Method for observing a sample, comprising the following steps: i) illuminating the sample using a light source, which produces a light wave that propagates along a propagation axis, ii) acquiring, using a photodetector, a plurality of images (I 1...I N) of the sample, said images being formed in a detection plane, the sample being placed between the light source and the photodetector, each image being representative of a light wave, called the transmitted light wave, transmitted by the sample under the effect of said illumination, each image (I i) being acquired in a spectral band (λ i) different from the others, the method also comprising the following steps: iii) determining, on the basis of each image (I i) acquired in a spectral band (Ai), an initial complex amplitude A i, p = 1 d r of the transmitted light wave, in said spectral band, in said detection plane, iv) back propagating each complex amplitude A i, p = 1 d r, A i, p d r, established in the detection plane, in a spectral band (Ai), to determine a complex amplitude A i, p 0 r of the transmitted wave, in each spectral band, in a plane (P 0) in which the sample lies, v) on the basis of the plurality of complex amplitudes A i, p 0 r determined in step iv), in various spectral bands, computing a weighting function F p 0 r, F p 0 → r, in the plane of the sample (P 0), the weighting function being computed on the basis of a weighted sum of said complex amplitudes A i, p 0 r determined in each spectral band in the plane in which the sample lies, or of their logarithms log A i, p 0 r or of their arguments arg A i, p 0 r, the weighted sum employing weighting factors, each weighting factor being positive, vi) propagating the weighting function F p 0 r, F p 0 → r,, to the detection plane, so as to obtain, for each spectral band (Ai), a propagated weighting function F i, p d r in said detection plane, vii) updating each complex amplitude A i, p d r of the transmitted light wave, in each spectral band (Ai), in the detection plane, using said propagated weighting function F i, p d r, in said spectral band (Ai), in step vi), viii) repeating steps iv to vii until a stop criterion is met. i) illuminating the sample using a light source, which produces a light wave that propagates along a propagation axis, ii) acquiring, using a photodetector, a plurality of images (I 1...I N) of the sample, said images being formed in a detection plane, the sample being placed between the light source and the photodetector, each image being representative of a light wave, called the transmitted light wave, transmitted by the sample under the effect of said illumination, each image (I i) being acquired in a spectral band (λ i) different from the others, iii) determining, on the basis of each image (I i) acquired in a spectral band (Ai), an initial complex amplitude A i, p = 1 d r of the transmitted light wave, in said spectral band, in said detection plane, iv) back propagating each complex amplitude A i, p = 1 d r, A i, p d r, established in the detection plane, in a spectral band (Ai), to determine a complex amplitude A i, p 0 r of the transmitted wave, in each spectral band, in a plane (P 0) in which the sample lies, v) on the basis of the plurality of complex amplitudes A i, p 0 r determined in step iv), in various spectral bands, computing a weighting function F p 0 r, F p 0 → r, in the plane of the sample (P 0), the weighting function being computed on the basis of a weighted sum of said complex amplitudes A i, p 0 r determined in each spectral band in the plane in which the sample lies, or of their logarithms log A i, p 0 r or of their arguments arg A i, p 0 r, the weighted sum employing weighting factors, each weighting factor being positive, vi) propagating the weighting function F p 0 r, F p 0 → r,, to the detection plane, so as to obtain, for each spectral band (Ai), a propagated weighting function F i, p d r in said detection plane, vii) updating each complex amplitude A i, p d r of the transmitted light wave, in each spectral band (Ai), in the detection plane, using said propagated weighting function F i, p d r, in said spectral band (Ai), in step vi), viii) repeating steps iv to vii until a stop criterion is met.