarter wave retarders 12a,
12b are on the right adjacent to the reflective surface. Thus, as shown,
the light from lamp 13 enters the linear polarizer 11a, exits vertically
polarized, passes through the quarter wave plate 12a and emerges rotating
CCW as viewed from the specular reflecting surface 14. The light reflects
off the surface 14 still polarized rotating CCW as viewed from surface 14
and passes through the circular polarizer 12b in the return direction path
to enter quarter wave plate 12b, from which it exits horizontally
polarized to the vertical linear polarizer 11b which blocks the light.
Linear polarizer 11b in the reception leg is distinct and separate from
the linear (vertical) polarizer 11a that was used in the transmit leg.
Because the polarization of the light ray along path 24 is horizontal, the
light does not pass through the linear polarizer 11b and cannot enter the
eye 26.
When a metallic surface, such as the wing of an air-craft, is illuminated
with circularly polarized light produced by the device of FIG. 1A and the
reflected energy viewed through the same circular polarizer the resulting
image is extremely dim since the circular polarizer is performing as an
isolator with respect to the specular reflection of the circularly
polarized light (of opposite hand) from the metallic surface.
A painted portion (non-specular) of the surface illuminated with circularly
polarized light does not reflect light in a polarized form. Instead, it
destroys the circular polarization and makes the reflected light
unpolarized. Thus, the unpolarized light reflected from a painted surface
portion when viewed through the same circular polarizer of FIG. 1A will
maintain its normal intensity. Time same holds true for circular polarized
light reflected from a wing covered by ice or snow. However, other common
harmless substances such as water or de-icing fluid that may be on the
wing do not destroy the circular polarization of the reflected light.
As explained with respect to FIG. 1B, the components of the circular
polarizer of FIG. 1A are flipped (rotated) such that the retarder plate 12
precedes the linear polarizer 11 with respect to the source of light 13,
so it no longer acts as a circular polarizer to an illuminating beam.
Accordingly, the reflection of circular polarized light from the metal
surface will pass back to the eye and will be of normal (bright)
intensity. The image intensity of such light reflected from a painted or
dielectric (non-specular) surfaces also will be unchanged as in the
previous case.
When a metallic surface is alternately illuminated and viewed by the
isolator and non-isolator devices of FIGS. 1A and 1B, the return images at
the eye 26 will alternate between dark and bright. A painted or dielectric
non-specular surface will remain uniformly bright to the alternation since
the light reflected from the painted or dielectric surface is not
polarized and will not be isolated.
Assuming that a metallic surface has a patch of ice thereon or is coated
with ice, the ice being either matte white due to snow covering or crystal
clear due to rapid even freezing, this will destroy the circular
polarization of the reflected light and therefore take on the property of
a matte painted surface with respect to the optical isolator. That is,
referring to FIG. 1A, if there is ice on any portion of the specular
surface 14, then the circularly polarized light 14 impinging upon such
portion of the surface will not have its polarization reversed. Instead,
it will have the effect of a painted surface so that the returned light
will be non-polarized and will pass to the eye, i.e., the returned image
will be bright.
Accordingly, upon alternately illuminating and viewing an ice-free metallic
surface 14 with the circular polarizer-isolator of FIG. 1A and the
non-isolator of FIG. 1B, the return viewed by the eye 26 will alternate
between dark and bright respectively. Any ice or snow covering a portion
of the metal surface 14 will cause that portion of the image to maintain
the same brightness regardless of whether it is viewed through an isolator
or non-isolator structure upon such alternate illumination and viewing.
Switching between an isolator structure, e.g., FIG. 1A, and non-isolator
structure, e.g., FIG. 1B, may be implemented by switching the light
illuminating the metallic surface between circularly polarized and
non-circularly polarized light while observing through a circular
polarizing filter of the same hand, i.e., CW or CCW, as required to
complete the isolator. As an alternative, the light illuminating the
metallic surface may be kept circularly polarized but viewed alternately
through a circular polarizer of the same hand and a non-circular
polarizing element having the same optical attenuation. This is most
easily accomplished by viewing through the same type of circular polarizer
flipped over (reflected light enters the polarizer element first) to keep
it from acting as the circular to linear polarizing element of an isolator
while simultaneously maintaining the slight light attenuation of its
elements.
Another arrangement is to maintain the illumination in a circular polarized
state. Thereafter, the surface would alternately be viewed through
right-handed and left-handed circular polarizers which alternately change
between the isolating and non-isolating states.
A non-isolating state also may be achieved by rotating either the receiver
or transmitter quarter wave retarder plate 12 by 45.degree.. This aligns
the slow or fast axis of the retarder with its polarizer. The net effect
is that, if done at the transmitter, linearly polarized light passing
through the quarter wave plate remains linearly polarized. If done at the
receiver, circularly polarized light (which passes through the retarder
plate first) emerges linearly polarized at 45.degree. to the original
direction. It can then pass through the linear polarizer to be viewed with
just slight attenuation.
Rotating either the transmitter or receiver quarter wave retarder by
90.degree. from the position in which it serves to operate as an isolator
also changes the state to non-isolating because the specularly reflected
circularly polarized wave is then exactly aligned with the receiver
polarizer as it emerges in linearly polarized form from the receiver's
quarter wave retarder.
The following table illustrates the implementation that may be used when
alternating either the illumination (transmitter) or receiver (detector)
polarizing elements or vice versa to change the overall path from an
isolator to a non-isolator structure:
______________________________________
Between Transmitter
Between Surface
and Surface (or
and Receiver (or
Surface and Receiver)
Transmitter and Surface)
______________________________________
CW only [CW, LP] [CW, UP] [CW, CCW]
CW, LP CW
CW, UP CW
CW, CCW CW or CCW
LP LP, UP LP+, LP- LP
______________________________________
In the table above the following abbreviations are used;
CW Clockwise polarization (Right handed)
CCW Counter Clockwise polarization (Left handed)
LP Linear polarization
LP+ Linear polarizer aligned with LP
LP- Linear polarizer at blocking angle (e.g. 90.degree.) to LP
UP Unpolarized
Alternating states are separated by commas. Equivalent sets of alternatin
states are isolated by square brackets. In any row CW and CCW may be
interchanged. In any row CW may be replaced by RH (right hand) and CCW by
LH (left hand). (The columns can be interchanged), i.e., the action can b
either on the transmitter or receiver leg.
The table shows that when using linear polarization, the isolating state
refers to the receiving polarizer being orthogonal to the polarization of
the transmitted energy beam, and the non-isolating state refers to any of
the following conditions: (a) non-polarized transmission; (b) no polarizer
in the receiver path; or (c) the polarizer in the receiver path is
approximately aligned with the polarization of the transmitted beam.
FIG. 3A is a schematic view of a monocular version of an ice detection
system suitable for night use based on direct visual observation. The
direct visual observation receiver uses a non-inverting telescope 50 with
a circular polarizer 40, like the circular polarizer of FIG. 1A, at its
entrance. Two spotlights 13a, 13b are used for the source of illumination,
i.e., the transmitter. One spotlight 13a has a circular polarizer 30
isolator, like FIG. 1A, mounted to it. The other spotlight 13b has a
neutral density filter 30a or a "same hand" circularly polarized filter
mounted backwards so that the light coming through is linearly and not
circularly polarized, i.e., like the non-isolator of FIG. 1B.
The two spotlights 13a, 13b illuminate a common overlapping area of a
surface shown as the entire area or a portion of an aircraft wing 15
having a patch 16 of ice thereon. The clear (no ice) portions of the wing
15 form a specular reflecting surface such as the surface 14 of FIGS. 1A
and 1B. The wing 15 is observed by the field of view 23 of the
non-inverting telescope 50. Both spotlights 13a, 13b and the non-inverting
telescope 50 are mounted on a support structure 52, which in turn is
mounted to a tripod or boom 54. A power supply and sequencer 51 for the
lights 13a, 13b is also located on the tripod boom structure. Two outputs
from the sequencer 51 are taken along wires 53a and 53b to connect with
and alternately energize the lamps 13a and 13b, respectively.
The eye 26 is shown looking through the telescope 50. The field of view of
the upper spotlight 13a is shown as 22a and that of the lower spotlight
13b as 22b. The region observed by the non-inverting telescope 50 is
formed from the fan of rays 23 reflected back from wing 15 into the
telescope 50.
In operation, the sequencer 51 alternates between sending a voltage to and
alternately energizing spotlight 13a and then spotlight 13b during
corresponding time periods "a" and "b". When the voltage is applied to
spotlight 13a the outgoing light is circularly polarized by polarizer 30
and the light emerges in fan 22a which illuminates the aircraft wing
surface 15. The light from fan 22a reflected from the aircraft wing 15
passes back through fan 23 into the circular polarizer 40 of non-inverting
telescope 50 where it may be viewed by the eye 26 during the interval "a".
During the period "a" an optical isolator arrangement is in place because
there are two circular polarizers 30 and 40 in the path. This is shown in
FIG. 2. That is, metal areas of the wing which produce a specular mirror
like reflection reverse the "hand" of the incident circularly polarized
light and prevent it from passing back through the isolator. Therefore,
the eye 26 sees a very dark region covering the aircraft wing, except
where there is ice, which is shown on area 16 of the aircraft wing and
which area will show brighter to the eye through the telescope.
When spotlight 13a is turned off and spotlight 13b is turned on during
period "b", the light emerging from spotlight 13b is not circularly
polarized. Now the reflection coming back to telescope 50 from both the
areas with ice or a metal area without ice will approximately maintain
their normal brightness. Thus as the sequencer 51 alternately energizes
the spotlights 13a and 13b, the image at the eye 26 from any area that is
metal, specular and ice free will appear to blink on and off. This will be
"on" (bright) when the optical isolator is not in operation and "off"
(dark) when isolation exists. However, areas that have ice will not blink
and will have essentially constant brightness, because the polarized light
produced during period "a" is depolarized upon impinging and being
reflected from the ice or the metal under the ice.
FIG. 3B shows another ice detection apparatus especially suitable for night
use, which is based on direct visual observation and uses only one
spotlight 13 with a circular polarizer 30, such as FIG. 1A. In FIG. 3B the
receiver telescope 50 has apparatus at its input for changing a circular
polarizer between the isolating (FIG. 1A) and non-isolating (FIG. 1B)
states. Here, the illumination source 13 and the telescope 50 are mounted
on a bracket 52 of a boom mount or tripod 68. A power supply 67 for lamp
13 also is mounted on the boom.
Power supply 67 supplies the power to lamp 13 along cable 66b. Lamp 13
incorporates a circular polarizer 30, such as of FIG. 1A. The field
illuminated by lamp 13 is shown as 22a and encompasses an aircraft wing
area 15 which has an area of ice 16. Telescope 50 has a field of view
encompassing the aircraft wing, or portion of the wing, and this is shown
in the ray fan 23 which enters the telescope. Telescope 50 alternates
between optical isolation and non-isolation to the reflected light using a
circular polarizer made of a fixed linear polarizer 41 and quarter wave
retarder plate 42. As shown in FIG. 3C, the quarter wave retarder plate 42
is rotated about its optical axis by drive 65.
FIG. 3C is a detail showing the apparatus for rotating the quarter wave
retarder plate 42. The quarter wave retarder plate is rim driven by
friction drive 65 attached to a motor shaft 64 driven by a motor 63 which
itself is attached to telescope housing 61. Bearings 62 between the
quarter wave retarder plate 42 and the housing 61 relieve friction so that
the quarter wave retarder plate may freely rotate about its optical axis.
When the quarter wave plate has rotated to such a position that its slow
and fast axes are at 45.degree. to the vertical, as shown in FIG. 2, the
unit acts as an optical isolator and any circularly polarized light that
is specularly reflected from the aircraft wing cannot pass through the
combination of the quarter wave retarder and the linear polarizer to the
eye 26.
A similar end may be achieved by rotating the linear polarizer 41 via rim
drive 60 and keeping the quarter wave retarder plate 42 fixed or by
keeping both linear polarizer 41 and quarter wave retarder plate 42 fixed
and rotating a half wave plate mounted between them with rim drive 60.
The position for optical isolation is achieved twice during two positions
spaced 180.degree. apart of each revolution of the quarter wave retarder
42. At any other position of rotation of plate 42, there is no isolation
and the circularly polarized light reflected from the various portions of
the wing, both metal and ice, is free to pass through to the eye with only
minimal attenuation. Therefore, the specularly reflective metal portion of
the wing that is not covered with ice will reflect light from the
illuminator 13, circularly polarized, back through the isolating mechanism
41, 42a and this specularly reflected light will be interrupted twice per
revolution and blink off completely. During the other positions of the
circular polarizer retarder plate 42 rotation the light will pass through
to the eye 26. Thus, the "on"-"off" blinking effect will be produced twice
for each rotation of plate 42.
On the areas of the wing 15 when there is ice present, the incident
circularly polarized light from lamp 13 and polarizer 30 will be
depolarized due to the surface of the ice or by passing through the ice.
This depolarized light will pass through the isolator 41, 42a at the
telescope 50 regardless of the rotational position of the quarter wave
retarder plate 42. That is, even when the plate 42 is in one of its two
isolating positions relative to reflected polarized light, the
non-polarized light reflected from the ice will pass through to the
telescope as well as when the retarder plate is in a non-isolating
position.
The eye 26, which is looking through the telescope 50, is able to
differentiate between the blinking effect produced by the ice free section
of the wing 15 and the non-blinking effect produced by sections 16 of the
wing with ice. That is, the sections of the wing covered with ice 16 will
appear to have constant illumination and the ice free sections of the wing
will appear to blink at a rate of twice the speed of rotation of the
quarter wave plate 42.
In either of the embodiments of FIGS. 3A and 3B, the apparatus can be moved
to scan all parts of the wing if the field of view is not large enough.
FIG. 4A shows an indirect viewing video-based ice detecting system that
employs two strobe lamp spotlights and is suitable for use with high
background illumination levels.
The system of FIG. 4A is similar to that of FIG. 3A in that it employs two
strobe lamps 93a, 93b. These lamps are of the type which produce a high
intensity output, for example a xenon lamp, for a short time period. Here,
both strobe lamp 93a and 93b have circular polarizers, such as in FIG. 1A,
attached. One is a right handed circular polarizer 30a and the other a
left handed circular polarizer 30b. The strobe lamps 93a and 93b are used
in conjunction with a conventional video camera 80 with a lens 81 having a
right handed circular polarizer 40 at its input.
The analog signals of the image produced by the video camera, which
observes the scene illuminated by the strobe lamps 93a, 93b, are sent to a
conventional frame grabber 70. The frame grabber 70 converts the analog
video signal from camera 80 to digital form and stores them in a digital
memory buffer 70a. Pulse generator 75 is used to initiate the strobing of
the lights and the grabbing of a single isolated frame by the frame
grabber from the video camera.
The system also preferably has a digital to analog converter and sync
generator so that the image stored in the buffer 72a can be sent from the
frame grabber video output to a video monitor and/or VCR 72 along cable
71. The video monitor and the video cassette recorder (VCR) are
commercially available. As an alternative, the video monitor may have a
disk recorder which is also commercially available. The frame grabber may
be purchased with additional memory attached and a computer as part of one
single image processor unit. This portion is shown as 70A. Frame grabber
70 and its memory, plus computer CPU 90A may be bought commercially as the
Cognex 4400.
A flip flop 85 alternates between states on every strobe pulse produced by
pulse generator 75. This allows selectively gating a strobe pulse to
either lamp 93a or 93b so that they are illuminated alternately. When a
pulse trigger input is received by the frame grabber 70 from pulse
generator 75 output 76, a camera synchronized strobe pulse is generated
which is fed from the frame grabber output 74 to the flip flop 85. The
strobe pulse toggles flip flop 85 and is also gated through one of two AND
gates 89a and 89b. When the flip flop 85 is in one state the strobe out of
the frame grabber is gated through AND gate 94a to the input 94a of strobe
lamp 93a. When flip flop 85 is in its other state, a pulse is sent along
wire 53b to input 94b of strobe lamp 93b. Thus, lamps 93a and 93b are
alternately illuminated.
The field of view from the strobe lamp 93a with right hand circular
polarizer 30a is shown as 22a. The illumination area from strobe lamp 93b
with left handed circular polarizer 30b is shown as 22b. The video camera
80 has a field of view 23 that covers the overlapping region between 22a
and 22b. In the video camera field of view 23 are the wing 15 with iced
area 16. The images that correspond to wing 15 and iced area 16 that are
shown on the video monitor 72 are labeled correspondingly as 15a and 16a.
During operation, the pulse generator 75 is set to provide trigger signals
at a constant rate, e.g., in a range between 1 and 10 Hz. When a trigger
signal enters the frame grabber input 77, it is synchronized with the
frame grabber internal cycle and at the proper time the frame grabber
provides a strobe to flip flop 85 which is passed on to strobe lamps 93a
or 93b. The strobe output is timed to be properly aligned with the frame
synchronization signal that is sent along cable 84 from frame grabber
output 83 into the video camera 82. Cable 84 provides a path from the
frame grabber 70 to the video camera 82 for synchronization and a return
path from video camera output 82 to frame grabber for the video signal.
If the pulse received by the AND gate 89a is enabled because flip flop
output 85 is high, the strobe will pass through AND gate 89a, enter the
strobe input 94a and fire the strobe lamp 93a. The strobe lamp will
produce a very short light pulse of approximately 10 microseconds length.
The light pulse from the strobe lamp 93a illuminates the wing area. The
reflected light from ice free specular area of the wing will be left hand
circularly polarized because of the right hand circular polarizer 30 at
the output of strobe lamp 93a. Because the video camera 82 has a right
hand circular polarizer 40 at its input, it acts as part of an isolator.
That is, any reflection from a clean metal specular area of the wing will
reflect left hand polarized light which will not be able to get through
the right hand circular polarizer 40 of the camera 80 and thus these areas
as viewed by the camera will be very dark. The image sent by the video
camera to the frame grabber will also appear very dark as well as the
stored image that is sent from the frame grabber buffer memory into the
video monitor 72 input 79 via wire 71.
Where there is ice present on the wing it will spoil the circular
polarization of the polarized incident light and the image scene of the
reflective light picked up by camera 80 and viewed on monitor 72 will not
be dimmed.
When the strobe signal passes through AND gate 89a, it simultaneously
resets flip flop 85 to the opposite state such that AND gate 89b is
enabled. Therefore, the next pulse from the pulse generator 75 into the
frame grabber 70 causes the corresponding strobe pulse to be genera | | |
