A plaque detection apparatus, comprising: an excitation source configured for outputting wavelength modulated light (λ ex) to an evaluation site, the wavelength modulated light being modulated at a wavelength modulation frequency to have a periodically changing wavelength, changing perioidcaly at said wavelength modulation frequency, centered around a wavelength corresponding to a non-linearity in one or more of an absorption and a fluorescence excitation spectrum for a chosen plaque exhibiting spectral characteristics different from spectral characteristics of one or more of (i) non-chosen plaque and (ii) interfering species other than the chosen plaque; a light detector configured for detecting light (λ site) received from the evaluation site, wherein the detected light (λ site) comprises both of (i) a site reflected light (λ refl) and (ii) a site emitted light (λ em); and a controller, operatively coupled to the excitation source and the light detector, comprising: an excitation control module for controlling, via at least one excitation control signal, the excitation source to output the wavelength modulated light; and a harmonic component detection module for detecting at least one harmonic component of the wavelength modulation frequency higher than a fundamental in at least one of (i) an absorption spectrum that comprises site reflected light (λ refl) and (ii) an excitation spectrum that comprises fluorescence emission in site emitted light (λ em). and wherein the controller is configured for detecting dental plaque based on both the detected light (λ site) and at least one detected higher harmonic of the wavelength modulation frequency higher than a fundamental in at least one of (i) an absorption spectrum that comprises site reflected light (λ refl) and (ii) an excitation spectrum that comprises fluorescence emission in site emitted light (λ em). an excitation source configured for outputting wavelength modulated light (λ ex) to an evaluation site, the wavelength modulated light being modulated at a wavelength modulation frequency to have a periodically changing wavelength, changing perioidcaly at said wavelength modulation frequency, centered around a wavelength corresponding to a non-linearity in one or more of an absorption and a fluorescence excitation spectrum for a chosen plaque exhibiting spectral characteristics different from spectral characteristics of one or more of (i) non-chosen plaque and (ii) interfering species other than the chosen plaque; a light detector configured for detecting light (λ site) received from the evaluation site, wherein the detected light (λ site) comprises both of (i) a site reflected light (λ refl) and (ii) a site emitted light (λ em); and a controller, operatively coupled to the excitation source and the light detector, comprising: an excitation control module for controlling, via at least one excitation control signal, the excitation source to output the wavelength modulated light; and a harmonic component detection module for detecting at least one harmonic component of the wavelength modulation frequency higher than a fundamental in at least one of (i) an absorption spectrum that comprises site reflected light (λ refl) and (ii) an excitation spectrum that comprises fluorescence emission in site emitted light (λ em). and an excitation control module for controlling, via at least one excitation control signal, the excitation source to output the wavelength modulated light; and a harmonic component detection module for detecting at least one harmonic component of the wavelength modulation frequency higher than a fundamental in at least one of (i) an absorption spectrum that comprises site reflected light (λ refl) and (ii) an excitation spectrum that comprises fluorescence emission in site emitted light (λ em). and wherein the controller is configured for detecting dental plaque based on both the detected light (λ site) and at least one detected higher harmonic of the wavelength modulation frequency higher than a fundamental in at least one of (i) an absorption spectrum that comprises site reflected light (λ refl) and (ii) an excitation spectrum that comprises fluorescence emission in site emitted light (λ em).