Optical device for characterizing a sample comprising - a source of parallel or collimated light having a continuous emission spectrum in an observation wavelength range of at least 50 nm width, for illuminating said sample, - a detector of light scattered by the sample, functioning in said observation wavelength range, - a spectral weighting filter placed on the optical path upstream of the detector of scattered light, and - a unit for processing the measurement signal in order to extract a parameter characterizing the sample, characterized in that the spectral weighting filter possesses, in said observation wavelength range, in reflection or transmission, a spectral filtering function T(λ) given by the following equation: T λ = η / λ 3 Ψ i 0 θ 0 λ F λ K λ where ▪ η is a constant of proportionality, ▪ λ is the wavelength, ▪ θ 0 is the direction of scatter of the light, ▪ i 0 is the angle of incidence at which the sample is illuminated by the light source, ▪ F(λ) is the spectral power density of the source in the observation wavelength range, ▪ K ( λ) is the spectral response of the detector in the observation wavelength range, ▪ Ψ( i 0, θ 0, λ)=C( i 0, θ 0, λ) in case of characterization of a surface of the sample and Ψ ( i 0, θ 0, λ)= D ( i 0, θ 0, λ) in case of characterization of a volume of the sample, ▪ C ( i 0, θ 0, λ) is an optical coefficient given by perturbation theories of the scatter of light by a rough surface, and ▪ D ( i 0, θ 0, λ) is an optical coefficient this time given by perturbation theories of the scatter of light by heterogeneous volumes. - a source of parallel or collimated light having a continuous emission spectrum in an observation wavelength range of at least 50 nm width, for illuminating said sample, - a detector of light scattered by the sample, functioning in said observation wavelength range, - a spectral weighting filter placed on the optical path upstream of the detector of scattered light, and - a unit for processing the measurement signal in order to extract a parameter characterizing the sample, characterized in that the spectral weighting filter possesses, in said observation wavelength range, in reflection or transmission, a spectral filtering function T(λ) given by the following equation: T λ = η / λ 3 Ψ i 0 θ 0 λ F λ K λ where ▪ η is a constant of proportionality, ▪ λ is the wavelength, ▪ θ 0 is the direction of scatter of the light, ▪ i 0 is the angle of incidence at which the sample is illuminated by the light source, ▪ F(λ) is the spectral power density of the source in the observation wavelength range, ▪ K ( λ) is the spectral response of the detector in the observation wavelength range, ▪ Ψ( i 0, θ 0, λ)=C( i 0, θ 0, λ) in case of characterization of a surface of the sample and Ψ ( i 0, θ 0, λ)= D ( i 0, θ 0, λ) in case of characterization of a volume of the sample, ▪ C ( i 0, θ 0, λ) is an optical coefficient given by perturbation theories of the scatter of light by a rough surface, and ▪ D ( i 0, θ 0, λ) is an optical coefficient this time given by perturbation theories of the scatter of light by heterogeneous volumes. ▪ η is a constant of proportionality, ▪ λ is the wavelength, ▪ θ 0 is the direction of scatter of the light, ▪ i 0 is the angle of incidence at which the sample is illuminated by the light source, ▪ F(λ) is the spectral power density of the source in the observation wavelength range, ▪ K ( λ) is the spectral response of the detector in the observation wavelength range, ▪ Ψ( i 0, θ 0, λ)=C( i 0, θ 0, λ) in case of characterization of a surface of the sample and Ψ ( i 0, θ 0, λ)= D ( i 0, θ 0, λ) in case of characterization of a volume of the sample, ▪ C ( i 0, θ 0, λ) is an optical coefficient given by perturbation theories of the scatter of light by a rough surface, and ▪ D ( i 0, θ 0, λ) is an optical coefficient this time given by perturbation theories of the scatter of light by heterogeneous volumes.