Аннотация:The effect of reflection and absorbtion of radiation in parametric light converter in the course of absolute spectral density measurements by quantum photometer is considered. The universal method is proposed for corrective factors calculation, which excludes systematic errors due to brightness measurements by parametric conversion in crystals using absorption and reflection of signal, measured and pump waves. The expression of corrective factor through coefficients of absorbtion and reflection from crystal facets of all the waves, taking part in parametric interaction, are found.