This a case of a patient wearing a hard contact lens and being imaged with the 3 different technologies of Visante Anterior Segment OCT (1310nm light source), PentaCam Sheimpflug system (475nm light source), and the Cirrus OCT (840nm light source).

The Visante OCT image is taken in the 1310nm.  The image above is taken in the standard resolution enhanced anterior segment imaging mode which is a 16mm by 6mm image.  The hard contact is clearly visible at both edges, the full with of the anterior segment, angle to angle is seem.  Note that the hard contact lens itself is clearly identifiable and cases minimal distortion of the anterior segment.

The Pentacam Scheimpflug image of the anterior segment is above.  The image is at 90 degrees to the axis of the central 475nm slit beam in the center.  As a results the entire edge of the hard contact lens is clearly seen in the image.  There is significant distortions of the anterior segment.  Given that the hard contact lens is also optically neutral to the 475nm light, the hard contact lens is not visible, yet its effects are clearly noted.

To extrapolate from this image, one can draw a conclusion about corneas that have been altered by refractive surgery.  Given that the image goes through an area that has optical power and able to change the image by magnification/minification, can the Scheimpflug system through its ray tracing compute for these changes?  I will present several cases that show the answer is no!

The above image is from the spectral domain Cirrus OCT with the 840nm light source.  The image is absolutely exquisite in detail.  The anterior surface of the hard contact lens is clearly visible.  The epithelium and the Bowman's are visible.  The spectral domain capability provides an enormous advantage, however, the capability is limited to a small 2 by 3mm image!