A Novel Technique of Measuring Distance Stereo-Acuity Using TNO Chart and Inverted Telescope

In a strabismus clinic, measurement of distance stereopsis is particularly useful in the management of intermittent exotropia and esotropia of divergence paralysis type. This is mainly because reduction in the distance stereopsis is a sensitive sign of early deterioration of these strabismic disorders and generally makes a case for the surgical intervention [10]. Various tests have been used for the measurement of stereoacuity for the near [11]. Titmus/Wirt Fly test, Randot test and TNO test are among the more popular ones. For the distance stereopsis, the Mentor BVAT SG [12] and Frisby Davis distance stereo test (FD2) [13] are fairly accurate and versatile. However, the distance stereoacuity tests are not commonly available in the clinics and they are relatively expensive. In the present study, we have explored the option of using the TNO test that uses red-green random dot stereopairs (anaglyphics) with an inverted telescope to measure the global, crossed disparity induced, central stereopsis. To our knowledge this is the first paper in the literature to optically induce the distance using a telescope in an inverted fashion and combine its use with a near stereoacuity assessment chart for the measurement of distance stereoacuity.