Lensometer with autocollimation and a square Ronchi grid.

ABSTRACT

A lensometer based on an autocollimation system and a square Ronchi grid was designed, constructed, and tested. Refractive powers of monofocal, astigmatic, bifocal, and progressive lenses were measured. The focal plane was identified when no spots, or a minimum number of fringes, are observed on the bironchigram (pattern with a square Ronchi grid). For cylindrical lenses, the spots were transformed in fringes along the $ x $x and ${y}$y directions from which the cylindrical and spherical powers were obtained. For the progressive lenses, a zero spots circle moved on the bironchigram plane along the umbilic zone while the square Ronchi grid was moved along the optical axis. This lensometer is compact, cheap, and precise. Our measurements and errors were very similar to those obtained with a commercial lensometer.