Application of the Partially Coherent Imagery method to the Laser Raster Output
Scanning (ROS) system with Acousto-Opto Modulator (AOM), appears to lead to a
nonlinear Modulation Transfer Function (MTF).
When using AOM for Laser ROS, it is necessary to consider not only
conventional parameters but also two more conditions, the illumination geometry of
AOM and the dynamic image of the sound wave in the AOM. The illumination geometry
of the ROS is similar to that of a microscope system. The calculation of partially
coherent image is useful for such systems. The most important factor of that
calculation is Relative Numerical Aperture (RNA), the ratio between illuminative
NA and objective NA. Replacing the laser beam diameter with the illuminative NA,
leads to the MTF curve for the static optical system. This system shows a
nonlinear response, due to a nonlinearity between light amplitude and intensity.
The response for the dynamic image of the sound wave at the AOM, enables to
calculate the MTF of the dynamic optical system which has constant velocity error.
Combining these and other MTF responses, the total system MTF curves are
The nonlinearity manifests itself by the peak value which is enhanced in
comparison to the continuous lighting level. As a total, the response of this
system is higher than the response of a conventional ROS system in practical
spatial frequency domain.