The modern retinoscope is designed as a very compact hand held instrument which has two main systems, these are:
1. Projection system
2. Observation system.
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| Fig :- Retinoscope |
PROJECTION SYSTEM
Projection system projects the light beam through the pupil and illuminates the retina. It contains the following components:
1. Light source
2. Condensing lens
3. Mirror
4. Focusing sleeve
5. Power supply.
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| Fig:- PROJECTION SYSTEM |
Light Source
The light source contains powerful halogen bulb and is at the bottom of the instrument. The streak retinoscope contains bulb with a linear filament to project a streak of light.
Condensing Lens
Condensing lens is a plus lens that lies in the path of light. It gathers the light emitting from bulb and focuses them onto the
mirror. The position of this lens in relation to the bulb can be changed by raising or lowering the focusing sleeve.
Mirror
The mirror bends the path of light at right angle such that the light beam is projected onto the retina. It also facilitates reflected light from the retina to enter the examiner’s eye.
Focusing Sleeve
Focusing sleeve varies the distance between the bulb and the lens. Changing the distance between the bulb and the lens is responsible for plane mirror effect or concave mirror effect. Sleeve up produces plane mirror effect and sleeve down produces concave mirror effect. Sleeve also controls streak rotation. Turning the sleeve rotates the bulb, which in turn rotates the projected streak.
Power Supply
Power supply needed to project the light beam may be generated either through battery or it may be electric operated.
OBSERVATION SYSTEM
Observation system lies towards the practitioner while using the retinoscope to observe the reflex. The system contains a peephole that allows the observer to see the retinal reflex from the pupil. When the observer wiggles the scope while looking through the peephole, he sees the movement of streak projecting on the retina through the peephole. The rays of light emerging from retina are affected by the optical components of the eye. The manner in which they are affected tells us about the optics of the patient’s eye.
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| Fig:- Observation system |
TYPES OF RETINOSCOPE
Basically there are two types of retinoscope:
• Spot Retinoscope
• Streak Retinoscope.
Spot Retinoscope has an ordinary light globe that gives a patch or spot of light . Streak Retinoscope has a special light bulb with linear filament that gives a streak of light. The reflex appears as a red to red-orange glow with a slight shadow around it. The streak has both plano mode and concave mirror mode. The spot has a plano mode only. When streak is used in the plano mode, it is basically a spot with sides cut off. The streak
retinoscope has less illumination and the optical phenomena that takes place away from the center of the streak are obscured. It will be more difficult to observe subtle changes in color or brightness. Generally this information is available with spot retinoscope. However, in practice streak retinoscope is more popular as it facilitates refinement of axis of cylinder correction. In all our further discussion we will consider the use of streak retinoscope.
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