The Instrument

• A lensometer measures the strength of an existing lens so opticians can best determine the prescription for a person's eyeglasses. The device also helps to properly orient and mark the uncut lenses in order to verify the correct mounting of lenses in certain frames. Lensometers use the diopter values--the refracting power of a lens--from the ophthalmologist in regard to sphere, cylinder, axis, ADD (mainly for bifocals from the difference in the power drum and sphere readings) and prism to set its parameters. The instrument checks the accuracy of progressive lenses, and has the capability of marking the middle of the lens. There are some lensometers that can examine contact lenses as well.
  
  

Function

• The technician puts the glasses onto the lensometer's platform and secures it to ensure accuracy with the readings. The axis turns along with the focus until small lines become pronounced and straight, then the technician records the diopter value for the sphere. The technician shifts the knobs again so now thick lines appear that creates a new diopter value, and when compared to the sphere's lines, makes the cylinder. Depending on the difference with the sphere, the cylinder can have either positive or negative diopter values. Both diopter values are used in a prescription for glasses.
  
  

Parts

• The lensometer has an eyepiece where the technician looks through to determine the diopter values of the cylinder and sphere. The lens holder holds the lens in place, and has a spring-loaded arm to read and mark diopter values. The lens table adjusts a metal shelf where the frames lie to ensure the correct measure of the axis readings for both the cylinder and the sphere. A power drum powers the lens through the use of a large wheel that ranges from -20 to +20 diopters marked in red and black numbers for the technician to read and record the values. Lastly, the axis drum measures from zero to 180 degrees in order to help find the axis location on the lens.