A Hartmann wavefront sensor measures optical wavefront aberrations
by using a lens array to break the input sample into subapertures.
The lens in each subaperture produces an image whose centroid position
depends on the average phase front tilt in the subaperture. The simplest
measurement implementation, a quad cell detector, is limited in tilt dynamic
range to ± 1 wave because the (diffraction-limited) image has a
radius equivalent to 1 wave of tilt. For greater tilts, the image is off-null
and no positional information is available. Dynamic range can be increased
by defocusing the spot but only at the expense of increased centroid
variance. However, if the spot image is optically intensified before being
detected, the intensified image can be defocused without increasing the
centroid variance. A theoretical derivation of sensor performance with and
without intensification is given. Results of simulations and laboratory experiment
are presented showing good agreement with theory. The use of
this technique may greatly extend the capabilities of wavefront sensor
systems at modest increase in cost or complexity.