The image formation of luminor body by lens is studied using geometrical optics. Considering the contributions due to viewed cone centred on the object point and extending through the luminnor body, the distribution of intensity in the image plane can be expressed by the integration of emitted powers of luminor body over the entire sightline under the paraxial approximation. Then following results obtained: 1) the contribution of emitted power in any plane perpendicular to the sightline is all the same. 2) the relative level of the intensity fluctuations measured by detector is smaller than the true value due to the integration, but the emission fluctuations of luminor body can be reconstructed under some conditions.