To the Editor.
—Thompson1 has studied the afferent pupillary defect and has increased the precision of its measurement by placing neutral density filters in front of the normal eye sufficient to equalize the direct and consensual reflexes.2 A portable device has been recently developed and marketed that uses rotating double polarizing filters to produce quantifiable neutral density filters over either eye. I have found it useful in quantification of afferent pupillary defects.A spectacle with bilateral, rotating, polarizing filters is available as a part of the Threshold Amsler Grid (Stereo Optical Co, Inc, Chicago, Ill). Each ocular is calibrated so rotation of the polarizing filter can be measured from 0° to 90° in 5° increments. The illumination through an ideal double polarizing lens, I, is related to the maximal illumination through a double polarizing lens, I0, by the relationship