A-scan biometry uses high frequency sound waves (ultrasound) to measure tissue thicknesses in the eye.  An ultrasonic measure of the length of the eye is used to calculate intraocular lens power for cataract surgery.
Alternative Names: IOL calculation, Intraocular lens calculation

What to Expect

  • Topical anesthetic (numbing) eye drops are administered. 
  • The patient is examined while lying on his/her back in an exam chair. 
  • A plastic shell is placed on the surface of the eye and filled with water. 
  • The patient is asked to lie on their back and look straight up at the ceiling.
  • The ultrasound probe is immersed in the water and measurements are obtained. 
  • The test will take approximately 15 minutes per eye.

How the Test Feels

Patient’s may experience stinging or burning with initial instillation of the topical anesthetic drop.  The sterile water may feel cool on the surface of the eye.   

Why the Test is Performed

A patient’s eye length is one component of the calculation required for intraocular lens power in cataract surgery. A-scan biometry may also measure thicknesses of lesions in the eye (i.e., calcification, tumor, etc).


  • Normal Values
    Normal length of the eye ranges from 22 to 24 mm.


  • Abnormal Results
    Abnormal results may indicate
    • Myopia
    • Hyperopia
    • Intraocular foreign body
    • Intraocular tumor
    • Intraocular calcification