Prior to ~ 1999 the only way to measure the length of the eye was ultrasound (U/S). About that time Zeiss invented a device that measured the length of the eye using partially coherent light (Optical). It was much more accurate, but only worked when the cataract was not too dense so there was an optical path … dense cataracts could and still cannot be measured optically. On the PREOP SCREEN the USER must select the U/S or Optical RADIO BUTTON (if both measurements have been inputed). Whichever he selects determines which appears on the IOL CALC SCREEN and on the printout.
U/S stands for Ultrasound and OPT stands for Optical, such as the IOL Master and LenStar. In our database there is a limited # of IOLs that have an Optical Constant … the rest are blank. If the USER does not have an Optical Constant and there is none in the database, then the Program uses the U/S Constant + 0.2, since there is no other value. After they have determined a PLC (Personalized Lens Constant) from 30 to 50 cases of post op information using Optical Axial lengths, the Program uses the Optical PLC.
PLC (personalized lens constant) takes precedence over SRG which takes precedence over MFG.