ORBSCAN II by ORBTEK sets a new standard for precision measurement of the anterior segment of the eye. Utilizing a scanning slit measurement system, the intrument provides the clinician with important diagnostic information about the cornea, iris, and lens.
ORBSCAN II provides anterior segment data in the form of topographic surfaces. Elevation topography of the anterior cornea enables clinicians to more accurately visualize the shape of abnormal corneas, which leads to more accurate diagnoses and more consistent surgical results. The reason for this is simple but important. When presented with maps of curvature or slope, you must must mentally translate these data into elevation, because we visualize shape as topography (hills and valleys). This translation can be very difficult, often counter-intuitive, if the curvature map is of an asymmetric or irregular surface. Curvature alone does not usually provide sufficient useable information for the mind to construct an accurate three-dimensional shape.
Traditionally, power maps (actually surface curvature) have been axis-centered, which are method dependent and only show one selected piece of the complete angle-dependent power. The new mean and toric decomposition of the complete curvature tensor, allow clinicians for the first time to see and quantify local curvature (and paraxial power) variations as well as the fundamental spherocylindrical components. Full corneal pachymetry allows surgeons to use regional blade settings, to measure the laser ablation depth from a fixed surface (the posterior cornea), and to assess the effect of the posterior surface on corneal power. Accurate knowledge of both surfaces will provide researchers with real data for corneal mechanics simulations. The point of minimum corneal thickness is a local center of axisymmetry, which for normal corneas aligns closely to the overall optical axis of the eye.
Four ocular surfaces: Anterior Cornea, Posterior Cornea, Anterior Iris, Anterior Lens
Geometry and shape maps: Relative Elevation, Inclination, Surface Curvature
Distance maps between surfaces: Full-Corneal Pachymetry, Anterior Lens Depth
Optical function maps: Optical Power, Point Spread Function, Effectivity
Clinical tools: Surgical Planning, Contact Lens Fitting
When a slit-beam intercepts an optically smooth surface, it splits into a specularly reflected beam and a refracted beam. The latter penetrates the surface and is volume scattered by internal scattering centers. Specular reflections are not used in slit-scan triangulation. Volume scattering is similar to surface diffuse reflection in that it scatters light omni-directional. This important property allows surface points to be independently observed and triangulated, and it gives ORBSCAN the capability to measure arbitrary surface shapes - convex or concave, aspheric and irregular.
The magnitude of volume (or diffuse) scattering is typically negligible from liquids, like the tear film and aqueous humor. In contrast, volume scattering is significant from the lens, the iris, and the relatively large collagen fibers of the corneal stroma. As a consequence, diffusely reflected images, which are projections of the scattering volume illuminated by the slit beam, are seen through the tear film and aqueous humor.
As typical internal scatters are generally smaller than the wavelength of visible light, the magnitude of scattering is inversely proportional to the third or fourth power of the optical wavelength (Rayleigh scattering). As a consequence, the diffusely scattered return consists of the shortest wavelengths found in the original beam, which is why corneal backscatter appears blue.
A planar slit beam, diffusely reflected from the convex shell of the cornea, appears as an annular arc in the video image. The outer and inner edges of this arc respectively correspond to the anterior and posterior surfaces of the illuminated corneal volume.
To locate a point on the anterior surface, an outer edge point is first detected to sub-pixel accuracy. From the video calibration, the detected edge point is then translated into the chief ray that entered the camera and formed the image. This camera ray is mathematically intersected with the slit-beam surfaces, which are precisely located during the calibration process. The result of this process, which we call direct triangulation, is an anterior corneal surface point located in (x, y, z) space.
Raytrace triangulation is required when a surface point lies behind an optical interface that refracts the slit beams and the camera rays of the edge points. As ocular surfaces are triangulated one at a time from front to back, all the refracting surfaces in front of a desired surface point are known a priori and can be used to calculate all the necessary optical refractions. The result is an undistorted measurement of an internal ocular surface.
For ophthalmic purposes, one of the most important geometric measures of a surface is its curvature. Curvature measures how fast a surface bends and is inversely proportional to its local radius of curvature. Within paraxial optics (which is an approximate theory valid in a thread-like region surrounding an optical axis), local interface power is proportional to surface curvature.
To calculate curvature (and other geometric properities), individual triangulated points are first used to construct an absolute height map of the surface. Differential geometry is then used to locally calculate curvature in any given direction.
Three-quarters of the refractive power of the eye occurs at the anterior corneal surface. Thus when performing raytrace calculations through the anterior segment (cornea, pupil, and lens), anterior surface accuracy is always of paramount importance. The hybrid technology of ORBSCAN II was developed to increase anterior surface accuracy by using the specularly reflected image of a placido or other reflective target to directly measure surface slope (recall, slit-scan technology directly measures surface height). Separate elevation and slope measurments are then unified in the construction of absolute height surface, accurate in elevation, slope, and curvature. These mathematical surface reconstructions are piecewise analytic with continuous second derivatives.
Because ORBSCAN II takes a series of data images for each exam (40 slit-scan and 3 placido images), the fixating but moving eye must be tracked during the exam. The figure below shows the eye movements (in x, y, and z) occuring during a typical exam (only the movement occuring for the first 20 images is shown here). As the (x, y) box is 400 microns on a side and the z range is 280 microns, typical fixating eye movement is considerable. Before the absolute height surfaces are constructed, data slices are first moved back to the starting position of the eye, which effectively removes eye movement from the exam.
ORBTEK is dedicated to provide you with the most up-to-date technology in ophthalmology. Our ORBSCAN II systems utilize the latest in microprocessor technology to acquire, analyze, display and archive the data. The basic configuration of an ORBSCAN image processing system is:
Computer hardware specifications are subject to change. ORBTEK continues to evalute the latest technologies to best meet our customers's needs.