System for detecting ice or snow on surface which specularly reflects light

What is claimed:

1. Apparatus for detecting on a surface which specularly reflects light, the presence of a light polarization-altering substance comprising:

means for transmitting light over a transmitting path to said surface;

means for receiving transmitted light;

a receiving path for said transmitted light from said surface and from said substance to said means for receiving;

optical means in at least one of said transmitting path and said receiving path;

means for alternating said optical means between an optical non-isolator state and an optical isolator state such that, when said substance is present on said surface, a difference in an intensity of light received at said means for receiving exists between said states, said difference resulting from more light being received from one of said surface and said substance than from the other thereof in one of said states than in the other of said states; and

means for detecting said difference.

2. Apparatus as in claim 1 wherein, when said optical means are utilized in both said transmitting and receiving paths, said optical means in one of said transmitting and receiving paths produces, light with one polarization characteristic and in the other of said transmitting and receiving paths produces light with two different alternating polarization characteristics.

3. Apparatus as in claim 2 wherein said optical means comprises:

a linear polarizer for receiving light in said at least one of said transmitting path and said receiving path; and

a means for changing a polarization characteristic of light received from said linear polarizer.

4. Apparatus as in claim 3 wherein said optical means further comprises means for alternating orientation of one of a quarter wave retarder plate and said linear polarizer relative to an optical axis of light it receives.

5. Apparatus as in claim 4 wherein said means for alternating further comprises means for rotating said linear polarizer and said quarter wave retarder plate relative to each other.

6. Apparatus as in claim 2, wherein said optical means comprises means for circularly polarizing light in said receiving path and a linear polarizer for changing the polarization of light received from said means for circularly polarizing.

7. Apparatus as in claim 6 wherein said optical means further comprises means for alternating orientation of one of a quarter wave retarder plate and said linear polarizer relative to an optical axis of light it receives.

8. Apparatus as in claim 7 wherein said means for alternating comprises means for rotating said linear polarizer and said quarter wave retarder plate relative to each other.

9. Apparatus as in claim 2 wherein said optical means in said receiving path further includes a quarter wave retarder plate, a light phase rotator, and a linear polarizer.

10. Apparatus as in claim 1 wherein said means for transmitting and said optical means alternate with each other to produce circularly polarized light of opposite hands.

11. Apparatus as in claim 10 wherein said means for transmitting comprises:

a first and a second lamp;

a means for alternately energizing said first and second lamp;

a first circular polarizer optically associated with said first lamp; and

a second circular polarizer, of opposite hand to said first circular polarizer, optically associated with said second lamp.

12. Apparatus as in claim 11 wherein said means for receiving comprises:

a video camera; and

at least one of said first, said second, and a third circular polarizer being associated with said video camera.

13. Apparatus as in claim 12 further comprising a means for capturing and storing each of a plurality of images produced by said video camera.

14. Apparatus as in claim 13 further comprising a means for displaying a stored image of said plurality of images stored by said means for capturing.

15. Apparatus as in claim 13 further comprising means for comparing a first stored image of said plurality of images stored by said means for capturing with a second stored image of said plurality of images stored by said means for capturing.

16. Apparatus as in claim 1 wherein said means for transmitting comprises:

a light source to produce a light;

means for sequentially strobing said light source;

means for circularly polarizing said light; and,

means for synchronizing the strobing of said light source with said means for alternating said optical means between said optical isolator and said optical non-isolator states.

17. Apparatus as in claim 16 wherein a part of said optical means in said receiving path is rotated on an axis transverse to an optical axis of said receiving path.

18. Apparatus as in claim 17 wherein said means for receiving comprises a video camera.

19. Apparatus as in claim 18 further comprising a means for capturing and storing each of a plurality of images produced by said video camera.

20. Apparatus as in claim 19 further comprising means for displaying said each of said plurality stored by said means for capturing.

21. Apparatus as in claim 20 further comprising means for comparing a first stored image of said plurality of images stored by said means for capturing with a second stored image of said plurality of images stored by said means for capturing.

22. Apparatus as in claim 16 wherein said optical means comprises:

a housing;

a linear polarizer within said housing;

a quarter wave retarder plate parallel to said linear polarizer within said housing; and

means for rotating said housing.

23. Apparatus as in claim 22 wherein said light source is a laser.

24. Apparatus as in claim 1 wherein:

said means for transmitting includes means for transmitting a circularly polarized light to said surface,

said optical means includes a retarder for changing said circularly polarized light, spectrally reflected from said surface to a linearly polarized light; and,

said means for alternating includes a linear polarizer, means for rotating a phase of said linearly polarized light to be transmitted to said linear polarizer between said optical isolator state and said optical non-isolator state; and said means for detecting receiving light from said linear polarizer.

25. Apparatus as in claim 1 wherein:

said means for transmitting and said optical means produce a polarized light; and,

said means for receiving includes

a first optical receiving means, optically associated with a first viewing means, for passing said polarized light specularly reflected from said surface, and,

a second optical receiving means, optically associated with a second viewing means, for blocking said polarized light specularly reflected from said surface from reaching said second viewing means.

26. Apparatus as in claim 25 wherein:

said means for transmitting and said optical means produce linearly polarized light; and,

said first and second optical receiving means respectively pass and block said linearly polarized light specularly reflected from said surface.

27. Apparatus as in claim 25 wherein:

said means for transmitting and said optical means produce a circularly polarized light; and,

said first optical receiving means comprises a first circular polarizer and said second optical receiving means comprises a second circular polarizer of opposite hand to said first circular polarizer, and said first and second circular polarizers respectively pass and block said circularly polarized light specularly reflected from said surface.

28. Apparatus as in claim 1 wherein said optical means in at least one of said optical isolator and said optical non-isolator states produces light having the following polarization characteristic(s) in said transmitting path and said receiving path, presented in tabular correspondence:

where:

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 to LP;

UP=Unpolarized;

and where:

said alternating optical isolator and non-isolator states are separated by commas;

equivalent sets of said optical isolator and non-isolator states are set apart by braces;

in any row, CW and CCW may be interchanged;

in any row CW may be replaced by RH (right hand);

in any row CCW may be replaced by LH (left hand); and

the headings over the two columns may be interchanged.

29. Apparatus for detecting the presence of a polarization-altering substance on a specularly reflecting surface comprising:

means for transmitting light over a first path to said surface,

means for receiving transmitted light returning on a second path from said surface, and

optical means in at least one of said first path and said second path;

said optical means including means for alternating between an optical isolator state and an optical non-isolator state for producing a difference in an intensity of said returning transmitted light between said returning transmitted light from said polarization-altering substance and said returning light from said specularly reflecting surface such that, when said substance is present on said surface, a difference in an intensity of light received at said means for receiving exists between said states, said difference resulting from more light being received from one of said surface and said substance than from the other thereof in one of said states than in the other of said states; and

means for detecting said difference, whereby the presence of said polarization-altering substance is detectable.

30. Apparatus for detecting the presence of a dielectric substance on a specularly reflecting surface comprising:

means for transmitting light over a transmitting path to said surface;

means for receiving returning light on a receiving path from said surface;

optical means in at least one of said transmitting path and said receiving path; and

means for alternating said optical means between an optical isolator state and an optical non-isolator state, whereby a brightness of said returning light from said specularly reflecting surface changes in a manner different from a change in brightness of returning light from said dielectric substance.

31. Apparatus for detecting the presence of a dielectric substance on a specularly reflecting surface comprising:

means for transmitting light over a transmitting path to said surface;

first means for receiving returning light on a first receiving path from said surface;

second means for receiving returning light on a second receiving path from said surface;

optical means in at least one of said transmitting path, said first receiving path and said second receiving path;

said optical means being effective for creating an optical isolator state in one of said first and second receiving paths, and an optical non-isolator state in the other of said first and second receiving paths, said states differing in an intensity of light received at said first and second means for receiving said difference resulting from more light being received from one of said surface and said substance than from the other thereof in one of said states than in the other of said states; and

means for detecting said difference.

Description Submit all comments and votes

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 cross and aligned linear polarizers, respectively.

OBJECT OF THE INVENTION

It is therefore an object of the present invention provide an apparatus for detecting the presence of a depolarizing 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 aircraft, 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. The 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 deicing 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 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+, LP- LP, UP 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 alternating 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 be 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 condition:

a) non-polarized transmission

b) no polarizer in receiver path

c) polarizer in receiving 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, 42 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 42a. 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