The image contrast is related to the sample's absorption coefficient formula_2. This is in contrast to bright or dark field microscopy, where the image contrast is due to transmittance or scattering. In principle, the contrast of fluorescence microscopy is proportional to the sample's absorption too. However, in fluorescence microscopy the fluorescence quantum yield formula_3 needs to be unequal to zero in order that a signal can be detected. In photoacoustic microscopy, however, every absorbing substance gives a photoacoustic signal formula_4 which is proportional to