What is Lensometer? It’s Working, Parts and 5 important Uses

What is a lensometer?

lensometer or lensmeter refers to an ophthalmic device . Sometimes, the lensometer is called a focimeter or vertometer. The procedures and techniques that are used for operating a lensometer are referred to as lensometry or focimetry. The lensometer is commonly used for verifying the accurate configuration of a pair of eye lenses by optometrists and opticians. 

PawduganLensmeter, marked as public domain, more details on Wikimedia Commons

Who invented the lensometer?

Antoine Claudet designed the photographometer in 1848. This device was used for measuring the intensity of photogenic rays. In the following year, he designed the focimeter that was used for finding the correct focus in photographic portraiture. Hermann Snellen designed an instrument known as the phakometer in 1876. This instrument has a set up similar to the optical bench and was used to measure the power and location of the optical center of a convex lens. It was in 1912 that Troppman proposed the idea of a direct measuring instrument.

A patent was filed in 1921, by Edgar Derry Tillyer from the American Optical Company proposing the first lensometer. This design projected the measurements of the target on a screen in order to eliminate the requirement for correction of the refractive error of the observer and to reduce the need of peering down a telescope into the device. 

What is the lensometer used for?

  1. The lensometer helps orient appropriately and mark uncut lenses, verifying the power of single vision, bifocal and trifocal lenses, and checking the right mounting of eye lenses in spectacle frames. 
  2. The device is used by the ophthalmologists or optometrists to evaluate certain parameters specified on the patient’s prescription, such as cylinder, axis, sphere, add, and in some cases, prism. 
  3. This instrument is often used to verify the accuracy of progressive lenses.
  4. It is also used to determine and mark the center of a lens and several other physical measurements that are essential for the proper functioning of the lens. 
  5. At times, the lensmeter is used for a prior examination of a patient’s eyes to match with the previous prescription that the patient was given. 

How does a lensometer work?

lensometer working principle:

A lens having a focal length (f) is used to image a target (usually a crossed set of lines). The user then places the spectacle lens (under test) at the lens’s rear focal point (f). The light rays emerging from the spectacle lens then pass into an eyepiece having an internal reticle. The target is axially shifted simultaneously by the user until it is in focus with the reticle. Then, a condition occurs when the light rays emerging from the test spectacle lens becomes collimated.

The formula relates the target position d and the power of the spectacle lens Φ: 

The axes of a spherocylinder lens can be aligned by rotating the target. Only one set of target lines can be focused on when the cylinder is present. To focus the orthogonal group of lines, the lensmeter needs to be readjusted. The cylindrical power of the lens is the difference between the powers of the two focus positions. Shifting the lens position in front of the eyepiece aperture can measure the different zones of a progressive addition lens. The recent version of the instrument is automated and digital.

What is the working principle of the lensometer?

The working of a manual lensometer is grounded on the Badal principle. According to the principle, when we place the eye at the focal point of a positive lens, we can detect an object’s virtual image, between the anterior focal point and the lens subtending the same visual angle.

The two most common lensometer varieties, as seen in optometric and ophthalmic practices and optical dispensing, are the manual lensometer and automated (digital)) lensometer. Generally, green or yellow lights are used in the lensometer as a target to remove the chromatic aberrations. The measurable power range of lenses lies between +20.00D (diopter) and -20.00D.

What are the parts of a Manual Lensometer?

1 .Adjustable eyepiece 2 .Reticle 3 . Objective lens 4 . Keplerian telescope 5 . Lens holder 6 .Unknown lens 7 .Standard lens 8 . Illuminated target 9 . Light source 10 . Collimator 11 . Angle adjustment lever 12 . Power drum (+20 and -20 Diopters) 13 .Prism scale knob
Image source: TamasflexLensmeter-2CC BY-SA 3.0

Parts of a Manual Lensometer

A typical lensometer contains the following parts:

1. Eyepiece

The eyepiece used for obtaining reading accuracy is mounted over a screw-type focusing mechanism. A rubber guard is also placed n the eyepiece to avoid the scratching of the user’s eyewear. 

2. Prism Compensating Device Knob

The Prism Compensating Device Knob helps in reading prism amounts that are greater than five prism diopters. 

3. Chrome Knurled Sleeve (Lensometer Reticle Adjustment Knob)

The Chrome Knurled Sleeve helps in rotating the lensometer Reticle to align the prism base. 

4. Lens Holder handle

The lens holder handle is present to hold a lens in place alongside the aperture. 

5. Spectacle Table Lever

The spectacle table lever helps in raising or lowering the level of the spectacle table according to the user.

6. Spectacle Table

The spectacle table provides a resting place for the spectacle frame when the lens power is being neutralized.

7. Marking device control (Lens Marker)

Lens markers are pins controlled by the handle and used to mark the lens at the optical center or the prism reference point.

8. Power Drum

The power drum is a handwheel that has numbered scale readings ranging from +20.00 to -20.00 Diopter. The scale of the reading interval is in 0.12 diopters steps, and for higher powers, the interval is of 0.25 diopters steps.

9. Prism Axis scale

The prism axis scale helps in the orientation of the prism axis.

10. Prism Compensating Device

Prism compensating device helps to verify a large amount of prism.

11.Prism Diopter Power Scale

The prism diopter power scale displays the prism amount.

12. Locking Lever

The locking lever helps in elevating or depressing the position of the instrument according to the user’s height or posture.

13.Cylinder Axis Wheel

The cylinder axis wheel helps in orienting and neutralizing the cylindrical axis.

14. Filter Lever

The filter lever can incorporate or remove the green filter.

15. Lamp Access Cover

The lamp access cover is used for changing/altering the manual lensometer bulb.

What is a Digital lensometer?

Digital lensometer or auto lensometer uses the green LED light capture techniques and auto lens detection techniques for determining the accurate power of single, multi-focal and progressive lenses. Once the lenses are aligned correctly with the lensometer monitor, the device automatically measures and gets the UV transmittance of the lens.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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