Currently, we are developing a computational optical biopsy technology for molecular sensing. We use the diffusion equation to model photon propagation but have a concern about the accuracy of diffusion approximation when the optical sensor is close to a bioluminescent source. We derive formulas to describe photon fluence for point and ball sources and measurement formulas for an idealized optical biopsy probe. Then, we numerically compare the diffusion approximation and the radiative transport as implemented by Monte Carlo simulation in the cases of point and ball sources. Our simulation results show that the diffusion approximation can be accurately applied if ${\mu }_s^{\prime }\gg {\mu }_a$ even if the sensor is very close to the source $(>1\mathrm{mm})$ . Furthermore, an approximate formula is given to describe the measurement of a cut-end fiber probe for a ball source.