Lensometry

Fig:- Lensometer

Aim

Lensometry is used to ascertain the spherical, cylinder and axis of an ophthalmic lens. It is also used to locate the optical center and to determine the base direction and the amount of prism present.

Targets

Reticle inside the lensometer is used for focusing the instrument with the help of eyepiece and to determine the prism power. The reticle is a permanently etched series of concentric rings.

It also contains orientation lines for each lens meridian and a protractor scale. Each ring denotes one prism diopter. Target consists of two sets of illuminated lines perpendicular to one another for reading the power of the lens. 

These lines are focused by the power wheel. They are closely spaced. Target types vary with different brands and different models of lensometer.

Three common types of targets are most commonly seen:

Fig:- Different target and reticles types

• Star burst type

• Cross line type

• AO type cross line.

Critical Factor

Lensometer does not read lens prescription, it measures the powers of the lens in each meridian. In order to distinguish the sphere and cylinder lines for your particular lensometer, with no lenses in place set the axis wheel to 180° and focus the lines. The vertically oriented lines are the sphere lines and the horizontally oriented lines are the cylinder lines. Keep the lensometer in switch off condition and focus the reticle with the eyepiece. Then switch on the instrument. At this time the power wheel should actually read zero diopter because there is only air in place. But often a small offset may be detected. Findings should be adjusted for any offset observed at this time.

Step by Step Procedure

1. Sit right in front of the lensometer and focus the black reticle with eyepiece. If it is not in focus, the reading will be erroneous. This is to be adjusted to accommodate the user’s own refractive error.

Fig:- Adjusting the eye piece of the lensometer

2. Now switch on the lensometer, set the axis wheel on 180° and bring the reading scale to zero and see whether the illuminated targets and the black reticle both are sharply in focus.

Fig:- Switching on the lensometer

3. Place the back vertex (ocular side) of the lens against the stop of the lensometer on the platform or if the lens is fitted in a spectacle frame place the spectacle lens so that, the temples of the frame is away from you. Make sure both eye rims-right and left are in contact with the platform.

Fig:- Placing spectacle lens onto the platform

4. Center the lens by moving it on the platform so that the illuminated target is aligned in the center of the reticle by moving the lens side to side or up and down. In case of prism, it will be difficult to do so.

5. Rotate the power wheel to focus the targets sharply and observe the two illuminated lines which are perpendicular to each other. If both the lines are in focus together, the lens is said to have only spherical power. The star burst ring is seen as a well-defined circular ring made of identifiable dots. The amount of spherical can be read on the power reading scale. Red letter shows minus power and the other shows plus power.

6. If only one set of lines is in focus and the other is blurred, the lens is said to have cylindrical power. Since cylinder lens forms the line image, the circular star burst ring made of dots is seen as oblong shape image made of a well-defined lines which are focused twice on two principal meridians.

Fig:- Power reading scale inside the lensometer

7. Cylinder power may either be planocylinder or spherocylinder. In case of planocylinder the spherical line will be focused when the power scale reads zero and cylinder line at some number which will show the amount of cylinder present in the lens. In case of spherocylinder lens, the spherical line will be focused first at some number which will show the spherical component and then the cylinder line at some other number which will show the summation of spherical and cylinder. The difference between the two will give the cylinder element.

8. Axis will be seen when the cylinder line is focused sharply.

Orient the axis wheel of the lensometer such that the cylinder lines are perfectly continuous together with the oblong illuminated image. Read the axis from the axis wheel. When the axis is not correctly positioned, the lines will appear “broken” or as if a “gates were left open.”

9. Axis can directly be seen by rotating the protractor ring. The

black line when parallel to focused cylinder line will point at the axis on the protractor.

10. Before removing the lens, put the dot on the optical center of

the lens using the ink marking device. While doing so, make sure target should be right at the center of the reticle.

11. To ascertain the power of the multifocal lenses, following

additional steps are needed:

a) Read and record the power of the distance portion as above.

b) Turn the lens around so that the ocular surface faces you.

The bifocal segment power is measured with temples pointing towards you (FVP) when the lenses are fitted in the spectacle frame.

c) Raise the platform up and check the power through the near segment area.

d) Compare the spherical power through the near segment to the spherical power through the distance portion. The difference between the two is taken as near “add”.

12. If the lens is ground with prism, it may be impossible to center the target in the reticle. Mark a dot at the point where the patient’s interpupillary distance coincides. 

Fig:- Prism shifts the star burst circular ring position

Align this dot at the center of the reticle. Read the position of the star burst circular ring with respect to the number of circle in the reticle. Each circle denotes one diopter of prism.

13. Record the readings so derived.