Science Behind Autorefractors for Biometry
Recently I was involved in a discussion on different makes of Keratometers by different companies. Lots of the participants voiced their concern on the difference of reading between numerous Keratometers, leading to confusion as to which data to be relied on. In a separate article named ” What to do when your K readings are different from different Keratometers,” this matter has been addressed. However, this informative article would pay attention to why different auto refractors and keratometers , optical or elsewhere, fail to own consistency between the readings. I’d also get you briefly through the science behind calculating the present-day optical biometers in answering so.
Once we understand, different Keratometry devices read different rings of the cornea. While the manual Keratometers would typically measure at central 3.0 to 3.2 mm of the cornea, the optical Biometry machines like IOL Master would measure 2.5 mm of the cornea. The Lenstar measures two separate rings of 1.65 and 2.3 mm approximately.
The Version from Alcon measures a peculiar central section of .8 to 1.2 mm of the cornea, although it takes the axis from the broader ring beyond this area. As you will see, none of the devices measure a fixed area, and it varies involving the label of the devices. One big basis for the differences in the readings is the heterogeneity of measurement areas.
Another element in the reading difference could be the Keratometry Index ( K Index ) employed by the devices. An indicate be noted here’s a Keratometer does not measure in diopters. It measures the Radius of Curvature in mm, then converted back to diopters via a formula ( D=n-1/r ) where n could be the Keratometry Index. The European and US make devices do not have the same K Index. This would therefore lead to further differences in the final K reading involving the devices.
The last but not the smallest amount of importance is always knowing how the present-day optical devices measure the cornea? The key here’s to know the Purkinje Reflexes from the cornea. None of them measure the Posterior Cornea (except for Zeiss Total Keratometry recently launched ).
The IOL Master version V would throw six light spots on the anterior cornea in a hexagonal pattern in the diameter of 2.5 mm. So the exact distance between each spot is 1.3 mm to the visual axis in a perfectly symmetrical and spherical cornea. The positioning of every spot reflecting from the cornea and following the other light spots will be found and analyzed by the computer. Light reflections closer to each other would signify a steeper cornea and vice-versa.
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