Placido Disk vs Scanning Slit/Scheimpflug imaging

Reflection doubles the angle, but scanning slits or Scheimpflug systems multiply by 0.5 (sine of 45 degrees). So for a given camera resolution, reflection based systems like Placido have 4 times higher precision in the measurements.

Placido systems generate high contrast images, so the image processing is highly accurate. Scheimpflug/Scanning slits image light scattered from the clear corneal tissue, so the images are low contrast. This further compromises the precision of Scheimpflug/scanning slit systems.

Scheimpflug and Scanning Slit systems take tens of images over a couple of seconds. Eye movement during this period can destroy the accuracy of the final result, and standard eye tracking cannot eliminate all these motion artifact errors. In AVS' Apex Topographer all of the information is in a single image, so there are no motion artifact errors. And no alignment errors!

Scheimpflug and Scanning Slit systems can measure the topography of the posterior surface of the cornea. Placido systems cannot. However, the refractive index difference between the corneal tissue and the aqueous fluid in the front of the eye, is very small. So the optical effect of this surface is very small, and patient-to-patient variations are not large enough to create a need to map the topography of this back surface. Even Dr Barrett has acknowledged measuring the astigmatism in the back surface did not improve his Toric IOL calculator, and claims of improved Keratoconus detection are probably related to artifact from the optics of the anterior topography affecting the posterior topography measurement.

Scheimpflug and Scanning Slit systems usually have one important advantage over Placido Disk systems; they directly measure the elevation of the corneal surfaces. So the result is accurate, if the eye is stationary during the imaging time. Placido systems do not have a unique solution; was it a flat cornea too far away or a steeper cornea closer to the device? Placido systems usually incorporate an alignment system to ensure the distance between the eye and the device was a known value at the time of the capture. These alignment systems require a skilled operator and do not work nicely when the device is hand-held. Assumptions about the smoothness of the corneal surface can then be applied to an accurate measure of the corneal topography. A skilled operator at a chin-rest can produce more accurate topography measurements of a normal eye with a Placido device. Edge detection and motion artifact errors in slit projection systems multiply when looking at Axial or Refractive maps (first derivative of shape) and Local or Tangential maps (2nd derivative of shape).

AVS' Apex Topographer use special prisms to allow the exact reflection angle of hundreds of points over the cornea to be measured directly. Giving the precision of reflection along with the accuracy of a direct measurement of corneal position and slope at those points. Assumptions can be adjusted or eliminated for the calculation of the hundreds of thousands of other measurement points. Highly accurate corneal topography maps can be measured without the need for a skilled operator nor stationary patient.