Lensometer

A lensometer is an instrument used to measure the spherical power, cylindrical power, and axis of an optical prescription in order to locate the optical center or measure the reference point and determine the base, direction, and amount of prism present.

It is a device for measuring the refractive power prescription of unknown lenses.

The lensometer is also known as a focus meter, vertexometer, refractometer, vertex focus meter, and lens meter, depending on the manufacturer.

There are two types of lensometers:

Manual Lensometer: A manual lensometer can be defined as an instrument used by an optometrist to find the back vertex or front vertex power of glasses. A manual lensometer is portable and can be taken anywhere. However, a person needs to have a better understanding to measure the power of a lens accurately.

Automated Lensometer: It is a fully automatic and well-programmed device primarily used in clinics. It is easy and fast and can print prescriptions. It is less precise compared to a manual lensometer.

Automated and Manual Lensometer

Principle of Lensometer:

Badal optics are disclosed in which the aperture array in an optical system under test is interrogated for light deflection between a set of detectors conjugate with the aperture array of the optical system under examination.

The excursion is measured in a plane normal to the axis of Badal optics instead of observing towards and away from the image focus along the axis of the Badal system.

In the case of an objective refractor, the eye is illuminated on the retina with a test point, preferably infrared light.

An image of the detector array is relayed by a relay lens through the Badal optics to the eye's cornea, the lens under test.

The light emanating from the test point on the retina passes through the eye's lens onto the image of the detector array.

This light undergoes excursion according to the power of the ocular lens at each aperture of the detector array.

The extent of this excursion is determined by a moving boundary locus between the image of the detector array itself.

This excursion is directly related to the ophthalmic prescription.

Parts Of Manual Lensmeter:

Parts Of Lensometer

Uses of Lensometer:

Manual Lensometer: Related Terminology

Rotate the power wheel to the plus sign. Slowly decrease the power and rotate the cylinder axis wheel until one set of lines is uninterrupted and in sharp focus. To read the lens in the form of negative cylinder, first focus on the sphere lines (i.e., the more positive of the two readings). If the cylinder lines come into focus first, rotate the cylinder axis wheel by 90 degrees – this will change the precise set of lines to the sphere lines. The power at which the sphere lines come into focus first is the power of the lens sphere. Record this measurement.

Continue focusing in or towards until the cylinder lines come into focus. The cylinder axis wheel should not need to be moved. (The cylinder lines should focus at a more negative power reading than the sphere lines. Suppose they do not change the axis by 90 degrees and return to step 6 (1). Note this power reading. The lens minus the cylinder power is the difference between this reading and the sphere power reading.

• The axis of the cylinder is read from the cylinder axis wheel.
• Record your final result. Example: -3.00 - 1.50 x 175.
• Repeat the measurement of the back vertex power for the left lens.

Note: When using an unknown lensometer, the following rule confirms the target sphere and cylinder lines: When the cylinder lines are in focus at the most negative power position, the angular position of the line will correspond to the reading on the cylinder axis wheel.

Procedure For Multifocal Lenses 

Note: Most bifocals and trifocals are front surface designs; therefore, front vertex powers must be measured to accurately measure the add power.

This involves rotating the lens so that the front surface is against the lens stop. While this is the most "correct" method for clinically measuring add powers, it is not frequently used.

When the distance and near powers are small, most people do not bother flipping their glasses.

However, as powers increase, significant differences occur between front and back vertex powers, making front vertex measurements highly necessary.

In the lab, we will measure front vertex add powers when the distance sphere power is greater than +/‐5.00 DS.

If the bifocal/trifocal is a back surface design, back vertex add powers should be measured.

b) Measure the posterior vertex power of the distance carrier as you would with a single vision lens and record your results (above the bifocal segment).

c) Focus on the more vertical set of target lines (either the sphere or cylinder lines). Note the power on the power wheel.

d) Slide the lens upward so that the add segment is over the lens stop.

e) Add more power on the power wheel beyond the focus point and slowly decrease the plus sign until the same set of target lines is again in clear focus. Note this power reading on the power wheel.

f) The add power is the difference between these two measurements (steps c and e). It is always a spherical add to the distance power. Therefore, there is no need to measure sphere power of an add segment, cylinder power, and axis location.

g) Record the total power of the lens. Example: OD: +2.00‐1.00×080 Add: +1.25 Repeat for the left lens.

h) Note: generally, but not always, add powers are equal in both lenses of a glasses prescription.

i) If the lens is trifocal, step f should be completed for each segment. The power of each segment is always the difference between the add power and the distance power.

How to Measure Prism on a Lensometer

Step 5: The total horizontal prism is the sum of the prism.