Giao thoa quang Một giao thoa kế quang học được thiết kế đặc biệt để tính năng lượng cấy ghép ống kính là thương mại (IOL Thạc sĩ, Carl Zeiss). Hệ thống này có thể được sử dụng để đo lường quang học của chiều dài trục, keratometry, và đo lường quang học của sâu khoang | CATARACT SURGERY millimetres 2-93 . For example IOL power would be 3-5 D for an axial length of 23-0 mm and 2-8 D for an axial length of 30-0 mm if using convex-plano implants . Optical interferometry An optical interferometer specifically designed for lens implant power calculation is commercially available IOL Master Carl Zeiss . This system can be used for optical measurement of the axial length keratometry and optical measurement of anterior chamber depth. In-built formulae Haigis Hoffer Q SRK T and Holladay 1 allow calculation of lens implant power. It can be used for measuring axial length in eyes in which visual acuity is 6 18 or better but dense cataract corneal opacification or vitreous opacities preclude measurement. The system is a non-contact one and is therefore ideal in terms of patient comfort and compliance. The patient sits with their chin on a rest and forehead against a band and is asked to fixate on a target light. The operator merely has to use the joystick to focus the instrument and to press a button to record the axial length. A measure of trace quality is given in a signal noise ratio which must be greater than 2-0 to be accepted by the machine. The system is ideal for use in those eyes that are difficult to measure using ultrasound for example eyes in which there are posterior staphylomata especially if eccentric or eyes with nystagmus. The system uses a low coherence Doppler interferometer to measure axial A collimated beam of near infrared 780 nm from a multimode laser diode is transmitted to the globe via a Michelson interferometer. Light is partially reflected at the ocular interfaces. Moving one of the interferometer mirrors varies the optical path difference between the two arms of the interferometer. When the path difference corresponds to the axial length of the eye concentric interference fringes are generated. The intensity of these fringes are plotted as a 78 function of the position of the mirror. The position of the
