VI.Practical Aspects – Contd.

How do you know that you are at the FP of the Subject's eye ?.

When you are sitting at 1 metre from the patient, how do you know that you have brought the FP of the patient's eye (by using lenses) to the nodal point of your eye at 1 metre ?.

The technique of Objective method of Refraction (Retinoscopy) will answer these questions.

Principle : Behaviour of the luminous reflex in the pupil of the patient is studied by moving the illumination across the fundus. This behaviour depends on the vergence of the light rays coming out of the pupil. It also depends on the position of the observer. (optics discussed later)

VI.a) Finding 1.When the observer is at 1 metre from the patient and if the rays coming out from the patient's eye form a focus (FP of the patient) behind the observer at a finite distance or at infinity or 'beyond infinity' (virtual point behind patient's retina), then the luminous reflex in the pupil will move in the same direction of movement of the illumination across the retina ('With Movement'). (This finding we get in myopia less than 1 D, in emmetropia and in hypermetropia)



VI.b) Finding 2.When the observer is at 1 metre from the patient and if the rays coming out from the patient's eye form a focus (FP) in front of the observer (between patient and observer) the luminous reflex in the pupil will move in the opposite direction of movement of the illumination across the retina ('Against Movement'). (This finding we get in myopia more than 1 D.)

VI.c) Finding 3.When the observer is at 1 metre from the patient and if the rays coming out from the patient's eye form a focus (FP) at the nodal point of the observer then the pupil of the patient will appear uniformly illuminated. By slight tilt of illumination across retina the pupil will appear uniformly dark. (This finding we get in myopia of 1D and also as the end point of retinoscopy with the observer at 1 metre from the patient.)

Reason for the differences in behaviour of the reflex in the pupil is explained along with the optics of retinoscopy.

Neutral Point or End Point or Point of Reversal in Retinoscopy is reached by bringing the FP of patient's eye to the Nodal Point of observer's eye by using lenses. (except in myopia of 1 D, where it is reached without any lens - already explained).