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Purpose: The HR (prism diopters [PD] per mm of corneal light reflection test [CLRT] asymmetry for strabometry) varies in humans from 14 to 24 PD/mm, but is totally unpredictable. Photo(grammetric) HR calibration in (of) each case facilitates acceptable strabometry precision and accuracy., Method: Take 3 flash photos of the patient with both the preferred eye and then the deviating eye fixating straight ahead and then again with the deviation eye fixing at (+/-5-10 PD) the strabismic angle on a metric rule (stick) one meter away from the camera lens (where 1 cm = 1 PD)., Results: On these 3 photos, make four precise measurements of the position of the CLR with reference to the limbus: In the deviating eye fixing straight ahead and fixating at the angle of deviation. Divide the mm difference in location into the change in the angle of fixation to determine the HR for this patient at this angle. Then determine the CLR position in both the deviating eye and the fixing eye in the straight ahead primary position picture. Apply the calculated calibrated HR to the asymmetry of the CLRs in primary position to determine the true strabismic deviation., Conclusions: This imaging method insures accurate Hirschberg CLRT strabometry in each case, determining the deviation in "free space", under conditions of normal binocular viewing, uncontaminated by the artifacts or inaccuracies of other conventional strabometric methods or devices. So performed, the Hirschberg CLRT is the gold standard of strabometry.

Purpose: To improve the usefulness of the Hirschberg Corneal Light Reflection Test (CLRT), using a photogrammetric technique independent of the size of the pupil, and to devise an easily remembered sequence of numbers corresponding to the fractional displacement of the reflection in various approximate angles of esotropia., Method: Photographs (slides) were taken of the right eye of 25 young adults using a centered strobe light source to produce the corneal light reflection in 5 prism diopters (PD) increments of adduction from 0 to 90 PD to standardize the Hirschberg CLRT for esodeviations. On a large, dual-projection screen, the eye of the subject looking straight ahead (primary position) was projected on one side and the various slides of the same eye of that subject looking into various degrees of adduction were projected on the other side of the screen. Measurements of the distance between the lateral limbus and the corneal reflection were made with a centimeter rule on the screen. The displacement (shift) of the corneal light reflection was expressed as a fraction of the distance between the lateral limbus and the position of the reflection (in relation to the lateral margins of the pupil) with the eye fixing in the primary position., Results: The values found were 1/4 = 30 +/- 3 PD; 1/3 = 41 +/- 4 PD; 1/2 = 59 +/- 4 PD; 2/3 = 75 +/- 6 PD; and 3/4 = 80 +/- 6 PD., Conclusions: An easily memorized similar series of values is 30, 40 60, 75 and 80 PD corresponding to 1/4, 1/3, 1/2, 2/3 and 3/4 PD. This is useful to know which prism to pick up to begin the prism alternate cover test efficiently in a small child.