FIELD OF THE INVENTION

The invention relates to a device for the detection of plaque on teeth.

BACKGROUND OF THE INVENTION US 6,186,780 Bl discloses a method and a device for the recognition of caries by directing an excitation radiation at a tooth. Returning fluorescence radiation is detected and evaluated at wavelengths larger than 800 nm to allow for a detection of carious tooth regions. SUMMARY OF THE INVENTION

It would be desirable to have a device which allows for an improved cleaning of teeth, particularly an improved cleaning from plaque, by detecting if plaque is present on the teeth.

This object is achieved by a device according to claim 1. Preferred embodiments are disclosed in the dependent claims.

According to the invention, a device for the detection of plaque on teeth is provided, which comprises the following components:

Light emission means for irradiating a tooth with excitation light at a first wavelength and a second wavelength different from the first wavelength. The irradiated light will be called "excitation light" in the following.

Light detection means for detecting fluorescence light coming from the tooth in response to the excitation light and generating a detection signal representing at least a part of the fluorescence spectrum of the tooth. On the basis of the fact that during operation, the tooth is irradiated with excitation light at a first wavelength and a second wavelength, a first detection signal and a second detection signal are obtained by means of the light detection means, wherein the first detection signal represents fluorescence light from the tooth that was excited by the excitation light at the first wavelength, and wherein the second detection signal represents fluorescence light from the tooth that was excited by the excitation light at the second wavelength. Evaluation and control means for controlling the light emission means, and for assessing whether plaque is present on the tooth by making a comparison between a first detection signal associated with the first excitation wavelength and a second detection signal associated with the second excitation wavelength, and evaluating whether the mutual relation of the signals following from the comparison corresponds to a mutual relation associated with the absence of plaque. The device according to the invention may be adapted to determine the mutual relation of the signals associated with the absence of plaque on the basis of the signals. According to another option existing within the framework of the invention, the mutual relation as mentioned may be predetermined, for example, on the basis of experiments, wherein it is practical for the evaluation and control means to be equipped with a memory in which the predetermined information is stored. In respect of the term "corresponds to", it is noted that this should be understood for its practical meaning, implying that the mutual relations are deemed to correspond to each other in case they deviate from each other to only a very small extent which does not exceed a certain (predetermined) threshold.

The "light emission means" may comprise any element by which light can controllably be generated and emitted such that it can reach the surface of teeth to be cleaned. The light emission means may for example comprise LEDs and/or lasers incorporated into the device. It may typically comprise or be supplemented by additional elements such as lenses, filters, light guides or the like.

In the emission of the light emission means, excitation light having the first wavelength shall be distinguishable from excitation light of the second wavelength. This may for example be achieved by modulating excitation light of the first wavelength differently from excitation light of the second wavelength.

It should be noted that in practice, the two wavelengths of the excitation light are actually two spectra of small bandwidth, characterized by a certain wavelength which may for example be the wavelength at the center of the corresponding bandwidth. Preferably, the light emission means are adapted to emit practically monochromatic light. In general, the first wavelength and/or the second wavelength may be quite arbitrary, wherein it is preferred to avoid wavelengths from the UV spectrum in view of the application of the device in the presence of living beings.

It should furthermore be noted that the irradiation with a first and a second wavelength and the corresponding generation of a first detection signal and a second detection signal are only minimum requirements. They can readily be extended to the application of more than two different excitations and the associated generation of more than two detection signals to be evaluated with respect to each other. Hence, the invention is in no way limited to the application of excitation light at only two different wavelengths and the associated generation of two detection signals, and the statement that the light emission means serve for irradiating a tooth with excitation light at a first wavelength and a second wavelength should be understood such as to leave the option open of having excitation light at (a) further wavelength(s).

The "light detection means" may comprise any device that allows for the generation of detection signals which indicate the presence and/or amount of fluorescence light coming from the teeth, optionally in a spectrally resolved way. Such device may for example comprise a photodiode, a photocell or any other light sensitive element.

Furthermore, the light sensitive element can be a single sensor, or a 1 D or 2D array.

The "fluorescence spectrum" of the tooth comprises values of a fluorescence characteristic such as intensity for a range of wavelengths of the fluorescence light coming from the tooth in response to the excitation light.

A "detection signal" may in general be any kind of signal, preferably an electrical signal such as a voltage. Moreover, the detection signal may encode its information in any appropriate way, for example as analogue or digital values. According to the invention, the information represents the whole measured spectrum or a part thereof, for example, by indicating an intensity value, or characteristic parameters of the spectrum such as a peak wavelength.

The "evaluation and control means" may for example be realized by dedicated electronic hardware, by digital data processing hardware with associated software, or by a mixture of both. The detection of plaque on teeth may be qualitatively or quantitatively. The result of this evaluation may be further processed in any appropriate way. The user may for example be provided with a corresponding feedback signal.

When the device according to the invention is applied, a method comprising the following steps is carried out:

Irradiating a tooth with excitation light at a first wavelength and a second wavelength.

Generating a first detection signal and a second detection signal corresponding to characteristics of the fluorescence light from the tooth that was excited by the first and the second wavelength of excitation light, respectively. Detecting the presence of plaque on the tooth by comparing the first detection signal and the second detection signal, and evaluating whether the mutual relation of the signals following from the comparison corresponds to a mutual relation associated with the absence of plaque.

In short, the invention relates to the detection of plaque by the evaluation of detection signals generated with different wavelengths of excitation light. The device according to the invention has the advantage that it allows for the detection of plaque with a high reliability. This is achieved by evaluating fluorescence light excited with two different excitation wavelengths. The fact is that the wavelengths may be chosen such that plaque on the one hand side and tooth material on the other hand side turn out to react differently to the wavelengths, so that their relative contribution to the first and the second detection signal will be different, too. This allows for a more reliable discrimination between fluorescence components originating from plaque and those originating from other components that are of no interest.

In the following, various preferred embodiments will be described that can be realized in connection with the device according to the invention described above.

In a first preferred embodiment, the light detection means are adapted to generate a detection signal associated with two parts of the fluorescence spectrum of the tooth, in particular two parts of which a wavelength difference is substantially the same as a difference between the first wavelength and the second wavelength of the excitation light multiplied by a predetermined factor, and wherein one of the two parts of the fluorescence spectrum of the tooth is a part associated with minimum plaque fluorescence. In this embodiment, the other of the two parts of the fluorescence spectrum of the tooth may be a part associated with maximum plaque fluorescence. In the following, for the sake of clarity, the part of the fluorescence spectrum of the tooth which is associated with minimum plaque fluorescence will be referred to as reference spectrum part, whereas the other part of the fluorescence spectrum of the tooth will be referred to as plaque spectrum part.

By adapting the mutual relation of the wavelengths of the two parts of the fluorescence spectrum of the tooth to the mutual relation of the excitation wavelengths, it is possible to estimate the contribution of plaque to the first detection signal by using the second detection signal in the interpretation of the first detection signal. This approach takes into account the fact that fluorescence light returning from a tooth typically comprises components of different origin, particularly a component that originates from plaque (if present) and components that originate from the actual tooth material. Hence, the fluorescence signals of plaque and tooth material superpose and cannot readily be separated. However, by taking the second detection signal into account, which was obtained with light of different excitation wavelength, it becomes possible to distinguish between these contributions. An actual method for determining these contributions will be described in more detail below.

In the interpretation of the detection signals, it is assumed that the second detection signal comprises substantially the same contribution from tooth material as the first detection signal. Particular procedures how this condition can be achieved will be explained in more detail below. It should be noted that boundary conditions during the generation of the first detection signal and the second detection signal are assumed to be substantially the same (besides the usage of different excitation wavelengths). For example, it is assumed that the excitation light of the first wavelength is irradiated onto the tooth with substantially the same total intensity as the excitation light of the second wavelength, or that operational parameters of detection circuits (e.g. the gain) are substantially the same during the generation of the first and the second detection signals, respectively.

Preferably, the above two conditions are simultaneously realized, i.e. (i) a part of the fluorescence spectrum where there is substantially no contribution from plaque (the reference spectrum part) is taken into account, and (ii) the second detection signal comprises substantially the same contribution from tooth material as the first detection signal. In this case, the reference spectrum part from the second detection signal immediately indicates the amount of fluorescence light from the tooth material that is contained in the plaque spectrum part of the first detection signal, thus allowing inferring the actual amount of fluorescence light originating from plaque as the difference between the plaque spectrum part of the first detection signal and the reference spectrum part of the second detection signal. Actually, a comparison is made in order to determine whether plaque is present, or not, as a difference is only found when plaque is present, indeed.

Within the framework of the invention, it is possible to modulate excitation light of the first wavelength and/or excitation light of the second wavelength (in its temporal course). The modulation may for example consist of an on/off switching with a given regular or irregular temporal pattern. Additionally or alternatively, the modulation may be continuous, for example with a sinusoidal amplitude at a given frequency. The temporal modulation of the excitation light provides the light with a characteristic fingerprint that can be exploited in the detection circuit. According to a first further development of the aforementioned approach, the first detection signal and/or the second detection signal is demodulated taking the modulation of the excitation light into account. Lock-in amplification may typically be used for this purpose in order to separate the components of detection signals one is interested in from disturbances and background.

According to a second further development of the above approach, excitation light of the first wavelength is modulated with a different temporal modulation than excitation light of the second wavelength. Thus, both kinds of excitation light are provided with different fingerprints allowing for distinguishing them on the detection side.

A particularly simple different temporal modulation that is useful in the aforementioned case is achieved if irradiation with the first wavelength is alternated with irradiation with the second wavelength. Fluorescence light detected during the irradiation with the first wavelength can thus uniquely be attributed to said first wavelength while fluorescence light detected during the irradiation with the second wavelength can uniquely be attributed to said second wavelength.

As mentioned above, in the first preferred embodiment of the device according to the invention, the first detection signal is used for obtaining information related to the fluorescence intensity at the plaque spectrum part of the fluorescence spectrum of the tooth. The second detection signal is used for obtaining information related to the fluorescence intensity at the reference spectrum part of the fluorescence spectrum of the tooth. In practice, the mean intensity in the plaque spectrum part, which is a first interval of the total fluorescence spectrum, may be determined. Similarly, the mean intensity in the reference spectrum part, which is a second interval of the total fluorescence spectrum, may be determined. In order for the light detection means to generate a detection signal associated with the two parts of the fluorescence spectrum of the tooth, bandpass filters may be applied in the path followed by the light coming from the tooth. Advantageously, the widths of the spectrum parts are comparatively small, preferably less than about 50% of the difference between the excitation wavelengths.

The wavelength difference between the spectrum parts may be determined by taking into account the wavelengths at the center of the respective parts. As mentioned above, the wavelength difference between the spectrum parts is preferably substantially the same as the difference between the first excitation wavelength and the second excitation wavelength multiplied by a predetermined factor QR. This means that the spectrum parts at which detection takes place are shifted with respect to each other by an amount A e _m that is proportional to the chosen distance Δλ^ between the excitation wavelengths:

A e _m = CXR.( <; _Χ 2 - λβχΐ) = OR. Δλ¾ _χ . Such an approach turns out to be particularly valuable due to a feature of the tooth material called "Red-Edge-Excitation-Shift" (REES), which refers to a shift of the fluorescence emission spectrum to longer wavelengths in parallel to a shift of the excitation wavelengths (if the latter are part of the longer wavelengths of the excitation spectrum). By exciting a tooth with a second excitation wavelength that is shifted with respect to a first excitation wavelength, a corresponding shift in the fluorescence response of tooth material can hence be achieved; observing this response in a correspondingly shifted detection interval then implies that substantially the same amount of fluorescence from tooth material is measured. If the wavelength of the reference spectrum part is chosen such that substantially no fluorescence emission of plaque is associated with that spectrum part, this approach allows for the separate and independent detection of fluorescence from tooth material that contributes to the plaque spectrum part of the first detection signal. Thus, the above-described isolation of the plaque components of interest from the first detection signal can be achieved.

The aforementioned factor o¾ is preferably larger than 0 and smaller than or equal to 1 (i.e. 0 < ο¾ < 1). A particularly preferred value is about 0.68. Suitable values can be derived from experiments and typically depend on the bandwidth and wavelength of the applied excitation spectrum.

In the above-described first preferred embodiment of the device according to the invention, the evaluation and control means are adapted to process the detection signal in order to obtain information related to the fluorescence intensity at the plaque spectrum part and the reference spectrum part, and to make a comparison between a fluorescence intensity associated with both the first excitation wavelength and the plaque spectrum part and a fluorescence intensity associated with both the second excitation wavelength and the reference spectrum part, and to determine that plaque is present if it is found that the fluorescence intensities are significantly different. Preferably, the evaluation and control means are furthermore adapted to calibrate the detection signals for a difference of a first gain associated with the first excitation wavelength and a second gain associated with the second excitation wavelength. This takes into account that the emission circuits, the detection circuits, and/or the materials themselves may have different properties at the different wavelengths. The gain may be predetermined, but it is also possible to determine the gain on the basis of a comparison of the reference spectrum parts of the first detection signal and the second detection signal, respectively. In particular, the evaluation and control means may be adapted to determine a proportion of the first gain and the second gain by determining a proportion of the fluorescence intensity associated with both the first excitation wavelength and the reference spectrum part and the fluorescence intensity associated with both the second excitation wavelength and the reference spectrum part.

In a second preferred embodiment of the device according to the invention, the evaluation and control means are adapted to process the detection signal in order to determine a characteristic wavelength of the fluorescence spectrum of the tooth. Information about a characteristic wavelength of a first emission spectrum of fluorescence light associated with the first excitation wavelength can be derived from the first detection signal, and information about a characteristic wavelength of a second emission spectrum of fluorescence

light associated with the second excitation wavelength can be derived from the second detection signal. The difference of the characteristic wavelengths of the two spectra can be used for determining whether plaque is present, or not, namely by assessing whether the difference is in conformity with a difference that may be expected if plaque is absent, or not. In practice, this approach involves a predetermination of the mutual relation of the characteristic wavelengths of the spectra for tooth material only.

In a first example of the aforementioned approach, the "characteristic wavelength" of the associated spectrum of fluorescence light may be the wavelength of a peak of said spectrum. Usually, the fluorescence spectrum has a single dominant peak only, in which case this definition of the characteristic wavelength is unique.

In a second example of the above approach, the "characteristic wavelength" of the associated spectrum of fluorescence light may be the center of gravity wavelength of said spectrum.

In a third example of the above approach, the "characteristic wavelength" of the associated spectrum of fluorescence light may be the wavelength of a difference between a long wavelength part and a short wavelength part of said spectrum. Preferably, a normalized value of this difference is used.

In order to realize plaque detection, the evaluation and control means are adapted to make a comparison between a characteristic wavelength of the fluorescence spectrum of the tooth associated with the first excitation wavelength and a characteristic wavelength of the fluorescence spectrum of the tooth associated with the second excitation wavelength. It is concluded that plaque is present if it is found that the mutual relation of the characteristic wavelengths is significantly different from a predetermined reference mutual relation associated with the absence of plaque. This predetermined reference mutual relation is determined by the shift of fluorescence spectrum of tooth material which is obtained by suitable excitation wavelengths and is disturbed when plaque is present, as the fluorescence spectrum of plaque practically does not shift with the excitation wavelengths.

The second preferred embodiment of the device according to the invention can exploit shifts of a spectrum that may for example occur in case of REES. To this end, the already mentioned shift factor ( R that relates a difference Δλεχ in excitation wavelengths to a difference A e _m in the characteristic emission wavelengths Xemi and of the first and second spectrum of fluorescence light, respectively, according to = (XR Δλε _Χ should be known in advance, for example from prior experiments.

A typical procedure for detecting the presence of plaque on teeth may then comprise the following steps:

1. Determining the location of the emission spectrum at an excitation wavelength, preferably an excitation wavelength where minimum plaque fluorescence can be expected. This excitation wavelength will be called "second excitation wavelength" in the following (for consistency with other embodiments of the invention). Moreover, the "location of a spectrum" is preferably defined via the above mentioned "characteristic wavelength", for example as the location of the peak of the emission spectrum.

2. Predicting the location of the emission spectrum (of tooth material without plaque) at another excitation wavelength, called "first excitation wavelength" λβ _χ1 . Preferably, this first excitation wavelength Ae _X i is chosen such that maximum plaque fluorescence is excited. The prediction may be based on the above mentioned predetermined excitation shift value (XR. For the predicted location of the spectrum the following formula can for instance be applied:

3. Measuring the actual location of the emission spectrum at said first excitation wavelength. This yields for example a value λ _6Π1 ι . 4. Determining the position difference (Ae _mliPIB d - ^ _ml ) in both said emission spectra as a measure for the amount of plaque present.

The described methods will typically be realized with the help of a computing device, e.g. a microprocessor or an FPGA in the evaluation and control means of the device. Accordingly, the invention further includes a computer program product which provides the functionality of any of the methods according to the invention when executed on a computing device.

Further, the invention includes a data carrier, for example a floppy disk, a hard disk, an EPROM, a compact disc (CD-ROM), a digital versatile disc (DVD), or a USB stick which stores the computer product in a machine readable form and which executes at least one of the methods of the invention when the program stored on the data carrier is executed on a computing device. The data carrier may particularly be suited for storing the program of the computing device mentioned in the previous paragraph.

Nowadays, such software is often offered on the Internet or a company Intranet for download. Hence, the invention also includes transmitting the computer product according to the invention over a local or wide area network.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-described and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 schematically shows components of a device according to a first embodiment of the invention;

Fig. 2 schematically illustrates excitation and fluorescence spectra involved in an embodiment of the present invention;

Fig. 3 schematically shows components of a device according to a second embodiment of the invention in which irradiation and fluorescence light paths partially overlap;

Fig. 4 shows measured emission spectra of tooth material (lingual side) for different excitation wavelengths;

Fig. 5 shows the dependence of the peak wavelengths of the spectra of Figure 4 on the excitation wavelength;

Fig. 6 shows measured REES shifts of extracted human teeth (peak-intensity based); Fig. 7 illustrates the dependence of fluorescence emission spectra of plaque (left) and tooth material (right) on the excitation wavelength.

Like reference numbers or numbers differing by integer multiples of 100 refer in the Figures to identical or similar components.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, preferred embodiments of the invention will be described that relate to oral healthcare, in particular a technology to support the hygiene and health of teeth.

The goal is to help users to clean their teeth, for example by informing users if they are indeed removing plaque from their teeth by means of a cleaning action as intended, and if they have fully removed the plaque, providing both reassurance and coaching them into good habits. Preferably, the information should be provided in real time during a cleaning action through suitable indicating means. For example, it will be useful if a toothbrush gives users a signal when the position at which they are brushing is clean, so they can move to the next tooth. This may reduce their brushing time, but will also lead to a better, more conscious brushing routine. A particular goal is to be able to detect plaque within an electric toothbrush, i.e. in a vibrating brush system surrounded with toothpaste foam. The detection system should provide contrast between a surface with the removable plaque layers, and a cleaner pellicle/calculus/dental filling/tooth surface.

Existing electrical toothbrushes do not detect the absence or presence of plaque. The present invention provides a way to detect plaque in real-time during the brushing routine. In a preferred embodiment, the idea is to implement plaque detection based on the autofluorescence properties of plaque, while reducing the influence of the background (enamel/dentin) fluorescence by using the specific fluorescence properties of this background fluorescence. An insight on which this is based is that excitation of the tooth site with two or more different wavelengths on the red edge side of the excitation spectrum (comprising wavelengths larger than 400 nm) enables distinction between plaque and tooth fluorescence based on the different spectral shift properties of both fluorescent types. Specifically: the tooth fluorescence will shift with excitation, while the plaque fluorescence will not, or to a considerably lesser extent. The shifting phenomenon is known as Red-Edge-Excitation-Shift (REES) and will be explained in more detail below.

Figure 1 schematically shows components of a device 100 according to a first embodiment of the above principles. In the following, it is assumed that the device 100 is a toothbrush which is primarily intended to be used for subjecting teeth to a cleaning action. According to the invention, the toothbrush 100 is furthermore adapted to carry out a process of plaque detection during a brushing action or prior to/following a brushing action.

Preferably, parameters of the cleaning process such as a mechanical cleaning intensity, the delivery of a cleaning agent or the like are automatically controlled/adapted in the toothbrush on the basis of the results of the plaque detection.

For the sake of completeness, it is noted that the term "toothbrush" should be understood such as to denote an implement for manually or electrically cleaning teeth (of a human user or of an animal). A toothbrush typically comprises a handle that is connected via a neck to a head that carries means for cleaning the teeth, for example a brush with bristles. An electrical toothbrush may additionally comprise elements such as a battery and a motor for moving the brush. Furthermore, it is noted that the application of the invention is not restricted to toothbrushes. A separate device for detecting plaque may be provided, for example, or the invention may be applied in another type of device for cleaning teeth, such as a water flosser.

In particular, the toothbrush 100 comprises the following components:

At least two different light sources 110 for generating excitation light L _ex of a first wavelength and a different second wavelength. The light sources 110 are preferably blue LEDs, but other sources (e.g. diode laser) are also possible.

- Optional cleanup filters 111 that are disposed in front of the light sources 110 and comprise for example a narrow bandpass filter which blocks any undesired wavelength from reaching the teeth (e.g. UV light) or the detector. The bandpass filters can be a single passband, multiple passband, or multiple single passband filters.

Optics 141 for directing excitation light L _ex generated by the

LEDs 110 onto the surface of a tooth T and for collecting fluorescence light La that is emitted in return from the tooth T and possibly from plaque P on the tooth T. The optics may for example comprise lenses, optical fibers and the like that are not shown in detail here.

A light detector 120 comprising two detector units, for example photodiodes with amplifiers. The detector may further optionally comprise focusing optics like lenses, CPCs (Compound parabolic concentrators) or both.

Spectral filtering 121 for filtering the fluorescence light Lg before it reaches the detector 120, for example two or more bandpass filters. The bandpass filters can be a single passband or multiple passband. Additionally or alternatively, spectral filtering 121 can be implemented using a wavelength dispersive element (e.g. grating or prism) followed by (optional) slit(s).

An evaluation and control unit 130 for evaluating the detector signals y _ls y2 and for controlling the light sources 110.

In general, plaque on teeth can be measured by exciting its fluorescence with appropriate excitation light and measuring the fluorescence response. A problem with this approach is however one of contrast: namely, distinguishing a weak plaque fluorescence signal in the presence of large background signals originating from dentine/enamel fluorescence. To make things worse, the emission spectra of plaque and teeth largely overlap.

It is therefore desirable to overcome the aforementioned problems and to provide a method to determine the amount of plaque on teeth by increasing the contrast between the measured fluorescence signals of teeth and plaque. According to the

embodiment proposed here, such an increase in contrast is achieved by using the REES fluorescence properties of dental hard tissue.

REES refers to a phenomenon according to which fluorescence of some fluorophores embedded in rigid media does not conform to classical rules. The fluorescence of such fluorophores can depend on the excitation wavelength. In particular, one of these properties is a shift in emission spectrum to longer wavelengths with increasing excitation wavelength. These shifts only occur on excitation from the longer wavelengths of the excitation spectrum (i.e. the red-edge). Hence, this phenomenon is called "Red-Edge- Excitation- Shift" (REES).

Dental hard tissue turns out to possess strong REES fluorescence properties, whereas plaque does not (or only to a negligible extent). Experiments show that the emission spectrum of the tooth material shifts with excitation wavelength, while that of plaque (having typically a peak at 635 nm) does not. This difference between the two types of fluorescence is exploited in the present embodiment of the invention. The approach is based on multiple wavelength excitations and comprises the following elements:

Two or more wavelength excitation means.

One or more wavelength emission detection means.

A preferred method to use the REES effect in a toothbrush 100 will now be described with reference to Figures 1 and 2, wherein it is noted that the curves shown in Figure 2 are only depicted for reasons of illustration of the principles of the method and do not represent actual emission curves. A first one of the band-pass filters 121 (e.g. 20 nm wide) around the plaque spectral peaks at e.g. mA = 510 nm, 635 nm or 700 nm is used to measure the intensity as function of excitation wavelength in a first band A (cf. Figure 2). Excitation is done at two excitation wavelengths λ _εχ ι , - . A second one of the band-pass filters 121 is located Δλ^ away from the first one at a position known to have minimum plaque fluorescence, measuring fluorescence in a second band B (cf. Figure 2). Δλ^ can be positive or negative, i.e. it is arbitrary if λ _βχ ι is chosen to be the longer or the shorter one of the excitation wavelengths.

Next, intensities in both pass-bands A, B are measured at the two different excitation wavelengths λβ _χ1 , λ _βχ2 being a distance

apart. The measured fluorescence intensity in the first passband A (detection signal yi) is the sum of plaque fluorescence Sp, _ex i and dental hard tissue fluorescence Si, _ex i . In the second passband B, the measured fluorescence intensity (detection signal y ₂ ) is mainly fluorescence ST, _eX 2 from the tooth.

One can determine the dental fluorescence part in the first passband A in the following way:

Itot,exl ⁼ Ιτ,εχΐ + Ip,exl where It ₀ t,exi is the total intensity measured in the first passband A, Ιτ,βχΐ is the fraction of Itot,exi originating from the tooth, and Ip, _ex i the fraction originating from the plaque fluorescence (at the first excitation wavelength λβχΐ).

The part Ii, _ex i can be determined from the second excitation measurement with the second excitation wavelength ^ _x2 . If one chooses the second excitation wavelength λ _εχ2 a distance Δλ^ away from the first, then the tooth intensity value measured in the first passband A at the first excitation λ^ι will shift to the second passband B. This shift is caused by the REES effect of the dental hard tissue.

Preferably, a gain compensation is added because the absorption and/or efficiency at both excitation wavelengths _{«χ1 |} will typically not be the same.

Figure 2 schematically illustrates the various spectra and wavelengths that are involved in the realization of the above procedures. The diagram depicts on its horizontal axis the wavelength λ and on its vertical axis the corresponding intensity of the spectra (arbitrary units). The values λβ _χ2 indicate a first excitation wavelength and the second excitation wavelength, respectively, that may for example be emitted by monochromatic LEDs of the light sources 110. The two excitation wavelengths are separated by an interval

When irradiated with the first or the second excitation wavelength plaque responds with more or less the same fluorescence spectrum Sp, _ex i = Sp, _eX 2.

Tooth material responds to an excitation by the first excitation wavelength βχ _ΐ with a fluorescence spectrum S-r,exi that overlaps the plaque spectrum. The total spectrum that can be measured in a first passband A at a first fluorescence wavelength λ _βηΐι Α is therefore a superposition of these two spectra,

Stot,exl ⁼ δτ,εχΐ + Sp, _ex l .

Hence the contribution of plaque, Ip, _ex i is not immediately observable. This problem can however be solved by the fact that the fluorescence spectrum of the tooth material is shifted by the amount Δλ^ when said material is excited with the second excitation wavelength λ _6Χ 2 resulting in the shifted spectrum Sp, _eX 2, while the fluorescence spectrum of the plaque remains more or less at the same position.

If, during excitation with the second excitation wavelength λβ _χ2 , detection is done in a second passband B at a second fluorescence wavelength λ _6Π1 Β that is the difference AAem apart from the first fluorescence wavelength the same amount I _P , _ex i = Ip, _eX 2 of fluorescence from tooth material will contribute to the signal. As no fluorescence from plaque is present in this second passband B, the observed amount is completely determined by the tooth material.

Accordingly, the unknown value from Sp, _exl is known from which the part originating from plaque in the first measurement can readily be estimated by subtraction.

Figure 3 illustrates a second embodiment of a toothbrush 200 comprising the following components:

At least two light sources 210.

Optional cleanup filters 211.

Optics 243 and 241 for shaping and directing excitation light L _ex .

A (dichroic) beam splitter 242.

A light detector 220 comprising two detector units.

Two bandpass filters 221. An evaluation and control unit 230.

As far as these parts are similar to those of the first toothbrush 100, reference is made to the above description for more details on this embodiment. A difference to the first toothbrush is that the second toothbrush 200 uses a combined optical path for part of the excitation and fluorescence light.

A further difference is the use of lock-in amplifiers 231 to increase SNR (e.g. rejecting brush- head movement artifacts). The lock- in uses a modulation of the light sources 210 by oscillators 212 and enables to use the different excitation wavelengths at the same time, by modulating them on different frequencies.

The lock-in embodiment can of course also be applied to the first toothbrush 100. Moreover, any lock- in embodiment can be done in the analogue and or digital domain.

In the following, an alternative approach for the evaluation of detection signals yi , y ₂ will be described with respect to Figures 4-7. This approach is based on the prediction of a "location" of the emission spectrum.

From experiments it is known that for a certain excitation (wavelengths range and bandwidth) the emission shift as function of the excitation wavelength is a constant value that can for example be described via a slope (

Figure 4 shows for example the emission spectra of an extracted human tooth at six different excitation wavelengths e _X from 405 nm to 455 nm. Also indicated are the "locations" of the emission spectra based on the peak emission value, i.e. the peak wavelengths p _ea k.

In Figure 5, these peak-based spectral locations peak are plotted as function of the excitation wavelength λ^. They show a linear relation between the emission peak and excitation wavelength. From Figures 4 and 5 it is clear that it is possible to predict the location of the emission spectrum of a tooth at a subsequent excitation wavelength after having measured the location at a prior excitation wavelength. Choosing the prior excitation wavelength (here ^ _x2 ) for minimum plaque fluorescence, and the subsequent excitation wavelength (λ^ι) for maximum plaque fluorescence will result in an offset between predicted and measured location of the emission spectrum. This offset is caused by the contribution of the plaque fluorescence to the second emission spectrum. Hence, said contribution can be estimated from the offset.

For the above approach, one needs a-priori knowledge of the REES shift slope . Figure 6 shows the measured shifts for an experiment (10 teeth measured at facial and lingual sides). The absolute location of the emission spectra varies (making the measurement at the prior excitation wavelength necessary), but the slope of the REES shift itself has a fixed value (measured average: c¾ = 0.68 nm _E M/nm _EX ). It should be noted that this shift can be different depending on excitation (e.g. a wider excitation bandwidth towards the blue edge of the excitation spectrum).

Figure 7 illustrates the above approach in two diagrams showing the emission spectra (intensity as function of emission wavelength ^) for plaque (letter "P", left diagram), and tooth material (letter "T", right diagram) in dependence on the excitation wavelength λ _εΧ (vertical direction). It can be seen that the emission peaks of tooth fluorescence shift proportional to the excitation wavelength, wherein the proportionality factor oiR is constant for all teeth (but the whole spectra might be shifted somewhat to the left or right dependent on the individual tooth, cf. Fig. 6). The spectra of plaque show no such REES shift.

The proposed method can be summarized as follows:

1. Determining the location (λ^) of the emission spectrum at a prior excitation wavelength λ _εΧ 2 (where minimum plaque emission occurs).

2. Predicting the location of the emission spectrum at a subsequent excitation wavelength λ _εχ ι (where maximum plaque fluorescence occurs). This determination is based on the predetermined excitation shift value o¾ and on ^ .

3. Measuring the location λ^ι of the real emission spectrum at said second excitation wavelength λ _εχ ι, i.e. of the superposition of plaque and tooth fluorescence.

4. Determining the position difference - mi) in both said emission spectra as a measure for the amount of plaque present.

The proposed procedure has some additional benefits, for example:

- Signals are measured in a wider bandwidth, yielding a better SNR.

- A wider part of plaque spectrum contributes to the total result, yielding a better sensitivity for young plaque.

- The method is not based on intensities and therefore independent of gain

(variations).

The method can be extended to using more (> 2) excitation wavelengths to achieve a more accurate prediction. Extending the number of excitations is also useful to compensate for a change of the expected REES shift due to the presence of calculus, fillings, staining, etc. One may for example use two (or more) excitations to determine the shift slope , and one (or more) to measure the spectral position with maximum plaque contribution. Also, the predetermined shift slope o¾ can optionally be adapted (slow auto- calibration) over a longer period of time (days) based on the fact that the amount of plaque measured should decrease during a brushing cycle. User (brush-head) identification is needed in case multiple users use the same handle.

The above discussion relates to peak-based "locations" of emission spectra.

Alternatively, the center-of-gravity (c.o.g.) wavelength of the emission spectra can be evaluated instead of the peak wavelength, which is similarly modified by the contribution of the plaque emission.

Another embodiment for the location measurement would be to split the emission spectrum into a long and short wavelength part and determine the normalized difference of both, i.e.

λ _ρ08 = (Ir-Ib)/(Ir+Ib) with pos being a measure for the location, Ir the intensity in a "long wavelength part" and lb the intensity in a "short wavelength part". Emission may for example be measured in a wavelength interval ranging from just above highest excitation, e.g. 460 nm (cf. Figures 5, 6), to a wavelength guaranteeing that most plaque emission peaks are included, e.g. 750 nm. Then this interval [460nm, 750nm] may be split at an intermediate wavelength, e.g. at 520 nm, to make "the intensity" lb the integral of photocurrents in the short wavelength part [460nm, 520nm] and Ir likewise in the long wavelength part [520nm, 750nm].

The aforementioned approach can readily be modified, for example by defining more than two wavelength intervals and/or weighting the intervals differently (e.g. with respect to known plaque emission bands).

While the above description referred to the use of discrete excitation wavelengths, a (fast) sweeping wavelength source could also be used.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Summarizing, the invention relates to a device 100, 200 for detecting plaque P on teeth T. According to one option existing within the framework of the invention, a tooth T is irradiated with excitation light L _ex of a first wavelength λ^ι and with excitation light L _ex of a different second wavelength λ _εχ2 , and the associated fluorescence light LA returning from the tooth T is recorded by a light detector 120, 220 as a first detection signal yi and a second detection signal y ₂ , respectively. An evaluation unit 130, 230 detects plaque P on the tooth T by evaluating the first detection signal yi with respect to the second detection signal y ₂ . Following the excitation at two different wavelengths λβ _Χ ι , separated by an interval Δλ _εΧ , the first and second detection signals yi, y ₂ may indicate the fluorescence response LA coming from the tooth T measured in two wavelength bands A, B separated by a wavelength interval A _em . Using the effect of Red-Edge-Excitation-Shift observed for fluorescence light Lfi from material of the tooth T but not from plaque P, the contribution of fluorescence light Lfi of the plaque P in the first detection signal yi can be separated from a background of fluorescence.