auxetometer

An auxetometer is an optical instrument designed to determine the magnifying power, or magnification factor, of a lens or lens system. It does this by comparing the angular size of an object as seen through the optical system under test against its apparent size when viewed with the naked eye from the same distance. The result is a dimensionless number, expressed as 2x, 10x, and so forth, indicating how many times larger the image appears than the object itself.

The instrument typically consists of an eyepiece aligned with a graduated reticle or scale, a target or test object at a fixed distance, and optical alignment apparatus. A user looks through the eyepiece at the test object, then without the lens system, and compares the angular extent of the object in each case. Some designs use a split-field arrangement where both images are presented side by side for direct comparison. The magnification is calculated from the ratio of the angular sizes, or read directly from a calibrated scale if the instrument uses that method.

Applications and variants

Auxetometers are most valuable in quality control for magnifying glasses, microscope objectives, telescope eyepieces, and projection lenses. They confirm that a lens system meets its rated magnification before it enters service or after maintenance. Variations exist: bench models with fixed geometry suit production testing, while portable designs allow field verification. Some auxetometers incorporate wavelength-specific illumination to account for chromatic effects, since magnification can vary slightly across the visible spectrum.

A key limitation is that an auxetometer measures apparent angular magnification only. It does not directly assess optical quality, aberrations, resolution, or light transmission. A lens can meet its magnification specification while still performing poorly due to spherical aberration, coma, or low contrast. Consequently, magnification measurement is one of several tests in a full optical acceptance protocol.

The name derives from the Greek auxein, meaning to increase or enlarge, and the Greek metron, meaning measure. The term has been in technical use since the 19th century, particularly in microscopy and precision optics trades. It is less common in modern parlance than in the early 20th century, partly because integrated magnification checking is now often built into production test stands rather than performed as a separate manual step.