particular pattern of traps the combined hologram u03c6 0 u03c1 u03c6 0 u03c1 u03c6 z u03c1 mod 2 u03c0 projects the same pattern of traps as u03c6 0 u03c1 but with each trap translated by u2212 z along optical axis 90 of the system 10 the resulting image obtained from the reflected light represents a cross section of the original trapping intensity at distance z from the focal plane of the objective lens 50 translating the traps under software control by computer 95 is particularly convenient because it minimizes changes in the optical train 39 s properties due to mechanical motion and facilitates more accurate displacements along the optical axis 90 images obtained at each value of z are stacked up to yield a complete volumetric representation of the intensity distribution as shown schematically in fig2 a 2c the objective lens 50 captures essentially all of the reflected light for z lt 0 for z gt 0 however the outermost rays of the converging trap are cut off by the objective lens 39 50 output pupil 105 and the contrast is reduced accordingly this could be corrected by multiplying the measured intensity field by a factor proportional to z for z gt.