Purchase the Digital Pupilometer from Gulden. This Pupilometer will not only measure the PD, but you can also set the patient's fixation distance with a lever that moves a pair of prisms inside the device. It is powered by AA batteries.
Measures pupil size and PD. The measuring range is 46mm to 82mm. Includes automatic standby after 2 minutes of non-use to preserve battery life. This lightweight unit is powered by 4 AA batteries (not included).