Theodre Scheimpflug, an Austrian army man worked extensively on a method for correcting arial skew distortion in perspective photographs. The non planar shape of cornea can potentially lead to spurious results and therefore the use of schiempflug principle in corneal imaging is a welcome new change. The triangulation between the reference slit beam surface and the reflected beam captured by the camera can be used to analyse the anterior and posterior corneal curvature and the pachymetry (Figure 2, right). In the Orbscan, 40 slits (20 each from nasal and temporal side) are projected on the cornea to assess 240 points on each slit. Multiple complimentary slits are used to perform an assessment of the corneal surface (Figure 2, left). It is one of the elevation based methods for assessment of topography. Whereas the original placido discs were aimed a qualitative keratoscopy, the videokeratoscope or the topographer uses mathematical formulae to provide a point to point quantitative gradient of these subtle changes in topography. Topographers use this technique to their advantage. Placido disc is a device made of concentric rings drawn on a device of a different color (generally white rings on a black background) (Figure 1a).The first refracting surface of the cornea (more accurately, the tear film –air interface and not the epithelium) also acts as convex mirror and reflects back light in a pattern dependent of the corneal pattern (Figure 1b). This is the principle used for Purkinje imaging as well in the Placido discs. However, all non-ideal refracting surfaces reflect some light off them. The primary optical aim of cornea is refraction and focusing of the light rays as it acts as a covering lens overall. Placido Disc Reflection for curvature analysis
#Reflex arena reflection orb free#
Recent advances in the technology use scanning slit methods for assessment of elevation data and incorporation of distortion free Scheimpflug photography techniques.Ĭorneal topography is most commonly used for the following purposes Placido disc has stood the test of time and the current placido based topographers work on the same principle of assessing the reflection of a concentric set of black and white rings from the convex anterior surface of the cornea. Placido’s disc was a major advancement in the late 19th century. In early 17th century, Schiener used reflection of marbles from the cornea as perhaps the earliest corneal topography.
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