An ophthalmic instrument. It is mainly used by optometrists and opticiansto verify the correct prescription in a pair of eyeglasses, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. Lensmeters can also verify the power of contact lenses, if a special lens support is used.
A simple lensmeter cross sectional view.
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
How to use a Focimeter:
Optics of Focimeter
How To Use A Lensmeter
1. Get Comfortable
Position the focimeter at a comfortable angle for viewing by using the locking lever or inclination control at the side of the instrument (Figure 2, label 10) to adjust the tilt of the focimeter. This is also a good time to ensure the prism compensator is at 0.
2. Focus the Eyepiece
as viewed through eyepieceIt is essential to focus the eyepiece for your eye if someone else has used the focimeter before you. The steps are:
- Keep the focimeter power off.
- Rotate the eyepiece counter-clockwise until the reticle is blurred. A white card or piece of paper held behind the eyepiece may make the reticle lines more visible.
- Turn the eyepiece clockwise until the reticle is just clear. The lensometer is now adjusted for your eye. Note: Do not turn past the point at which the reticle is first clear.
3. Check Power Calibration
Ensure the power calibration of your focimeter is accurate. This should be done occasionally.
- Turn the focimeter on.
- Turn the power wheel into the plus, then slowly decrease the power until the focimeter target (sphere and cylinder lines as seen in Figure 2) is sharply focused. Do not oscillate the wheel back and forth to find the best focus. The power wheel should read zero if the instrument is in proper calibration.
- If the power wheel does not read zero, re-focus the eyepiece and re-check the calibration. If the power wheel still does not read zero, the error must be compensated for on all future measurements made with the lensometer, or the lensometer needs maintenance. (Note: algebraically subtract the calibration error from the power measurement to compensate for calibration errors.)
4. Position the Spectacle Frame
- Turn the spectacles so that the front of the spectacles is facing towards you. The temples (the arms of the spectacle frame) should be pointing away from you.
- Put the spectacles on the frame table. The bottom rim of the spectacles should rest on the frame table.
- Clamp the spectacle lens to keep it pressed against the lens rest. Note:always start with the right lens (for consistency)
- Look through the eyepiece and move the spectacles side-to-side and up-and-down until the target is in the centre of the black graticule.
- Change the height of the frame table to keep the frame horizontal in this position (that is, with the bottom rims of the spectacle against the table to make sure one side does not drop down)
5. Mark the Optical Centre
- When the lens is positioned as above – where the centre of the focimeter target is over the centre of the graticule – then you are at the optical centre of the lens. Note: if there is a ground prism in the lens, despite movements of the lens up-and-down or side-to-side, the focimeter target will not be centred over the graticule – so there is no optical centre of such a lens.
- Mark the optical centre. Focimeters usually have an ink well and marking pins. When the lens is centred correctly, you can use the marking pins to put a mark (usually small dots) on the lens surface. If the vertometer does not have an ink well or marking pins, you can use a marking pen (felt tipped pen) to mark the optical centre of the lens yourself. You will need to make the mark on the lens directly over the lens rest.
6. Determine the Right Lens Power
- With the right lens positioned at its optical centre, turn the power wheel to a high plus reading.
- Slowly decrease the power (reduce the plus by turning the power wheel) until the target lines just become clear (Figure 3).
a. If the Lens is a Sphere:
- All the target lines will come into focus at the same time (Figure 3)
- Simply read off the power drum to determine the spherical power
b. If the Lens is Astigmatic:
- Only some of the lines are clear at a given power (Figure 4)
- Turn the power wheel to a high plus reading and slowly decrease the power until some of the target lines just become clear.
- Turn the axis wheel until the three parallel target lines are straight and unbroken.
- The number on the power wheel at this point is the most positive meridian of the lens. This will be the spherical powerwhen you write the astigmatic lens prescription.
- Slowly turn the power wheel to decrease the power until the other line is clear. The number on the power wheel will now tell you the power of the least positive meridian of the lens.
- Find the cylindrical power of the lens: this is the difference in powers, ie the more positive power subtracted from the more negative power (negative cylinder)
- Find the axis of the cylinder. The axis of the cylinder is the direction of the second power reading, ie the least positivepower. The direction of this line is measured by aligning the graticule and looking at the axis numbers on the graticule inside the eyepiece.
- Example: Reading 1 is +3.00D x 90 and Reading 2 is +2.00D x 180. The lens is: +3.00/-1.00 x 180
7. Determine the Left Lens Power
- Without changing the height of the lens table, clamp the left lens in position
- Centre the lens by moving it left-and-right as appropriate
- There should be no need to move the lens up-and-down: the focimeter target should be vertically aligned with the graticule at the table height used for the right lens. If it is not, then there is vertical prism between the lenses, which can be measured by the markings on the graticule.
- Once aligned at the optical centre, mark the optical centre of the left lens and measure the power of the left lens using the same steps as for the right lens above.
8. Determine the Prism
a. Vertical Prism
- As mentioned above, the vertical prism is determined by keeping the lens table at the same height on swapping from right lens to left lens
- If the focimeter target (the green cross) is not centred vertically about the reticule, then there is vertical prism present
- If the focimeter target is below the horizontal then there is base-down prism
- If the focimeter target is above the horizontal then there is base-up prism
- The size of the prism can be read from the reticle markings
- Note: this method determines the total vertical prism between the two lenses. In order to determine the precise contribution of the left and right lenses to the total prism it is necessary to know the position of the patient’s pupils relative to the optical centres of each lens. In practice, the total vertical prism is usually divided evenly between the two lenses
b. Horizontal Prism
- Mark the optical centres of each lens
- Determine the patient’s inter-papillary distance (IPD)
- Mark the left lens at the IPD distance away from the optical centre of the right lens
- Clamp the left spectacle lens on the lens rest so that the PD mark that you made on the lens is over the centre of the lens rest.
- Measure the horizontal prism and direction using the reticule. This is the total horizontal prism. Remember that:
- if the target is to the left (nearer the right lens) the prism is base-in
- if the target is to the right (away from the right lens) the prism is base-out
- Note: this method determines the total horizontal prism between the two lenses. In order to determine the precise contribution of the right and left lenses to the total prism it is necessary to now the position of the patient’s pupils relative to the optical centres of each lens. In practice, the total horizontal prism is usually divided evenly between the two lenses.
9. Determine the Near Add
- Turn the spectacles around. The near addition is a measure of the front vertex power – as opposed to the distance prescription which is a measure of back vertex power. It is therefore necessary to turn the spectacles around so that the arms of the spectacles point towards you
- Measure the power of the distance section and compare this to the power of the near section – the difference is the near addition
- For astigmatic lenses, simply compare one meridian in the distance to the equivalent meridian in the near – again the difference is the near add
-eyedocs.co.uk/ophthalmology-learning/articles/optics-and-refraction/645-how-to-use-a-focimeter – en.wikipedia.org/wiki/Lensmeter
Focimeter (Lensmeter) Notes