Exophthalmometer

Exophthalmometer is an instrument used for measuring the degree of forward displacement of the eye in exophthalmos. The device allows measurement of the forward distance of the lateral orbital rim to the front of the cornea. Exophthalmometers can also identify enophthalmos (retraction of the eye into the orbit), a sign of blow-out fracture or certain neoplasms.

There are several types of exophthalmometers, Hertel and Luedde measure the distance of the corneal apex from the level of the lateral orbital rim while Naugle measures the relative difference between each eye:

The normal range is 12–21 mm. Upper normal limit for people of African origin is a little higher, about 23–24 mm.

A difference greater than 2 mm between the eyes is significant.

In children and teenagers mean exophthalmometric measurements increase with age: Less than 4 years old (13.2 mm), 5–8 years old (14.4 mm), 9–12 years old (15.2 mm) and 13–17 years old (16.2 mm).

Axial Length of the eye affects exophthalmometer reading. Pseudoproptosis may be seen in severe myopia.

...
Wikipedia