Abstract This work describes an exact radiometric model for experimental demonstrators of the detection of exoplanets by the transit method. This model generalises the calculation of the depth of occultation from the standard transit method to the case of a finite size demonstrator apparatus. Results show that, for demonstrator apparatuses of moderately small sizes, a significant accuracy improvement in the determination of the size of a planet model can be achieved using the proposed method in comparison to using the formula from the standard transit method. The radiance distribution of the star model is found to be of crucial importance, as deviations from a Lambertian radiance distribution can lead to significantly different results.