Interpupillary Distance and Microscopy

The center to center distance between the pupils of your eyes when focussed at normal reading distance (approximately 16 inches or 40 millimeters) describes your interpupillary distance or IPD. In microscopy, this physical attribute must be accomodated to realize the greatest image potential when using a microscope fitted with a binocular (or trinocular) head. There are two critical issues that need to be understood. First, there is a significant difference in IPD between individuals. Variances can typically range from 45-75 millimeters and about 95% of the population has been noted to fall in the 55-72 millimeter range. A microscope head must adjust for these anatomical differences so that there is coincidence between the images formed by the right eye and the left eye. This is exactly the same issue that must be addressed when one is being fitted for eyeglasses. If this condition is not met, not only are the images unclear if visible at all, but nausea to varying degress is experienced as the eyes try to accomodate for this disparity. Long term microscope use under these circumstances becomes difficult if not impossible. Additionally, if the microscope is used to make quantitative measurements, differences in IPD must be compensated for in order for the results to be both reliable and reproducible. This is easily accomplished with a Seidentopf type head, because the IPD adjusment is accomplished without changing the image path length and thus the magnification (i.e. artifact size) of the sample being examined. A well designed, well built modern microscope will almost certainly employ the Seidentopf design. NIKON, UNICO and KONUS all offer binocular and trinocular microscopes that incorporate Seidentopf viewing heads.