a. What are these two instruments used for? b. How do these two intruments from each other?
b. How do these two intruments from each other?
The Bausch and Lomb keratometer uses the image of a single illuminated mire, which it focuses with a condensing lens and passes back through a diaphragm with 4 apertures, the sizes are calculated so that the ones in the vertical orientation together are equal to the area of a single one of the apertures in the horizontal orientation. The B&L keratometer uses the Scheiner principle to determine the end point of focusing the image i.e. the image of the mires passes undeviated through the vertical apertures thus producing 2 images until the point at which they are in exact focus. The J-S Keratometer uses 2 coloured mires set along a graduated arc and are able to be moved closer together or further apart, adjusting the size of the object until the image is a predetermined size and thus the doubled images are just touching end to end. At this point the image size is known, as is the distance from the corneal surface and the object size can be measured, thus radius of curvature can be calculated and corneal dioptric power deduced.
The J-S Keratometer uses 2 coloured mires set along a graduated arc and are able to be moved closer together or further apart, adjusting the size of the object until the image is a predetermined size and thus the doubled images are just touching end to end. At this point the image size is known, as is the distance from the corneal surface and the object size can be measured, thus radius of curvature can be calculated and corneal dioptric power deduced.
