Prism reflex test first described by Krimsky has become universally accepted as a practical method of estimating the size of the angle of squint in patients with a blind or deeply amblyopic eye with or without eccentric fixation.
Patient is asked to fixate on a point light and prisms of increasing strength (with apex towards the direction of manifest squint) are placed in front of the normal fixating eye till the corneal reflex is centered in the squinting eye.
The power of prism required to centre the light reflex in the squinting eye equals the amount of squint in prism dioptres.
To avoid errors from parallax, the examiner must observe the corneal reflex with one eye by sitting directly in front of the deviating eye while keeping his other eye closed.
In an alternative method, prism of increasing power can be placed in front of the deviating eye until the corneal reflexion is centred.
However, since the observation of the corneal reflexion through prisms is difficult, therefore, the method described above is preferred.
Since the test requires only that the patient fixate the light, being entirely objective otherwise, it is useful in testing small children.
It is quicker to perform than the prism cover test.
It can be used in patients in whom the deviating eye has a low visual acuity or has lost central fixation.
Since the angle kappa is included in the measurement, the test is inaccurate.
It is impossible to perform the test for distance fixation, since the position of the examiner's head required to obtain an accurate observation prevents the patient from seeing the fixation light.
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