An instrument to measure the distance between the anterior pole of the eye and a fixed reference point, often the zygomatic bone.   http://www.audea-med.com

Characteristics:   
The product can precisely measure the level of exophthalmia over the orbital margins, on the principal basis that both vertexes of cornea and scaleplate are projected to the front of prism by total reflection on the bevel, with the special structure of the isosceles right-angled triangle prism.   

Specifications:   
Measuring Range of the Exophthalmia: 0～35mm   
Adjustable Range of the Orbital Distance：80～120mm