Overview of Lensometry
Lensometry is a process that checks the power and prescription of your eyeglasses or contact lenses. This is done using a machine called a lensmeter, which is sometimes also called a focimeter, vertometer, or Lensometer™. The main purposes of lensometry are to measure the power of your glasses during an eye exam and to check the accuracy of eyeglass prescriptions made in an optical lab.
Lensometry works based on the idea of lens neutralization. This means the lensmeter measures how much power is needed to balance out the effect of the lens so it acts as if it has no power at all. The machine uses adjustable lenses within it to do this. There are different types of lensmeters – some are manual, while others are semi-automated or fully automated.
With the help of a lensmeter, it’s possible to measure different aspects of a lens – like its dioptric power (how strong it is), its optical center (where on the lens you should look through), cylindrical axis (the orientation of the astigmatism), and prism (decentring of the lens). Some lensmeters can even measure how dark the tint is on a lens, how much light it lets through, and how much it blocks ultraviolet light.
Briefly, lensometry is important in eye care because it allows professionals to check your glasses or contact lenses for correcting vision issues accurately. This not only helps to ensure you are getting the best possible vision correction, but also that you are using the right glasses or lenses.
Why do People Need Lensometry
Lensometry is a process used for various reasons including: figuring out the strength, central point, and prismatic power of a lens, making a mark on the optical axis and aligning the lens before it is fitted. Simply put, this process helps us understand how powerful a lens is, where its center is, its refractive power and how it might bend or refract light. This information is crucial before fitting the lens to make sure it’s suitable and correctly aligned.
When a Person Should Avoid Lensometry
There are no situations or conditions that would prevent a person from having lensometry. This means anyone can safely undergo this procedure.
Equipment used for Lensometry
You can find different types of manual devices for checking the strength of your eyeglasses lens available in the stores (see Image. Manual Lensmeter). A typical lens-checking device usually has the following parts:
- Power switch: This part is used to turn the device on and off.
- Illumination system: This is the device’s light source, which can either run on batteries or be connected to the power supply in your home. Those lens-checking devices that you need to plug in a power source might even include a transformer.
- Focusable eyepiece: This component allows you to view the measurement scales, and you could adjust it according to your own vision before using the equipment.
- Power wheel: This is a rotary wheel that indicates the strength of your lens in a unit called diopters. The strength numbers might either be carved on the wheel or indicated inside the device.
- Axis scale: It’s a semi-circular part of the device marked with degrees from 0 to 180°.
- Reticle: This is a black target inside the device that helps you align the lens correctly and read the lens strengths.
In addition, there’s a special table or opening (lens stop aperture) where you read the lens if it’s flipped with its back surface away from you. The lens needs to make full and proper contact with the table or opening to avoid any mistakes in reading.
- Lens holder: This is a spring-loaded attachment used to hold the lens against or release it from the lens stop aperture or instrument table.
- Lens height adjustment knob or lever: You can use this part to raise or lower the instrument table. It allows you to switch easily between checking near and distant parts of a lens sample.
- Lens ink well or marker: There are pins set in temporary ink wells that you can raise and adjust on the lens to mark 3 reference points. This is particularly helpful in marking the lens’s optical center after checking.
- Prism control system: This optional accessory includes a prism control knob, axis scale, and power scale.
Automated or semi-automated lens-checking devices work slightly differently but are fairly similar to manual ones (see Image. Semi-Automated Lensmeter). These types of devices typically include:
- Power switch: This part is used to turn the device on and off.
- LCD screen: This screen displays the lens parameters.
- Lens-holding device: This is used to hold the lens in place.
- Lens frame table: This provides a place to rest the lens.
- Printer: A printer might be built into the device or be a separate add-on.
- Lens marker lever and pins: These are used to mark the lens.
- Nosepiece holder
Who is needed to perform Lensometry?
A person who has been properly taught and has proven their skill can do a lensometry. Lensometry is a test that measures how strong your glasses need to be. This includes anyone from eye doctors to trained medical staff. As long as they have received the right training and have shown they can do the process correctly, they are allowed to carry out this test.
Preparing for Lensometry
Before using a device called a lensmeter, it’s important to ensure the lens you’re measuring is clean and ready. A lensmeter is a machine that measures prescription lenses, and can tell if a lens is of single-vision or multifocal. It can also show if any color tints or special coatings that darken the lens when exposed to sunlight are present. The machine can be used to measure different types of eyeglasses lenses, including naturally occurring materials such as CR-39, as well as synthetic lenses like those made of polycarbonate.
For accurate results, the lensmeter needs to be calibrated and properly prepared before use. Here’s how it’s done:
First, the user needs to look into the eyepiece towards the reticle, which is a black target inside the machine. By doing this, the machine is adjusted specifically for the user’s eyesight. The reticle also includes a protractor tool to figure out the axis of the cylinder-shaped lens. Now, the adjustment dials must be zeroed out and the light source should be correctly aligned. The eyepiece is then rotated until the reticle is at its sharpest view. Placing a white piece of paper on the measurement table can make it easier to see.
After the lensmeter is switched on, you then rotate the power wheel to zero position. At this point, the illuminated lines known as mires should be in their sharpest focus. If they aren’t, adjust the power wheel in both directions until the sharpest focus is found. This number, if it’s not zero, is recorded as an instrument inaccuracy . This information regarding instrument inaccuracy can either be used to adjust the device’s calibration or subtracted from the final prescription’s spherical power, which is the lens’s curvature, for each lens measured with this device.
How is Lensometry performed
Measuring the strength of eyeglass lenses involves using a manual lensmeter, a device that has been calibrated to ensure accurate readings. This process consists of the following steps:
1. Put the lens onto the lensmeter stage with the outward-curving side facing upwards.
2. Arrange the lens by placing a target in front of it and aligning it in the center of the lensmeter’s viewfinder.
3. Adjust the sphere and cylinder dials on the lensmeter to focus on the target and determine the lens’ refractive power, which is measured in units called diopters.
4. If the person has a prescription for two lenses, repeat the process for the second lens.
5. Check your measurements against the known—usually prescribed—strength of the lenses, making adjustments to ensure accuracy.
6. Adjust any slight mistakes in the device’s results to get the final strength value of the lens.
Cylindrical lenses, which bend light differently at different angles, are a little trickier to measure. Their strength can vary depending on the direction, so you need to measure two primary directions (or ‘meridians’) separately before combining these into a final strength. Here’s how it’s done:
First, record the strength of one meridian. Then, rotate the lens 90 degrees and measure the other meridian. The strongest value will be the sphere, or the part of the prescription that corrects for nearsightedness or farsightedness. The difference between the two measured values is the ‘cylinder,’ which represents the astigmatism, or inability of the eye to focus light evenly. The final part of the prescription is the ‘axis’ or the angle at which the cylinder is oriented.
For example, if the two measured powers are +1.00 (at 90°) and +1.50 (at 180°), the sphere is the higher value (+1.50) with the cylinder being the difference between the two strengths (-0.50) and the axis the direction at which the weaker power was measured (90°). So, the final prescription would be +1.50 -0.50×90.
Multifocal lenses, which include varifocal and bifocal lenses, contain different strengths for different purposes, typically for distance-viewing and reading.
To measure them, you first measure the strength in the upper section of the lens for distance viewing. Then you measure the lower or reading section strength. The difference between the two readings is the additional strength added for reading purposes. For varifocal lenses, if the manufacturer’s guidelines aren’t present, you can find the midpoint of the horizontal markings on the lens where the distance-viewing power can always be found.
If your measurements for the lens’ center and the prescribed center don’t match, it could lead to what’s called the prism effect. This can be calculated using a formula (P=DxC) where P is the prism value, D is the strength of the lens and C is the amount the center of the lens has shifted. This prism effect might be helpful for managing double vision, but it can also be a source of eye strain.
Possible Complications of Lensometry
There are no known risks or problems associated with lensometry.
What Else Should I Know About Lensometry?
Recently, the development of adjustable lenses has led to more research into the study of dynamic focimetry, which is the measurement of lens power changes in real time, with minimal or no need for human assistance. Such technology can be useful for measuring the power of lenses placed inside the eye during cataract or refractive surgery, as described by Garcia-Domene and colleagues in their research.