Viewing Objective

Figure 1-12. Schematic of the optical system for OCT imaging in the anterior eye. Operator viewing is provided by a standard imaging system with the image plane on the anterior eye, A computer controlled scanning mirror is used to position and scan the OCT measurement beam. The OCT measurement beam is focussed with a lens onto the same image plane as the operator view.

Instrumentation for Anterior Segment Imaging

Figure 1-12 shows a schematic diagram of the optical imaging system for tomographic imaging of the anterior segment. The basic diagnostic instrument is similar in design to a slit-lamp biomicroscope. In this case, the microscope images structures in the anterior segment directly and the operator can view them through an ocular or with a video camera. The OCT measurement beam is coupled along the viewing path of the device using a partially reflective mirror and the focal point of the OCT probe beam is coincident with the image plane of the biomicroscope. The position of the OCT optical beam is controlled by a scanning mirror or pair of mirrors which aims the beam position the coronal plane (x,y directions).

The operator s view of the anterior segment is determined by the magnification of the ophthalmic instrument or slit-lamp biomicroscope, The depth of field or range over which the (]CT beam remains focussed is also determined by the imaging optics used to focus the beam onto the coincident image plane for the slit-lamp biomicroscope. For studies in the anterior segment, the depth of field is often set to be relatively large and thus the transverse spot size of the OCT beam is usually large. The resolution of the OCT image in the transverse direction is coarse with low transverse magnification of the OCT image.

Instrumentation for Retinal Imaging

The second general type of ophthalmic imaging which can be performed using OCT is imaging the posterior segment of the eye or the retina. In this application, the instrument operates analogously to a fundus camera. Figure 1-13 shows a schematic diagram of the imaging optics which are used for retinal OCT imaging. In this case, a high power condensing lens is used so that the retina may be imaged onto a plane within the instrument. The instrument then reimages the retinal plane to permit viewing by the operator The OCT measurement beam is coupled into the optical path of the instrument using a partially reflective mirror or beamsplitter. The magnification of the retinal image is determined by the refractive power of the condensing lens and the magnification of the ocular The OCT measurement is focused onto the retinal image plane using a focusing lens. Since the retinal thickness is quite small, the depth of focus is relatively shallow, and the spot size for the OCT beam can also be made small

The transverse position of the OC measurement beam is controlled by a mechanical scanning mirror system within the instrument. A typical field of view at lowest magnification is 30°. The fundus image can

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