Field of the invention

The invention relates to a method and device for controllably reflecting radar signals via one or more reflection members, which may be used for several applications, including identification tags, gaming and military applications.

Background US 5,819,164 discloses a modulating retroreflector device. The device contains an electro-responsive reflecting surface, with a reflection coefficient that depends on applied DC voltage. The device is used for communication to an external device. The retroreflector device applies a time dependent voltage in time to the electro-responsive reflecting surface in order to modulate reflection, while the surface is exposed to microwave radiation from the external device. Electro responsive reflective materials are well known per se. US 5,819,164 discloses specific electro-responsive materials, but other ways of providing electro-responsive reflectors are known for example from US 6,657,580, and US 6,300,894.

US 2006/145853 discloses an electro-responsive retroreflector tag for use by persons or to identify objects. The retrorefelector tag controls its reflection coefficient by connecting an antenna to different load impedances using an electronic switch. When the tag is exposed to RF radiation from a reading device, the tag controls of the switch in a time dependent way, according to an internal identification code. As a result the reading device can derive the identification code from the time dependence of the reflected radiation. In the art of radar baffles it is known to provide for microwave absorbing surfaces to adapt the spatial radiation pattern of antennas. US 5,847,672 discloses the use of photo-sensisitive baffles for this purpose. GaAs, GaN, silicon germanium alloys and/or photoconductive polymers are described as examples for use in such photo-sensisitive baffles. When not illuminated such a photo-sensisitive material is electrically non-conductive and, hence, transparent to RF energy. When illuminated the photo-sensisitive material is electrically conductive, so that it absorbs or reflects RF energy. US 5,847,672 proposes to use the photo-sensisitive material at selected positions in an antenna, where it affects the spatial antenna pattern, in combination with lamps to switch the conductivity of the material on or off, dependent on whether the antenna is in a transmission or reception mode for example.

EP 524 878 similarly describes a microwave absorber surface that is controlled by light. A set of optical fibers on the surface, coupled to a laser, is used to switch the surface between an absorbing state and a non-absorbing state.

Summary Among others, it is an object to provide for a radar retroreflector from which information can be read without requiring active modulation in the

retroreflector.

A method according to claim 1 is provided, for detecting information from an object coding module, wherein the object coding module comprises a light controllable radar reflection member whose radar reflection characteristics depend on lighting properties of light to which the radar reflection member is exposed. The method comprises steps of:

irradiating the radar reflection member by a radar signal; irradiating the object coding module with light having time variable lighting properties between different time points at which the radar reflection member is irradiated by the radar signal;

receiving reflected radar signals reflected by the radar reflection member,

processing the reflected radar signals to detect differential light activation of the radar reflection member from a temporal difference between radar reflection at times when the object coding module is irradiated with light having mutually different lighting properties.

In an embodiment, the light controllable radar reflection member comprises a strip of photo-sensitive semi-conductor material, of a material type wherein light is able to excite movable charge carriers in the conduction band.

Photoresistors of this type are known per se. The conductivity strip increases when it is lighted, causing it to reflect radar signals, or at least to do so more strongly. In another embodiment a combination of a photovoltaic element and an electro-responsive material, such as known from the cited documents being used, the light causing the photovoltaic element to generate a voltage, which is applied to the electro-responsive material to change its reflection of radar signals. In an embodiment the reflection member may comprise a plurality of conductors, interconnected by such photoresistors and/or photovoltaically controlled material. In an embodiment the radar reflection member may have as shape and or size that is resonant at the frequency or in the frequency range of the radar, when made more conductive by lighting,

In an embodiment, lighting properties may be time variable because the intensity of the light is varied, for example between no intensity and a nonzero intensity. In an embodiment, lighting properties may be time variable because the spectral content is varied as a function of time, for example by using narrowband light and sweeping the wavelength of the light, or using a light source with a plurality of lighting devices, such as different color LEDs, that are designed to provide light with mutually different spectral content and varying the relative intensity of the lighting devices. In an embodiment, lighting properties may be time variable because the light source comprises a broadband lighting device and a color filter or filters that are applied to light from the lighting device in a time dependent way. In an embodiment, lighting properties may be time variable because the spatial configuration of the light is varied (the intensity and/or spectral content as a function of spatial direction from the light source), for example by scanning structured light, like a plane of light with a scanner, e.g. using a rotating mirror. In each case, the lighting properties having a time dependent effectiveness to modify the reflective characteristics of the radar reflection members for said radar signals

The time dependent lighting properties result in time dependent radar reflection. The radar reflection signals are processed to detect the radar reflection member from a temporal difference between the radar reflection at times the object coding module is irradiated with light having mutually different lighting properties. The radar signal may be transmitted in a series of sweeps (e.g. frequency sweeps), the lighting properties being changed between sweeps. In another embodiment the radar signal may be transmitted continuously while the lighting properties are changed, so that the lighting properties are different at different time points during the continuous transmission. In another embodiment, the radar transmission may be interrupted between such time points. Reflected radar signals are detected and used to detect the radar reflection member. The reflected signals at the different time points (e.g. time points where the lighting is off and on respectively), or averages of the reflected signal strength during time intervals including the time points, may be subtracted and the radar reflection member may be detected from the resulting difference. Alternatively, the received reflected signal strengths, or their averages, may be compared, for example to determine whether or not there is at least a predetermined difference between the signal strengths. Instead of using the received radar signals directly, they may be processed to determine radar reflection (radar reflection values), i.e. quantities proportional to ratios between the transmitted signal strength and received signal strength, optionally corrected in the same way for distance effects or other effects, and the radar reflection member may be detected by comparing these radar reflections.

In an embodiment, a code programmed in the object coding module may be determined using detection of the reflection differences under lighting with different lighting properties. Detection of a difference may be used, which confirms that an activatable radar reflection member representing a code or part of a code is present, or the absence of a difference in a detected object may be used to detect that the radar reflection member has been deactivated. In an embodiment a plurality of such radar reflection members may be provided in the object coding module, in a way that makes it possible to detect them individually, for example by filtering control light to each of the radar reflector members with a light color filter that passes a different wavelength band, or positioning the radar reflector members at mutual distances that makes it possible to light them distinctly by a scanning light source. The detection of the presence or absence of reflections from these members may be used to derive respective bits of the code.

In embodiments, the radar reflector member may be detected by selectively comparing radar reflections for corresponding radar distances and/or directions, by comparing radar reflections values for corresponding radar delays and/or corresponding radar image positions obtained for different time points. When a plurality of radar reflector members is used to detect a multi- bit code from an object coding module, detections for corresponding radar distances and/or directions are preferably used for all bits. A radar image may be formed by sweeping a direction of a beam or plane of said light, and assigning respective reflected radar signals to respective different positions in a radar image according to the direction of the beam or plane of said light at a time of detection of the reflected radar signals.

Structured light scanners may be scanned for example by rotating a light source and/or a mirror or lens in the light path. During the sweep of the direction of the light, a radar beam may be used with a beam width for detecting reflections from a direction range that covers at least part of the directions of the light scan. Thus a higher radar image resolution than said beam width is obtained. The radar direction range may be kept fixed during the light sweep.

In an embodiment a coding module with the light controllable radar reflection members having mutually different radar resonance wavelengths may be used. The radar reflection members may be irradiated by radar signals at respective ones of said radar resonance wavelengths, while the light has equal lighting properties. In this case light activation at respective ones of said resonance frequencies may be detected. Thus more readable information is provided in the coding module. Different resonance frequencies may be provided by using radar reflection members of different size and/or shape. Irradiation at the radar resonance wavelength involves irradiation at a wavelength within a resonance peak in the wavelength dependent response function of the reflection member, not necessarily at the top of the peak.

The lighting and radar detection may be supplied under common control, for example from an interrogation device for identifying objects, so that it is known in advance of processing which radar reflection corresponds to which of the different lighting properties. In another embodiment independent control may be used. In this case a light detector may be used to detect when which lighting properties are applied, for use in the detection of the reflection members.

A detection system with an interrogation system and an object coding module is provided, that uses light controlled activation of radar reflection members to detect whether activatable radar reflection members are present, as well as an interrogation system and object coding modules for such a detection system.

A method is provided for controllably reflecting radar signals via one or more reflection members, the one or more reflection members have reflective characteristics for the radar signals which are different when irradiated by a light source or when not respectively, the method comprising steps of:

irradiating the one or more reflection members are by at least one radar signal;

- irradiating at least part of the one or more reflection members by a light source during at least part of the time they are irradiated by the at least one radar signal;

receiving a first radar reflection signal or signals reflected by the one or more reflection members during the time that the at least part of the one or more reflection members are irradiated by the light source,

receiving a second radar reflection signal or signals during the time that the at least part of the one or more reflection members are not irradiated by the light source;

processing the first and second.radar reflection signals and to detect differences between the first and second.radar reflection signals.

In applications where reflection members are used having different distances to the radar transmitter, those distances can be measured by the (e.g. FMCW) radar. Recognition of the reflection members amidst other reflective objects thus can be enhanced by applying the method, i.e. by irradiating the reflection members by light, making a radar image, turning the light off and making a radar image again and determining the difference between both radar images. Only reflection spots which solely appear in de radar image when the light was turned on represent the reflection members, as those reflection members solely reflect radar signals when they are irradiated by light.

Radar is not very suitable to discriminate between reflection members which are located at about equal distances to the radar source and/or in each other's vicinity. When such reflection members e.g. are used for remote coding purposes, it may be preferred that the reflection members, used as (binary) coding elements, have reflective characteristics for radar signals which are different when irradiated by a light source having a predetermined wavelength or wavelength range or when not respectively. Using such discrimination in light wavelength (colour) sensibility of the reflection members, coding means may be formed having two or more reflection members having similar reflective characteristics for radar signals when irradiated by a light source (reflective for radar) or when not (not reflective for radar) respectively, the reflection members, however, having those similar reflective characteristics for radar signals when irradiated light or when not respectively, for mutually different wavelengths or wavelength ranges of the light. An exemplary embodiment for a system using light wavelength discrimination for reflective coding members will be discussed hereinafter as an application example.

Brief description of the drawing

These and other objects and advantages will become apparent ferfronm a description of exemplary examples, using the following figures

Hereinafter the invention will be discussed more in detail with reference to some figures, wherein Figure 1 shows an exemplary embodiment of a system for controllable reflecting radar signals via one or more reflection members, applied for object identification;

Figure 2 shows an embodiment of a coding module including light dependent reflection members, in uncoded (left) and coded (right) state;

Figure 3 illustrates, as functions of the time, two optional radar signals, a preferred ON/OFF light signal, processed radar signals reflected by any objects including the reflection members, and processed radar signal solely reflected by the reflection members.

Figure 4 illustrates the reflection of an electromagnetic wave off a surface;

Figure 5 shows a relation between the wave reflection and the surface's

characteristic impedance;

Detailed description of exemplary embodiments

Figure 1 shows a system for controllably reflecting radar signals via a number of reflection members 1 included by an object coding module 2. Each of those reflection members 1 have reflective characteristics for radar signals which are different when irradiated by a light source 4 or when not respectively. All individual reflection members 1, however, have that characteristic for different (light) wavelengths, indicated as coi - con respectively. A light source 4 is able to radiate light between wavelengths co _a and cob, in a way as shown in figure 3.

The system in figure 1 further shows an FMCW radar transceiver 3 which able to irradiate the reflection members 1 with a radar signal which periodically sweeps from frequencies f _a to fb. Due to this frequency sweep the distance between the radar transceiver 3 and the reflection members 1 can be measured in a way which is generally known from the FMCW radar technique. When, however, it is not necessary to measure the distance between the radar transceiver 3 and the reflection members 1, a radar transceiver 3 may be used which simply transmits a signal having a continuous frequency f _c , in stead of a "frequency sweeping" radar signal.

The radar wavelength may lie between one meter and one millimetre for example. The wavelength of the light may be a wavelength lying between 0.4 and 1.5 micrometer for example, an optical or infrared wavelength may be used.

The light source 4 and the radar transceiver 3 are under control of a control unit 5. Under control of the control unit 5 the reflection members 1 of the code module 2 are irradiated by the light source 4 during always part of the time they are irradiated by the radar signal. As illustrated in figure 3, the light is, alternately, switched on and swept from co _a to cob within one period P, and switched off then (dotted lines) during next period, switched on and swept from coa to cot, again, and switched off, etc. When an FMCW radar signal is used, it may be preferred that the period P of that FMCW signal is equal to the light on/off period P. However, if stated before, FMCW radar (including periodical frequency sweep) is not strictly necessary for remote reading the code value of the code module; however, it may be used for determining the distance from the radar transceiver 3 to the coding module 2.

Figure 2 illustrates the configuration of the coding module 2. The left side of figure 2 shows a basic code module, which has not been coded yet. It is constituted by a carrier 6, a number of separated layers in any shape, e.g. having the form of stripes 7, made of a material which is reflective for radar signals when irradiated by light. Without being irradiated by light, they are not reflective for radar beams. The material may be a photoresistive material. Photoresistive materials are well known per se. They have light dependent resistivity. Optical excitation of conduction band charge carriers in a semiconductor material may be used for example. The material may include cadmium sulphide, cadmium selenide, silicium, germanium, gallium arsenide, the conductivity of which depends on the intensity of incident light. In this way the characteristic (wave) impedance Zi will change when the intensity of incident light changes, and hence the reflection rate of the stripe surface will change.

To make the stripes 7 selective for certain light wavelengths or wavelength areas, the stripes 7 are provided with colour filters 8, i.e. filters which are selective for one wavelength of wavelength range. In that way the coding module includes a number of stripes, each being sensitive for one wavelength (area) coi ... con. The stripes 7 and the filters 8 may be manufactured by evaporation or printing upon the carrier 6.

After manufacturing, the coding module 2 can be coded, e.g. for an object to be identified remotely, by e.g. etching away some stripes 7 and filters 8, or removing them in another way, e.g. mechanically or by selective heating. The right side of figure 2 shows a module 2 coded with binary code 10011101101, viz. by etching away or covering (e.g. by printing) the stripes (i.e. either their relevant stripe layer 7 or filter layer 8 or both) the sensible for the light wavelengths C02, C03, co ₇ and coio respectively.

In another embodiment, light blocking layers may be provided on the stripes, which may be removed from selected stripes to activate selected stripes as light controlled radar reflectors to code the coding module. In another embodiment light blocking layers may be selectively deposited to deactivate selected stripes. Conversely, in another embodiment light blocking layers may be applied selectively to (e.g. printed on or applied with a marker pen on) those stripes for which no radar response is desired. In an embodiment coding module 2 may be manufactured in large batches, each with an array of light sensitive radar reflectors, or a layer of light sensitive radar reflecting material with a mask printed on it that leaves open radar reflectors, and optionally color filters on the reflectors in each module. In this embodiment, each modules being provided with an individual code only in a last printing step wherein code dependent reflectors are printed over, or selected color filters are printed on the reflectors.

When each of the remaining stripes 7/8 is irradiated by light having the (pass) wavelength for which its filter layer 8 is sensitive, the stripe 7/8 will be reflective for a radar signal which irradiates that stripe at the same moment (the radar frequency is not relevant as such). So, when the coding module 2 is by the light source 4 with a light sweeping from co _a and cob and, at the same time, by the radar transceiver 3, each of the stripes 7/8 not etched away will reflect the radar signal, while each stripe 7/8 which was etched away will not. The result is that, while the light sweeps from co _a to cob, subsequently, the stripes 7/8 -indicated by their filter wavelengths before they were etched away- coi will reflect the radar signal, C02 and C03 will not, co4, cos and coe will reflect, co7 will not, cos and cog will, coio will not reflect and con will reflect the radar signal. The result is that subsequently a radar signal is reflected having in the form of 10011101101, in which 1 represents "reflecting" and 0

represents "not reflecting". Assigning 1 to "reflecting" and 0 "not reflecting" is arbitrary: as an alternative it could be chosen that 0 would represent

"reflecting" and 1 "not reflecting", which would result in a code 1100010010.

The radar transceiver 3 (including processing means such as a microprocessor with a program memory containing a signal processing program, which are not shown explicitly) is arranged for receiving and processing any radar reflection signals, i.e. portions of the radar signal which are reflected by the reflection members 1, and/or by any further reflective objects (not shown explicitly) to wit during the time that the reflection members 1 of the coding module 2 are irradiated by the light source 4, and during the time that the reflection members 1 are not irradiated by the light source 4. Figure 3 illustrates, as functions of the time, two optional radar signals, a preferred ON/OFF light signal, processed radar signals reflected by any objects including the reflection members, and processed radar signal solely reflected by the reflection members.

The transmitted radar signal may be an FMCW signal, having a saw tooth like frequency/time shape and including periodic frequency sweeps fro m f _a to fb. It is not necessary to use FMCW radar signals; only if the distance between the radar transceiver and the coding module should be measured, FMCW could be used. However, the radar signal could comprise a continuous signal having any frequency f _c .

Figure 3 further illustrates that a light source is switched on and off, one or more times. When switched on, the light wavelength sweeps from co _a to cob, after which the light source is switched off (indicated by the dotted lines).

During the period(s) the light is ON the radar signal are reflected by several objects having reflective properties for radar frequent signals, including the reflection members as long as they are irradiated by the light source (each reflection member only when the light has that reflection member's specific (filter) wavelength ωι.,.ιι). Those reflections are seen in figure 3 in the first and third period: the reflections of the reflections members 1 are indicated, for intelligibility, in full black, any further reflections caused by objects different from reflection members 1, in white. During the period(s) the light is OFF, i.e. during the second and fourth periods in figure 3, the reflection members do not reflect the radar signal. Only further the reflections, caused by objects different from reflection members 1, will be received by the radar transceiver. When the reflection signals, received by the radar transceiver, are processed - e.g. within the control module 5- as illustrated in the at the bottom of figure 3, i.e. by subtracting the ("white") reflections picked up by the radar transceiver 3 during a period (e.g. the second or fourth period) the light source 4 is OFF (thus disabling the reflectivity of the reflective members 1 of code module 2) from the ("white and black") reflections, picked up by the radar transceiver 3 during a period (e.g. the first or third period) the (sweeping) light source 4 is ON (thus enabling the reflectivity of the reflective members 1), resulting in solely the "black" reflections, i.e. solely the reflections caused by the reflection members 1. In other words, a series of radar reflection measurements is performed, each accompanied by illumination with light at a respective different wavelength of light, and one or more reference radar reflection measurements are performed without such illumination. The radar reflection measurements at the respective light wavelengths are compared with a reference measurement (e.g. by subtracting one from the other). From the respective comparisons bits of a code are determined. When FMCW is used, the radar reflection measurements at the respective light wavelengths are part of radar results obtained with a frequency swept radar signal, but instead measurements at a single radar frequency may suffice. Although an example has been shown wherein the light is switched on and off, it should be appreciated that instead light intensity may be changed between different non-zero values to make a comparison between the radar reflections possible. Also, a comparison may be used between the radar reflections obtained with light with different spectral color content instead of between reflections with the light on and off. The comparison may be realized by subtracting measured radar reflection (normalized for transmitted radar irradiation strength), or time averaged radar reflection, or by subtracting detected reflected signals directly. Instead of subtraction a comparison between radar reflections or the reflected signals may be used.

The presence of an activatable (activation enabled) radar reflection member (i.e. a radar reflection member whose radar reflection properties can be changed by under the influence of light), can be detected by comparing radar reflections measured at respective time points, or in respective time intervals, during with lighting with respective lighting properties are used whose difference affects radar reflection. When a plurality of radar reflection members is used, each may be detected individually by comparing radar reflections under lighting conditions where only that radar reflection member is affected and lighting conditions where none of the radar reflection members is affected. But individual activatable radar reflection members can also be detected by comparing radar reflections obtained under respective ones a set of lighting conditions that give rise to activation of respective different

combinations of radar reflection members, or at least different combinations of activation strength. Instead of using detection of individual activatable radar reflection members to set or clear bits in a code, detection of differences between radar reflections under light with pairs of different lighting properties may be used directly to set or clear bits. It is not necessary to decide on individual activatable radar reflection members to do so. Preferably the size and or shape of radar reflection members is designed to provide that their radar resonance frequency is below or at the radar radiation frequency. Stripes of at least a quarter radar wavelength, or a half wavelength may be used for example. Although an embodiment has been described wherein each radar reflection members if formed y an integral stripe, it should be appreciated that instead a radar reflection member may comprise a set of conductor patches coupled by material that becomes more conductive when irradiated by light. In this case conductor patches may be used that they have a resonance frequency above the radar frequency, the resonance frequency of the radar reflection member being lowered to the radar frequency or below when the patches are conductively coupled under the influence of light.

Although an embodiment has been described wherein the photosensitive material of the patches directly becomes more conductive under the influence of light, it should be appreciated that alternatively a combination of a first material that becomes more conductive under an applied voltage and a second material that produces the voltage under the influence of light may be used. Although an embodiment has been shown wherein the entire radar reflector member is exposed to light (or covered by a filter), it should be appreciated that instead it may suffice to expose or cover part of the radar reflector member. This may be sufficient to produce measurable light dependent reflection differences.

The light source may comprise a broadband light source and an adjustable color filter, the color filter being adjusted to change the wavelength range of the irradiating light towards the coding module 2. A set of lighting devices, such as LEDs, with mutually different output color range may be used. In this case, time dependent control of the lighting devices may be used to generate light with time dependent spectral content. Although an embodiment using narrowband light, activating or leaving deactivated one radar reflection member at a time has been shown, it should be appreciated that alternatively has been shown, it should be appreciated that alternatively wider band light may be used that activates, or leaves deactivated groups of radar reflection members may be used.

Although the use of FMCW is not necessary, it may be used to distinguish different coding modules 2 at different distances. In an embodiment, the light induced reflection differences detected for corresponding distances are used selectively to detect activatable radar reflection members. The distance may be selected by FMCW or simply using pulse delay time directly. Similarly a radar image may used (which need not be displayed) to detect light induced reflection differences detected for corresponding directions of transmission and/or reception from the radar antenna may be used selectively to detect activatable radar reflection members. In an embodiment a radar difference image is formed from differences to between measured radar reflection for corresponding directions of radar transmission and/or reception with and without lighting. A plurality of such images may be formed, obtained with and without light of respective wavelengths.

Instead of using a radar that forms radar reflection images as a function of radar direction, an image of radar reflections from different coding modules within a broad radar beam may be formed by using a narrow beam or plane of light, and scanning the direction of this beam or plane within the radar beam. In this case the time dependence of the radar reflections during the scan of the direction of light beam of plane can be used to provide one or more dimensions of the radar image, instead of scanning the radar beam. Reflections obtained at different time points during such a scan may be attributed to different coding modules and used distinguishing code bits from different coding modules.

Respective bits of a multi-bit code may be derived from the differences between measured reference reflections when the light source is off and respective reflection measurements when the light source is on at respective

wavelengths. Each difference may be used to set a respective bit of the code. Optionally the resulting code may be translated to a second code, for example by means of error correcting decoding, or look-up from a table. However it is not strictly necessary that differences between radar reflection measurements with and without light are used, or radar reflection

measurements with single wavelength light. For example, differences between measured radar reflections when the light source is on at successive pairs of wavelengths may be used to determine successive up down transitions. If underlying bits represented by reflectors with different color filters are 10011 for example, deltas obtained for the differences between measurements at successive colors would be -1, 0, 1, 0. The underlying bits could be recovered from such deltas by bit integration, or the delta may be used to address a look up table that stores such underlying bits. The deltas may also be used to look up any code. Similarly, when radar reflectors are used that are covered with filters that are covered with filters that pass different combinations of plural wavelengths, deltas between differences between measured radar reflections when the light source is on at successive pairs of wavelengths may be used as a code.

As another example, if a coding module contains radar reflectors with red, green and blue filters, a first difference between radar reflection

measurements with red+green light and without light, a second difference between radar reflection measurements with green+blue light and without light and a third difference between radar reflection measurements with green light and without light may be used. If these measurements produce results 1, 0, 1 respectively then it may be determined that the red and green filter covered radar reflectors are active, but not the green covered radar reflector. With results like 0 1 for the first and second difference, the third difference would be superfluous. But also in this case the differences may be used directly as a code, without translation back to individual radar reflectors.

Any differences between measurements obtained at different lighting conditions such as different wavelength lighting or lighting with more and less intense light may be used to derive a code, which does not need to represent the activity of individual radar reflectors with individual bits.

A interrogation unit may be provided with a radar transceiver, a light source and a processing and control circuit. The radar transceiver may comprise an antenna or antennas, a radar signal generator and a receiver. The processing and control circuit may be coupled to the radar transceiver. The processing and control circuit may be programmed to obtain radar measurements with the radar transceiver at different time points or in different time intervals. The processing and control circuit may be coupled to light source. The processing and control circuit may be programmed to control the light source to apply light with mutually different lighting properties at the different time points or in the different time intervals. The processing and control circuit may be programmed to compare radar reflections derived from reflected radar signals at the different time points or in the different time intervals to detect light activatable radar reflection members for which the difference between the lighting properties affects radar reflection. The processing and control circuit may be programmed to set respective bits of a read out code according to whether reflection from a respective activatable radar reflection member, or group of respective radar reflection members, is detected to be affected by the difference between the lighting properties. The read out code may

subsequently be used to compute a result code. Although a programmed processor with a program memory such as a semi-conductor memory or a disk is preferably used, it should be appreciated that alternatively a specially designed circuit may be used to perform part or all of these steps. As used herein "configured to" will be used to refer to both these implementations.

Light controllable radar reflectors are not limited to radar reflectors with light controllable conductivity. Figure 4 illustrates that the reflection of an electromagnetic wave to a surface between media is determined by the wave impedance of the material in the media (z ₀ and zi). In the figure Ei indicates the electric field vector of incident light, E _r that of reflected light. The reflection coefficient Gamma of the interface between two media follows from a ratio between the difference of the impedances and the sum of the impedances:

7 - 7

Because the wave impedance depends from the permittivity, permeability and conductance a reflector is realized with a material if at least one of these quantities differs from that of air. A photosensitive reflector is realized when the wave impedance is light sensitive, i.e. if any one or a combination of permittivity, permeability and conductance is light dependent in a way that changes the wave impedance.

In figure 5 it can be seen how the reflection changes as a function of the characteristic (wave) impedance Zi of a reflector

It is known that of certain semiconductive materials like cadmium sulphide and cadmium selenide the conductivity depends on the quantity of incident light. Besides, it is known that undoped semiconductive materials like silicium, germanium gallium arsenide have this property. These materials are suitable to be used as a coating for optically controlled radar transponders as proposed in the present patent application.

According to one aspect a method for controllable reflecting radar signals via one or more reflection members (1), is provided that comprises the steps of: - providing that said one or more reflection members have reflective characteristics for said radar signals which are different when irradiated by a light source (4) or when not respectively;

providing that said one or more reflection members are irradiated by at least one radar signal (3); providing that at least part of said one or more reflection members are irradiated by a light source during at least part of the time they are irradiated by said at least one radar signal;

receiving and processing any radar reflection signal or signals, i.e. the portion or portions of the at least one radar signal reflected by said one or more reflection members and/or by any further reflective objects, during the time that said at least part of said one or more reflection members are irradiated by said light source, and any radar reflection signal or signals during the time that said at least part of said one or more reflection members are not irradiated by said light source.

An embodiment uses one or more reflection members having reflective characteristics for said radar signals which are different when irradiated by a light source having a predetermined wavelength or wavelength range or when not respectively. In an embodiment coding means (2) are provided including two or more reflection members having similar reflective characteristics for said radar signals when irradiated by a light source or when not respectively, the reflection members, however, having those similar reflective

characteristics for said radar signals when irradiated by a light source or when not respectively, for mutually different wavelengths (ωι.,.ιι) or wavelength ranges of the light.

In one aspect a system for controllable reflecting radar signals via one or more reflection members (1) is provided, wherein said one or more reflection members have reflective characteristics for said radar signals which are different when irradiated by light or when not respectively, the system, moreover, including radar means (3), providing that said one or more reflection members are irradiated by at least one radar signal, and including light emission means (4, 5), providing that at least part of said one or more reflection members are irradiated by a light source (4) during at least part of the time they are irradiated by said at least one radar signal; at least part of said radar means being arranged for receiving and processing any radar reflection signal or signals, i.e. the portion or portions of the at least one radar signal reflected by said one or more reflection members and/or by any further reflective objects, during the time that said at least part of said one or more reflection members are irradiated by said light source, and any radar reflection signal or signals during the time that said at least part of said one or more reflection members are not irradiated by said light source. In an embodiment of the system one or more reflection members are arranged to have reflective characteristics for said radar signals which are different when irradiated by a light source having a predetermined wavelength or wavelength range or when not respectively. In an embodiment the system comprises coding means (2) which include two or more reflection members (1) having similar reflective characteristics for said radar signals when irradiated by a light source or when not respectively, the reflection members, however, having those similar reflective characteristics for said radar signals when irradiated by a light source or when not respectively, for mutually different wavelengths (ωι.,.ιι) or wavelength ranges of the light.

In another embodiment light respective light controllable radar reflection members may be used in the coding module that are resonant at mutually different radar wavelengths (e.g. stripes of mutually different length). In this case the presence different active radar reflection members may be detected by comparing radar reflections at respective radar wavelengths with and without lighting without requiring light at different colors, or color filters. A

combination of color filters and radar reflection members that are resonant at mutually different radar wavelengths may be used to increase the information capacity of the coding module.