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PubMed1998

Methods to obtain quantitative parametric descriptions of the optical surfaces of the human crystalline lens from Scheimpflug slit-lamp images. I. Image processing methods.

Of the methods developed (e.g., phakometry, magnetic resonance imaging, etc.) for noninvasive measurement of the geometry of the anterior segment of the human eye, Scheimpflug photography offers the best resolution and the highest precision. The primary obstacle encountered with this or any other image-based method has been in obtaining quantitative measurements directly from the images. Image enhancement (gray-scale gradient analysis) and pattern recognition methods (Hough transform and recursive least-squares algorithms) are developed so that parametric representations of lens surfaces and zone boundaries can be obtained directly from the images. Methods to correct for nonlinear Scheimpflug camera reproduction ratios and provide error estimates for geometrical parameters are also developed and will be presented separately. Combined, these techniques yield representations of lens geometry having sufficient precision, to which paraxial ray tracing can be applied to determine lens optical properties by using well-posed optical models with one unknown.

Algorithms↗
PubMed2001

Safe exposure times for slit-lamp fundus biomicroscopy with high plus lenses.

PURPOSE: The purpose of this study was to calculate the retinal irradiance and maximum permissible exposure time (MPE) using high plus fundus biomicroscopy lenses. METHODS: Four Volk handheld condensing lenses (+78 D, +90 D, Superfield NC, and Super 66) were tested with biomicroscopes from three manufacturers (Nikon, Topcon, and Zeiss) on both high and medium illumination. Using the conservation of radiance theorem, the retinal irradiance was calculated and. with guidelines from the American National Standard Institute (ANSI). the maximum permissible exposure time was determined. RESULTS: The range of MPE time across all lenses and biomicroscopes was from 23 seconds to 74 seconds on high illumination setting. The average MPE times were: for the +78 D, 36 seconds; the Superfield NC, 57 seconds; the Super 66, 32 seconds; and the +90 D, 52 seconds. CONCLUSION: Retinal irradiances and safe exposure times are useful guidelines in both the clinical and academic setting. Fundus biomicroscopy with non-contact high plus condensing lenses produced calculated retinal irradiances and MPE times similar to those of other commonly used ophthalmic equipment. Awareness of the maximum permissible exposure time increases the safety of this valuable technique.

Equipment Design↗
PubMed2001

[Ocular biometric measurements with a slit-lamp method (Orbscan)].

PURPOSE: To evaluate the accuracy and reproducibility of the biometric measurements obtained with the Orbscan((R)) device in normal eyes. METHODS: Anterior chamber depth, axial length, and corneal diameter were studied in 50 normal eyes of 25 patients using the Orbscan((R)) device and A-Scan ultrasonography. For each eye, two exams were done successively and reproducibility was calculated by analyzing the difference between both measurements. RESULTS: The reproducibility of anterior chamber depth and corneal diameter measurements obtained by Orbscan((R)) was high (respectively 0.721.25% and 0.831.40%). The anterior chamber depth measurements by Orbscan((R)) and A-Scan ultrasonography were strongly correlated (rs=0.92;p<0.001). The axial length measurement obtained with the Orbscan((R)) device was less reproducible (3.554.11%). For the axial length, the Orbscan((R)) measurements and A-Scan ultrasonography measurement was poorly correlated. (rs=0.64; p<0.001). CONCLUSION: The anterior chamber depth measurement obtained with the Orbscan((R)) device showed high reproducibility and it was strongly correlated with the measurement obtained by A-Scan ultrasonography. Axial length measurement obtained with the Orbscan((R)) device was less reproducible and poorly correlated with the A-Scan ultrasonography measurement. It was not relevant for clinical application.

Adult↗