Measurement of lens power by Lensometer Faculty Aravind School of Optometry

Corrective lenses Lens type Lens form Corrective use Sphere Convex ( + ) Concave (--) Hypermetropia Myopia Cylinder & Sphero-cylinder Hyperopic Astigmatism Myopic Astigmatism Prism Correcting squinting eyes Relieving eye strain

Measuring Lens Power Hand neutralisation by trial lens method. Lens power (normally back vertex power, can be determined by using the technique of hand neutralisation or, more commonly today, the use of a focimeter. This lecture will examine the principles of neutralisation and the workings of the focimeter. Hand neutralisation by trial lens method. By Lensometer.

Trial lens method View cross target at distance Procedure Movement is neutralised using a trial lens of opposite sign. That is, a 'with' movement is neutralised by holding a positive trial lens in contact with the unknown lens. The power of the trial lens is repeated until no movement of the cross hairs is seen. If the unknown lens is spherical, the movement will be in the same direction in both principal meridians and will be of the same speed. View cross target at distance Hold lens on visual axis, close to eye. Align lens such that cross target is continuous. Move lens vertically along line of vertical limb of target Determine direction of horizontal limb movement Place trial lens flush with ‘unknown’ lens No movement = neutral Repeat the same for line of horizontal limb of target.

Neutralization of lens power using trial lenses Hand Neutralisation Hand Neutralisation In the absence of a focimeter, it is useful to be able to neutralise the power of a lens with trial lenses. Infrequent occasions arise when this technique is used in full because of the availability of portable focimeters. But hand neutralisation is used qualitatively in many clinical and dispensing situations- first examination of a lens often involves simply identifying if it is a positive, negative or toric lens. Hand neutralisation can be done by viewing a distant cross target through the lines whose limbs extend beyond the lens edge. The lens is moved up and down, and left and right to ascertain the presence of a 'with' or and 'against' movement. A 'with' movement is seen with negative powered lenses and an 'against' movement is seen with a positive powered lens. Neutralization of lens power using trial lenses Lens Movements Cross Target “With” “Against”

Hand Neutralisation of Toric Lenses If the lens is cylindrical or sphero-cylindrical then each axis will have to be neutralised separately. To find the principal meridians hold the lens against the cross-hairs. Rotate the lens about it optical axis. At some positions, the appearance of cross-hairs through the lens will not be continuous with those outside the lens and will not be at right angles to each other. Rotate the lens such that the lines within the view of the lens are continuous with those outside the lens. This will constitute the principal meridians; these can be marked and individually neutralised.

Lensometer Measures the corrective lens power Optic center Sphere Cylinder and its axis Prism. Optic center

Components of a Focimeter The Focimeter The focimeter may also be called a lensometer or a vertometer. The function of the focimeter is to measure the back or front vertex power of a lens or lens system. The optical centre of the lens is located in order to position the lens correctly, relative to the visual axes and the centre of rotation. When the optical centre is positioned correctly in the focimeter, the spherical power, cylindrical power and its orientation and prism power can be measured. For ophthalmic lenses, the back vertex power of the lens is measured when the back vertex of the lens is resting on the lens stop. Similarly, the front vertex power of the lens is measured when the front vertex of the lens is resting on the lens stop. Eyepiece Spring Clip Table Lens Stop Axis Dial Optic Centre Marker Power Wheel

Constructing a Focimeter Optics of a Focimeter For a plano lens or no lens under test the target is positioned at the first principal focus (fo )of the standard (collimating) lens and the lens rest is positioned at the second principal focus (f’o). This ensures that the movement of the target along the system’s principal axis is proportional to the lens under test. The proof is shown on the next slide. For a minus lens under test the target will be moved away from the standard lens, decreasing the divergence of the light incident on the standard lens. The resulting convergent light exiting from the standard lens will be rendered parallel by the lens being tested. For a plus lens under test the target will be moved towards the standard lens, increasing the divergence of the light incident on the standard lens. The resulting divergent light exiting will be rendered parallel by the lens being tested. Zero Position Target at first principal focus Telescope Lens holder for unknown lens With Negative Power Lens Unknown lens at 2nd principal focus Distance between standard lens & target is increased Standard lens With Positive Power Lens Unknown lens at 2nd principal focus Distance between standard lens & target is reduced Target

Constructing a Focimeter x fo f’o x’ Optics of a Focimeter The determination of lens power based upon the position of the target from the standard lens is based upon Newton’s Relation Newton’s relation states that: -x.x’ = f’o2 where x is the distance of the target from the first principal focus of the standard lens (fo), x’ is the distance of the image from the second principal focus of the standard lens (f’o), which is coplanar with the back vertex of the lens under test, and f’o is the focal length of the standard lens. Since, in the case of the focimeter, x’ represents -f’v, where f’v is the second principal focal length of lens under test, then:  x. f’v = f’o2  x = f’o2 / f’v  x = F’v / F’o2  F’v = x F’o2 Therefore, the distance travelled by the target from the first principal focus of the standard lens is directly related to the back vertex power of the lens being measured.. Lens holder for unknown lens (holding negative lens in figure) Light Source & Moveable Target Standard lens Telescope

Calculation x=1000 / Fo² = 1000 / 25² = 1.6mm per Dioptre Constructing a Focimeter Travel of the target per dioptre If a standard lens of +25.00DS is selected, over what distance is the target required to travel when lenses of power 20.00D are to be measured? The movement of the target from the focus of the standard lens (the distance x) per dioptre is: x = 1000 / Fo2 For example, if the standard lens is +25D, then: x = 1000 / 252 = 1000 / 625 = 1.6mm If 20.00D is to be measured by the focimeter, then this represents a total of 40 dioptric steps. Therefore, the total target travel required to measure the range between 20.00D is given by multiplying the number of dioptric steps by the number of millimetres per Dioptre that the target must move. So, Total target travel = 40 1.6 = 64mm Movement of the target per dioptre Fo = +25.00D Target travel (mm) per dioptre: x=1000 / Fo² = 1000 / 25² = 1.6mm per Dioptre So, for a focimeter required to measure  20D the total required travel of target = 40 x 1.6 = 64mm

Target system American optical system – crossed line target. European optical system – Ring dots system

Focimeter Preparation Focusing the eyepiece The eyepiece should be focused at each use as the setting will vary between individuals. Rotate the eyepiece until fully extended from the instrument (usually by rotating the eyepiece as far as possible in an anticlockwise direction). The graticule visible through the eyepiece will now appear blurred. The eyepiece should then be rotated in a clockwise direction until the target crosshairs and the graticule just come into focus. Continued rotation of the eyepiece will force the observer to accommodate in order to keep the graticule in focus. Accommodating whilst viewing the target can cause variability in the power measurement. Check Calibration With the power wheel at the zero position, the cross hairs and the target should be in clear focus. Focus the eyepiece Ensure that all the readings at Zero. Calibration

Focimeter – crossed line target system Focimeter Use – Line Targets Insert the spectacles To measure the BVP, the back pole of the lens should be positioned at the lens stop and the spectacles should be secured with the aid of the table and the spring clamp. The optical centre of the lens should be positioned over the centre of the aperture of the lens stop. Determining the lens power (spherical lenses): Rotate the power wheel until the lines become clear. Note the power on the power wheel. If the lens power is spherical, both sets of lines will be clear regardless of the position of the axis drum. Marking the optical centre: Check that the centre of the crosshair coincides with the centre of the target. When this is so, the lens is correctly positioned and the optical centre should be marked. Measuring the power of the second lens: Without moving the position of the table, proceed with the insertion of the spectacles and the determination of the power as for the first lens. If the lens target is displaced up or down from the horizontal portion of the crosshair, there is a vertical prismatic correction incorporated for the two eyes (see later). Insert the spectacle. Determine the lens power. Mark the optical centre. Measure the power of the second lens. Line Target 1 3 5 Reticule scale Prism dioptres delineation

Focimeter - Ring of Dots Targets Determining the Lens Power (Sphero-cyl Lenses) Step 1 (finding the sphere power) Rotate the power wheel until one set of lines (stretched dots) becomes clear. Start with the higher positive power (or lower negative power). The axis drum will need to be rotated to ensure that the lines are unbroken. Note the power on the power wheel. Step 2 (finding the cyl power) Rotate the power wheel until the second set of lines (stretched dots) becomes clear. The second power reading minus the first reading will give the power of the cyl (and its correct sign). Step 3 (finding the axis) Note the direction of the lines (stretched dots) at the second reading. This is the axis. The rotatable line in the graticule is used to line up with the stretched dots to determine the axis. Reticule scale Prism dioptre delineation 180 90

Procedures Focus the eyepiece by focusing the hair-line black reticule. Place the back vertex (the ocular side) of the unknown lens against the lens stop of the lensometer. Make sure both eye wires of frame, right & left, are touching the stage. Move the lens side to side to align with center the target that places the optical center of the lens at the stop. Notice that the target lines are a cross composed of two sets of lines oriented 90 apart. Try focusing the target lines. If all lines are in focus simultaneously, the lens is spherical.

How to measure the cylindrical power and axis? If only one set is in focus (the other set 90 away is blurred), the lens is cylindrical. If this is the case, focus one set of lines with the power drum, while simultaneously rotating the axis wheel so that the lines are not only clear, but they are also unbroken. Note the power on the drum. If the unknown lens is cylindrical, the set of lines 90 away will be out of focus. The power of the first set of clear lines is called the sphere reading. Now focus the other set of lines (90 away from the first set) so that they are clear and unbroken. Note the new power on the drum & also note the axis on the outside wheel.

How to measure the cylindrical power and axis? If the 2nd set of lines was focused at a lower + (more --) power, you have measured in ‘—’ cylinder form. Since our convention is ‘—’ cylinder form, get used to making the less ‘—’ measurement in 1st measurement. That would mean that the 2nd measurement would be more +, thus allowing to record the reading in ‘—’ cylinder form. In this case (minus cylinder form), the first measurement is the sphere, and the increase, or change in minus power you had to travel to the second, (higher minus power) focus is the cylinder power. Record the values.

Example - Line Targets +1.00 / -2.00 x 120 Power wheel Sphere setting Examples: Line Targets Step 1 (finding the sphere power) Rotate the power wheel until one set of lines becomes clear. Start with the higher positive power (or lower negative power). The axis drum will need to be rotated to ensure that the lines are unbroken. Note the power on the power wheel. In this case the power reading is +1.00D. Step 2 (finding the cyl power) Rotate the power wheel until the second set of lines becomes clear. The second power reading minus the first reading will give the power of the cyl (and its correct sign). In this case the second reading is –1.00D. So the cylinder power is: -1.00 - +1.00 = -2.00D. Step 3 (finding the axis) Note the direction of the lines at the second reading. This is the axis. The lines are lying at 120. So the lens power is +1.00/-2.00 X 120 NB: In this case the manufacturers of focimeters selected the thin lines to correspond with the sphere power, so that the axis drum would read 120. If, however, the practitioner did not know this and used the three thick lines for the first reading then the three step rule would give the correct result but the axis drum would be reading 90 degrees off axis. +1.00 / -2.00 x 120 Power wheel Sphere setting Power wheel Cylinder setting Axis - +2.00 - - +1.00 - 0.00 - - -1.00

Example - Ring of Dots Targets Examples: Ring of Dots Targets Step 1 (finding the sphere power) Rotate the power wheel until one set of lines (stretched dots) becomes clear. Start with the higher positive power (or lower negative power). Note the power on the power wheel. In this case the power reading is +1.00D. Step 2 (finding the cyl power) Rotate the power wheel until the second set of lines becomes clear. The second power reading minus the first reading will give the power of the cyl (and its correct sign). In this case the second reading is –1.00D. So the cylinder power is: -1.00 - +1.00 = -2.00D. Step 3 (finding the axis) Note the direction of the lines at the second reading. This is the axis. The lines are lying at 120. So the lens power is +1.00/-2.00 X 120 +1.00 / -2.00 x 120 Power wheel Sphere setting Power wheel Cylinder setting Axis 90 90 180 180 - 0.00 - - -1.00 - +2.00 - - +1.00

Focus of cylindrical lens

Measure the bifocal add Lift the front of the lens against the lens stop. Take a reading of one set of unbroken, focused lines through the distance portion, then move the stage up until the bifocal add is against the stop. Re-focus the same lines again, but in segment. The difference between the 1st & 2nd reading is the add power.

Vertical Prism 2.0 Base Up Determination of Vertical Prism With the Focimeter Vertical Prism If the first eye is correctly positioned with the optical centre over the aperture stop vertical prism, relative between the two eyes, can be noted if the target is displaced up or down when the second lens is placed over the aperture stop. No adjustment of the table should be made when the second lens is positioned for measurement The direction of the displacement of the focimeter target indicates the base direction of the vertical prism. Thus, if the target is displaced upwards from the centre of the crosshair, base up prism is present in the second lens relative to the first lens measured. The magnitude of the prism is measured from the centre of the target to the centre of the eyepiece scale. 2.0 Base Up

Horizontal Prism 1.5  Base In Determination of Horizontal Prism With the Focimeter Horizontal Prism Horizontal prism is more difficult to determine, particularly if it has been achieved by horizontal displacement of the optical centres. If the spectacle wearers interpupillary distance is known, the distance should be marked on the second lens and this mark should be placed over the aperture of the lens stop. Any horizontal displacement of the target when the lens is so positioned indicates the presence of horizontal prism. Alternatively, the wearers interpupillary distance should be compared with the distance between the spectacle lens optical centres. The prim being determined using Prentice’s Rule. If the spectacle wearers interpupillary distance is not known, the optical centre of the lens should be marked at the position where the target is coincident with the centre of the crosshair. The wearers interpupillary distance should subsequently be measured and compared with the distance between the spectacle lens optical centres. The prim being determined using Prentice’s Rule. 1.5  Base In

Oblique Prism Determination of Oblique Prism With the Focimeter Vertical & Horizontal Prism If vertical and horizontal prism are present in the second lens measured, the target will appear to be displaced obliquely from the centre of the focimeter crosshairs. The vertical and horizontal prism components can be corrected separately such that the target becomes centred to the crosshair or can be neutralised with prism at an oblique angle, the axis of which should be noted by aligning the crosshair with the centre of the target. 3.0 up @ 150

Focimeter Use - Sources of Error Eyepiece Failure to focus the eyepiece will result in incorrect readings of vertex power. Dependent on the degree to which the eyepiece is incorrectly focused for a given observer, the error in the power reading may classify completed lens spectacles as outside the acceptable tolerances for optical appliance standards. Zero setting & axis alignment With the eyepiece in focus and the power wheel at zero, the reticule and target should be clearly in focus. The clarity of the reticule and the target should also be checked for additional lenses of known back vertex powers. Axis alignment should be similarly checked with a cylindrical or sphero-cylindrical lens of known cylinder axis orientation Centration of the graticule and target When rotated, the graticule and target should remain centred relative to each other. If this is not the case, check that any variable prism incorporated into the focimeter is set to zero. If the target and graticule are still in misalignment, the focimeter needs servicing! Corrective measures The calibration of the focimeter should be checked frequently. If the error in the BVP is systematically incorrect, it is likely that the aperture stop is in misalignment and is no longer positioned at the second principal focus of the standard lens. The power reading should be corrected for a systematic error in the short-term. Replacement and repositioning of the aperture stop presents a long-term solution Failure to focus the eyepiece Zero setting & axis alignment Centration of the reticule & target

Projection & Automatic Focimeters Projection and Automatic Focimeters Projection focimeters The target is usually projected on to a screen which is advantageous as no focussing of the eyepiece in a telescopic system is required, thus removing a source of error from the focimeter reading. The majority of projection focimeters are semiautomatic, have line targets and axis alignment is usually achieved by rotating the target such that the triple lines become clear. This is then denoted as the sphere component and, if refocusing is required, the cylindrical component and axis are calculated by the focimeter. Thus, sources of arithmetic errors in determining the cylinder magnitude are removed. Automatic focimeters The operator is required only to centre the lens correctly. The focimeter scans the lens to determine the maximum and minimum powers and displays the result. This type of focimeter removes the majority of sources of errors from focimeter readings.

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