Fluorescence spectra measured from biological samples, such as tissues or cell suspensions, are usually distorted due to the light absorption by intrinsic chromophores. These distortions are aggravated by strong scattering of light inside the samples. A new method is described for a fast correction of these spectral distortions, using only steady-state spectroscopic measurements. The method is based on the formulas derived from a simplified photon diffusion model, in the isotropic one-dimensional approximation applied to a semi-infinite, highly scattering, and moderately absorbing medium with a refractive-index-matched boundary. The formulas describe the spectral distortions of the fluorescence emission and excitation spectra, together with the diffuse reflectance spectrum, as the functions of one spectral characteristic of the medium, the darkness, which is the ratio of absorption coefficient and reduced scattering coefficient. The algorithm does not involve any iterative procedures, and offers a direct, simple, and fast method for real-time spectral correction. The true fluorescence emission or excitation spectrum is directly calculated from a pair of experimental spectra: the fluorescence emission or excitation spectrum and the diffuse reflectance spectrum, measured from the same position on a sample. The correction produces the profile of the true fluorescence spectrum, the same as the one measured from the corresponding sample with an infinitely low absorption and no scattering. The restoration of the spectral profiles of true fluorescence emission and excitation spectra was tested experimentally, using highly scattering phantoms with a fluorescent dye and a deliberately added nonfluorescent dye producing strong inner-filter distortions.