Lensometer Definition, Parts Names and Functions, Reading

Content:

• What is Lensometer?
• Lensometer Definition
• Lensometer Parts
• Lensometer Names
• Lensometer Functions
• Lensometer Reading

What is Lensometer?

A lensometer, also known as a lensmeter or focimeter, is an optical instrument used to measure the prescription of eyeglass lenses. It determines the sphere, cylinder, axis, and in some cases, the prism of the lenses. This device is crucial for optometrists and opticians to verify the correct prescription in a patient's glasses, ensure proper alignment of multifocal lenses, and to measure and mark the optical centers of lenses.

Lensometer Definition

A lensometer, also known as a lensmeter or focimeter, is an optical instrument designed to measure the prescription of eyeglass lenses. It determines the sphere, cylinder, axis, and, in some cases, the prism of the lenses. This device is essential for verifying the correct prescription in glasses, aligning multifocal lenses, and marking the optical centers.

Lensometer Parts

A lensometer consists of several key components, each serving a specific function in the process of measuring and verifying eyeglass lenses. Here are the main parts of a lensometer:

1. Eyepiece: The part through which the operator looks to focus on the internal scales and reticle of the lensometer.
2. Lens Holder (Lens Table): The platform where the eyeglass lens is placed and held securely during measurement.
3. Lens Stop (Lens Rest): A small support that ensures the lens is properly centered and positioned in the optical path for accurate readings.
4. Power Drum/Scale: A rotating dial or digital readout that indicates the lens power in diopters. It helps measure the sphere and cylinder power of the lens.
5. Axis Wheel: A control used to determine the axis of the cylinder component of the lens prescription.
6. Prism Compensator: A component used to measure the amount and direction of prism present in the lens.
7. Marker Pens/Ink Pads: Used to mark the optical center and other reference points on the lens.
8. Adjustment Knobs: These knobs allow for fine-tuning the position and alignment of the lens to ensure precise measurements.
9. Illumination System: Provides the necessary lighting for viewing the internal scales and reticle clearly. It typically consists of a light source and mirrors or lenses to direct the light properly.
10. Reticle (Target): The pattern seen through the eyepiece, usually consisting of crosshairs or concentric rings, used for aligning and focusing the lens.
11. Viewing Window (Field Lens): The area where the lens is viewed and the measurements are taken, often including a built-in magnifier to aid in precision.
12. Base and Stand: Provides stability to the instrument and may include leveling feet to ensure accurate readings.

Lensometer Names

A lensometer is also known by several other names, which can vary depending on regional preferences or specific use cases. Here are the common alternative names for a lensometer:

• Lensmeter
• Focimeter
• Vertometer

These terms are generally used interchangeably in the optical industry to refer to the same instrument designed for measuring the prescription of eyeglass lenses.

Lensometer Functions

A lensometer is an ophthalmic instrument primarily used by optometrists and opticians to measure the characteristics of a lens, including:

• Power (sphere, cylinder, axis) 
• Optical center location 
• Measure reference point location 
• Prism power and direction 

The lensometer is used to verify the correct prescription on a pair of eyeglasses, properly orient and mark uncut lenses, and confirm the correct mounting of lenses in spectacle frames.  It can also be used to check the power of contact lenses if a special lens holder is used. 

Lensometer Reading

Here is how to read a lens using a lensometer:

1. To neutralize a spherical lens:
2. Rotate the power wheel to a high minus reading where the lines are blurry.
3. Rotate the power wheel counterclockwise until the lines come into focus. The number on the power wheel indicates the spherical power. Remember to indicate whether the reading is plus or minus (e.g. +4.00 or -2.00).

To read the cylindrical power:

1. After finding the spherical power, turn the power wheel to lower the power slowly.
2. As you do this, the dividing line will go out of focus and two other similar lines will move in and out of focus.
3. Turn the power wheel until the third line is completely in focus and read the scale.
4. Subtract the second reading from the first to determine the cylindrical power of the lens.

To read the axis:

1. The axis is indicated by the angle on the axis adjustment wheel.

To read the prism power:

1. The prism power is indicated by the position of the center of the target with respect to the eyepiece reticle. The innermost ring represents 1 prism diopter, the second ring 2 prism diopters, and so on.

To read the bifocal add power:

1. Move the glasses so the bifocal segment is being read.
2. Turn the focus knob again until the fat lines become focused.
3. The amount of change needed to refocus the lines will give you the bifocal add power.