INSTRUMENTS USED TO OBTAIN AND CORRECT REFRACTIVE ERROR

             

  Refractive errors occurs when the shape of the eye prevents light emanating from infinity to form image on the retina. The length of the eyeball, changes in the shape of the cornea, or aging of the lens can cause refractive error.The most common types of refractive errors are myopia, hyperopia, presbyobia, and astigmatism. Below are the instrument used to obtain refractive status of the eye.

    

1.  

   Lensmeter:

              The lensmeter is an instrument used to measure the power of a patient’s present spectacle lenses. Both manual and automated lensmeter are available. The lensmeter measures four principal properties of spectacle lenses:

·        Spherical and cylindrical power.

·        Cylindrical axis if cylinder is present.

·        Presence and orientation of prism.

·        Optical centration.

2.  

   Retinoscope:

                The handheld streak retinoscope comprises a viewer (peephole), a mirror assembly, and a light bulb with a delicate filament that can be rotated and focused by manipulating a sleeve on the instrument handle. It produces a streak of light, as differentiated from the round dot of light produced by a spot retinoscope, which is used less frequently. The vergence of the slit (that is, the focus of the beam) on the streak retinoscope is adjusted by moving the sleeve up or down the instrument’s handle. To perform retinoscopy, the examiner looks through the retinoscope peephole viewer and aligns the retinoscope streak with the patient’s visual axis. By shifting the position of the instrument, manipulating its light characteristics in specific ways, and observing the reaction of a light reflex from the patient’s eye, the examiner can determine the patient’s refractive state and estimate the corrective needs.

3.     Trial lenses and frames:

                 During retinoscopy, the examiner has the patient look through a variety of lenses until an appropriate optical correction is determined. One way to this is with the use of trial frames- eyeglasses that can hold a variety of lenses from a set of spheres, cylinders, and prisms. Trial frames have adjustable eyepieces, temple pieces, and nose pieces, and the examiner should become proficient in adjusting these elements to align the frame properly on the patient’s face. Halberg clips, which can be affixed onto the patient’s own glasses, also hold trial lenses and can be used to allow modification of the patient’s glasses with them in place, a procedure referred to as overrefraction. This term is also used to refer to the refraction, with a trial frame or refractor, of a patient who is wearing contact lenses.

4. 

    Phoroptor:

                The phoroptor, or refractor, provides an alternative to a trial frame and loose lenses that the examiner can dial into position. Most refractors have a variable setting for interpupillary distance and have a tilt feature to allow the eyes to converge for determining near vision correction.

5. 

    Jackson cross cylinder:

                  This instrument is a special lens used as part of the refinement process to confirm first the axis and then the power of a correcting cylindrical lens for astigmatism. The handheld device consists of a handle attached to a lens containing two cylinders of equal power, one minus and one plus, set at right angles to each other. A cross cylinder is usually built into the refractor.

6.  

   Distometer:

                  A distometer is a small handheld device used for determining vertex distance (that is distance between the patient’s eye and the back of the corrective lens). A distometer is used to measure vertex distance accurately. A vertex distance of 13.5mm is considered average but can vary among patients. It is important to keep the vertex distance for the patient’s eyeglasses constant during refractometry. If the vertex distances used for the two eyes differ, the effective power of the patient’s corrective lenses will be different and, probably unacceptable. When the prescription is filled, unless the optician has been informed of a specific measurement, a distance of 13.5mm is assumed for each eye. Vertex distance is especially critical in patients with high refractive errors (more than 5.00D of plus or minus sphere).