Effective collecting area The effective collecting area is, in the simplest approximation, the area of the front lens as seen from the voxel. The variation of the effective area with angle $\theta$ relative to the optical axis is: $$A(\theta) = A_{L}\,\cos\theta$$ (5.16) Here $A_{L}$ is the maximum area of the front lens or the aperture of the optics and $\theta$ is the angle relative to the optical axis. This formula is good enough for single thin lens optics. For more complex optical systems with a large number of lenses and apertures, the effects of mechanical vignetting -- the decrease of effective aperture with increasing angle relative to the optical axis -- must be corrected for. The higher order corrections that are needed must be determined from measurements. This can be accomplished with a simple experiment in which a narrow weak beam of parallel light illuminates the front lens at varying angles relative to the optical axis and with varying translation from the centre of the front lens. This has yet to be done for the ALIS cameras.