Biometry
Axial length measurement and IOL-calculation support — optical and ultrasound A-scan technique.
Measuring the Eye for Surgery
Why it matters — axial length plus K readings drive the IOL power calculation; a 1.0 mm axial-length error shifts the postoperative refraction roughly 2.5–3 D.
Optical biometry — light-based (IOLMaster/Lenstar class), non-contact, fast, highly repeatable; today's first choice whenever media permit.
Ultrasound A-scan — contact technique risks corneal compression, which falsely shortens the eye (postop myopic surprise); immersion technique avoids probe-to-cornea contact and is the more accurate ultrasound method. Dense cataracts that defeat optical biometry still need ultrasound.
Quality checks — measure both eyes, compare (large asymmetry gets re-measured), confirm the velocity setting matches the eye's status (phakic vs pseudophakic), and repeat until readings cluster tightly.