To the Editor.
—Drs Dandona et al1 have alerted clinicians to the very limited ability of the current generation of computerized optic nerve analysis devices to reliably perform three-dimensional measurements.Accuracy is limited by the absolute resolution of the computer system. The device described in this article used a display of 512 × 512 pixels. It was noted that, with this system, 10 pixels is equivalent to 100 μm on the disc surface. The angular disparity between the axes of the two images was 5.8°. Geometric examination shows that, with this construction, a depth difference of 97.8 μm is needed to create a 1-pixel disparity on the retinal surface. This factor, by itself, is capable of accounting for the documented variability. It is likely that three-dimensional topographic analysis will become more useful as computer systems with 2000 lines of resolution become available.
Lempert P. Limitations of Computerized Depth Measurements. Arch Ophthalmol. 1990;108(6):779. doi:10.1001/archopht.1990.01070080021010