Know Your Trial Lens Set

A standard trial lens set used for clinical refraction contains the following gadgets—

TRIAL LENSES

A comprehensive trial lens set contains a set of lenses that are designed to fit a standard trial frame and encircled by a metal rim for protection in spherical and cylinder powers. Spherical trial lenses have handles, whereas cylinder trial lenses do not have any handle. The set contains following lenses

• A pair of plus sphere lens ranging from +0.12D to +20.00D
• A pair of minus sphere lens ranging from –0.12D to –20.00D
• A pair of plus cylinder lens ranging from +0.12D to +6.00D
• A pair of minus cylinder lens ranging from +0.12D to +6.00D.
The trial lenses are put in the trial frame and the patient is asked to read the test chart through them.

PRISM LENSES

Prism lenses are triangular or wedge-shaped piece of glass or plastic that displaces the bundle of light towards the base. When they are placed in front of the eyes, an object viewed through them will appear to be displaced towards its apex. During the clinical refraction, they are employed to measure the presence and amount of any tropia or phoria. A standard trial lens set contains the following prism lens measured in diopter –0.5Δ, 1Δ, 2 Δ, 3Δ, 4Δ, 5Δ, 6Δ, 8Δ.10Δ,12Δ

PINHOLE DISK

Pinhole disk is a small disk with central opening that prevents peripheral rays of light to pass through. The optimal size of the central opening is 1.2mm. Larger size pinholes do not effectively neutralize refractive error and smaller size pinholes significantly increases diffraction and decreases the amount of light entering the eye. Some trial lens set also contains multiple pinhole disk which also serves the same purpose. The patient finds easier to use multiple pinhole disk as he does not require searching for a single tiny hole. The disk permits the examiner to differentiate between the reduced visual acuity caused by refractive error and the reduced visual acuity caused by some pathological changes within the eyes. In general visual acuity that is not improved with pinhole disk, cannot be improved by the use of lenses.

STENOPAIC SLIT

Stenopaic slit is an elongated pinhole which is used to allow the light transmission in one meridian and is a useful tool for the correction of astigmatism. The width of the slit aperture ranges between 0.50–1.00mm and the length is 15mm and is assumed to limit the light admission to one meridian. When slit is used for meridional refraction, only spherical trial lenses are used to correct 

the refractive error in different meridians.

RED AND GREEN LENS

The standard trial lens set also contains one red and one green filter lens (Fig. 2.5). Some trial lens also contains one yellow filter. These filter lenses are used to check over-correction and under-correction of refractive error. Under ordinary condition of illumination, the eyes are emmetropic for yellow, myopic for green and hypermetropic for red. With each eye occluded in turn, the uncovered eye views the best line first through the red filter, which is rapidly exchanged for green filter uccessively.The patient is asked to make a comparison of definition through the filter to notice the clarity of letters. If the patient states:

• No apparent difference between the two, it implies optimum correction is present.

• Vision is clearer with red, the eye is slightly fogged. A small minus can be added until equality is achieved.

• Vision is clearer with green, the eye is hypermetropic and a plus lens can be added until equality is reached.

• If red and green appear clearer alternatively before any adjustment is made in the correction, this indicates accommodation is active. In such case it is better to abandon the test. Red and green filters are also used for macular perception test. If the patient can identify red and green color, macula can

be taken as functioning. They are great tool for Worth 4 Dot test to ensure eye teaming or to detect diplopia. Yellow filter is sometimes used to break the confusion of the patient to identify correct observation.

MADDOX ROD

Maddox Rod is a group of either red or colorless parallel rods taken together act as a cylinder. The purpose of the Maddox Rod is to dissociate the two eyes and prevent the images to fuse. It accomplishes its function by changing the size, and shape of a point source of light. It is useful tool to detect tropia or phoria.

OCCLUDER

During a routine eye examination, the practitioner often needs to check one eye’s acuity compared to other. Examiner uses a set of special eye covers known as occluders. Occluders usually prevent the full penetration of light. There are also translucent occluders which creates blurry effect over the eye.

TRIAL FRAME

A trial frame is used to use the trial lenses for refraction. It is designed to accommodate 3–4 lenses for each eye. It can be adjusted from its earpiece and bridge to rest behind the ears and to alter interpupillary distance. Some trial frame also have an adjustment facility to alter its pantoscopic tilt. The front surface of the front lens cell of trial frame is marked with degree from 0 –180°. The new design of trial frames have a thumbscrew mechanism on the side of the trial frame to rotate the front lens carrier. Modern Phoropters or refractors are an alternative to trial lens set. All the lenses in the phoropters are inbuilt with the help of motors. There are basically two types of phoropters—Manual

and Automatic. Two spherical lens assemblies and two cylinder lens assemblies are housed in the phoropter for each eye. Cylinder axis is altered by turning a knob for each eye that rotates the axis through meridian from 0–180°. Two cross cylinders, two rotary prisms, pinhole, stenopaic slit, occluders, red lens and polarizing analyzer are also included on accessory post. The

patient is asked to be seated behind the phoropter. Horizontal and vertical position of the lens apertures are determined by the mechanical arm adjustment suspending the phoropter. The

adjustment knob for IPD is used to adjust the geometric centers of apertures before the eyes. Pantoscopic tilt can be incorporated by adjustment of the swing connection between the phoropter and the mechanical arm. The light mounting provides illumination of the near test card. Newer generation phoropters also provide additional feature to tilt during the course of reading to ensure

natural reading posture. The only drawback of phoropter is the eyes of the patient is hidden behind the phoropter which means the practitioner cannot see patient’s reactions and operational flexibility is less as it is fixed at one place. It is difficult to have eccentric viewing which may be a reason why it is not suitable for visually impaired patient. However, there are certain advantages of the phoropter. They can also measure phoria (natural resting position of the eyes), accommodative amplitudes, horizontal and vertical vergences, and many more.

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