Hand Neutralisation and Vertometry
Hand Neutralisation and Vertometry
Hand Neutralisation and Vertometry
AND VERTOMETRY
THINK
A widow comes to see you for an eye examination. She brings a pair of spectacles that she bought elsewhere many
years ago. She tells you that she thinks her vision has changed.
you can see if her refractive error has changed, and by how much
it can give you a starting point for your refraction
it will help you decide whether or not she needs to get new spectacles.
If you decide to order new spectacles for this widow, you will need to check the new spectacles when they come back
from the optical workshop. To measure the power of the spectacles you will need to use hand neutralisation or
vertometry.
AIM
This unit will show you two methods for measuring the power of spectacle lenses.
LEARNING OUTCOMES
When you have worked through this unit you should be able to:
Sphere power
+2.00 D
or: +2.00 D / 1.25 DC x 70
1.25 DC x 70
or simply: +2.00 / 1.25 x 70
Cylinder Axis
power
A trial set is a collection of spherical, cylindrical and prism lenses, as well as some
accessory lenses.
TRIAL LENS SETS Spherical plus and minus trial lenses are either:
- Labelled with a “+” or ““ sign, or
- Surrounded by a coloured rim
(usually plus lenses are black and minus lenses are red).
1. Hand neutralisation: a simple technique that can be performed using just a trial lens set.
2. Vertometry: a more accurate technique that uses a special instrument called a vertometer.
HAND NEUTRALISATION
Hand neutralisation is a good way to measure spectacle lens power when you do not have a vertometer. Vertometers
can be expensive and need electricity to work.
Hand neutralisation can be performed using only lenses from a trial lens set.
If you look at an object through a lens and move the lens from side to side (right and
left), the image that you see through the lens will also move.
A plus lens will make the image move in the opposite direction to the lens movement:
If you move the lens to the right, the image will move to the left.
If you move the lens to the left, the image will move to the right.
IMAGE MOVEMENT A minus lens will make the image move in the same direction as the lens movement:
THROUGH LENSES If you move the lens to the right, the image will move to the right.
If you move the lens to the left, the image will move to the left.
The image of an object seen through a plano lens will not move.
ADDING LENSES
When lenses are placed on top of each other, their powers can be added together.
TOGETHER
If you have a +1.00 D lens and you put a +4.00 D lens on top of it, the total power is:
+1.00 D + +4.00 D = +5.00 D.
A +1.00 D and a +4.00 D lens together are the same as a +5.00 D lens.
EXAMPLE 1
Image movement:
A +1.00 D lens gives against movement.
A +4.00 D lens gives against movement.
A +5.00 D lens gives against movement
(the same as a +1.00 D lens and +4.00 D lens together).
If you have a +1.00 D lens, and you put a 5.00 D lens on top of it, the total power is:
+1.00 D + 5.00 D = 4.00 D.
A +1.00 D and a 5.00 D lens together are the same as a 4.00 D lens.
EXAMPLE 2
Image movement:
A +1.00 D lens gives against movement.
A 5.00 D lens gives with movement.
A 4.00 D lens gives with movement
(the same as a +1.00 D lens and 5.00 D lens together).
If you have a +3.00 D lens, and you put a 3.00 D lens on top of it, the total power is:
+3.00 D + 3.00 D = 0.
A +3.00 D and a 3.00 D lens together are the same as a plano lens.
EXAMPLE 3
Image movement:
A +3.00 D lens gives against movement.
A 3.00 D lens gives with movement.
A plano lens gives no movement
(the same as a +3.00 D lens and 3.00 D lens together).
If you have a lens of an unknown power, you can find out what power it is by
“neutralising” it with another lens of a known power.
Neutralisation occurs when two lenses are held together and there is no movement of the
image through the combined lenses. This will only happen when the two lenses are of
equal but opposite power.
Examples:
HAND A +3.00 D lens and a 3.00 D lens held together will give no movement. We can thus
NEUTRALISATION “neutralise” the power of the +3.00 D lens by using a –3.00 D lens.
A 7.00 D lens and a +7.00 D lens held together will give no movement.
We can thus “neutralise” the power of the –7.00 D lens by using a +7.00 D lens.
A 2.75 D lens and a +2.75 D lens held together will give no movement.
We can thus “neutralise” the power of the –2.75 D lens by using a +2.75 D lens.
If you know the power of one of the lenses you have neutralised, you will also know the
power of the other lens.
Draw a cross on the centre of a piece of paper. Make sure that the lines are perpendicular
(at 90) to each other, and that each line is at least 15 cm long.
Place the cross approximately 1 m away from you (sometimes it is easiest to put it on the
floor).
Hold the lens close to your eye and look at the cross through the lens.
Make sure that the cross is in the centre of the lens.
Hold the lens so that the lines of the cross that are seen through the lens
line up with the lines outside the lens.
The lines of the cross should look aligned through the lens and outside the
lens edge – then the centre of the cross is at the optical centre of the lens.
SET-UP
Lens
Cross Target
Figure 9.1: The lines of the cross inside and outside the lens are aligned,
so the centre of the cross is at the optical centre of the lens
To find out whether the lens you are holding is a spherical or an astigmatic lens, you need to
rotate the lens in front of your eye. To rotate the lens you turn it clockwise or anti-clockwise,
like the steering wheel of a car.
If the lines of the cross stay perpendicular when you rotate the lens, it is a spherical lens.
If the lines of the cross do not stay perpendicular when you look through the lens, the lens is
a cylindrical or sphero-cylindrical lens. This movement is known as “scissors” movement.
Scissors movement
SPHERE OR
CYLINDER?
Lens
Once you know that you have a spherical lens (by rotating it), you need to find out if it is a plus
or a minus lens.
Move the lens up and down and from side to side (right and left) in front of your eye.
If the lines of the cross move in the opposite direction to the movement of the lens (“against”), you
have a plus lens. If they move in the same direction (“with”), you have a minus lens. If the lines do
not move at all, you have a plano lens.
PLUS OR MINUS
SPHERE? Lens
Lens Movement
Figure 9.3: A cross is viewed through a lens, and the lens is moved upwards.
A minus lens will give “with” movement and a plus lens will give “against” movement
Now that you know whether you have a plus or minus lens, hold a trial lens of the opposite
power against your unknown lens.
If you have a plus lens, you will choose a minus trial lens.
If you have a minus lens, you will choose a plus trial lens.
Remember that high power plus lenses are thicker in the centre, and high power minus lenses are
thicker at the edge. Looking at the shape of the lens you want to neutralise might help you estimate
the power of the lens needed to neutralise it.
Now hold the unknown lens and the trial lens that you have chosen to neutralise it together in
front of your eye. Move the lenses up and down, and from side to side.
FINDING THE If there is still movement when you look through both lenses, you will need to choose a different
POWER OF A trial lens.
SPHERICAL
LENS If there is against movement and your unknown lens is a:
plus lens → you need to choose a higher powered minus trial lens
minus lens → you need to choose a lower powered plus trial lens.
If there is with movement and your unknown lens is a:
plus lens → you need to choose a lower powered minus trial lens
minus lens → you need to choose a higher powered plus trial lens.
Continue to try different trial lenses with your unknown lens until there is no movement of the
cross lines when you look through both lenses together. When there is no movement, you have
neutralised your lens. The power of your unknown lens will be equal and opposite to the power
of the trial lens that neutralises it.
The power of an unknown minus lens that is neutralised by a +4.00 D trial lens is: 4.00 D.
EXAMPLES
The power of an unknown plus lens that is neutralised by a 2.75 D trial lens is: +2.75 D.
You can save time when hand neutralising by using a technique known as bracketing.
Bracketing is a logical method that helps you to choose your next trial lens.
Example:
You have an unknown lens that you rotate: you find that it is a spherical lens.
When you move the lens you see against movement: it is a plus lens.
Because it is a plus lens, you choose a 4.00 D trial lens from your trial lens case.
The two lenses together still give you against movement.
This means that your unknown lens is a plus lens that is stronger than +4.00 D.
You choose a 8.00 D trial lens next.
This time, the two lenses together give you with movement.
This means that your unknown lens is a plus lens that has a power between +4.00 D
and +8.00 D.
Now, you can choose any trial lens between 4.00 D and 8.00 D to try and neutralise
your unknown lens, but you will be more efficient if you “bracket”
BRACKETING → which means using a logical way of choosing the next lens power to try.
To bracket, you choose a lens power that is half way between 4.00 D and 8.00 D
→ you chose a 6.00 D trial lens.
Together, your unknown lens and the 6.00 D lens give you against movement.
This means that your unknown lens has a power between +4.00 D and +6.00 D.
Bracketing again, you choose a lens that is half way between 4.00 D and 6.00 D
→ you choose a 5.00 D trial lens.
This time you get with movement.
This means that your unknown lens has a power between +5.00 D and +6.00 D.
You now choose a 5.50 D trial lens.
This time you get no movement when you look through both lenses.
This means that you have neutralised your unknown lens.
The power of your unknown lens is: +5.50 D.
REMEMBER:
An astigmatic lens has two principal meridians that are perpendicular
(at 90) to each other.
To find the power of an astigmatic lens, the power of each principal meridian must be
found separately.
The principal meridians can be found by rotating the lens until the lines of the cross (that
show scissors movement) are lined up perpendicularly. When this happens, the lines of
the cross are lined up over both principal meridians of the lens.
FINDING THE
PRINCIPAL
MERIDIANS OF AN
ASTIGMATIC LENS
Principal lens
meridians
Figure 9.4: To find the principal meridians of a sphero-cylindrical lens, rotate the lens
until the lines of the cross appear perpendicular through the lens
Hold the astigmatic lens with its principal meridians lined up with the cross:
To find the power of the vertical meridian, move the lens up and down.
To find the power of the horizontal meridian, move the lens from side to side.
You now need to neutralise each meridian separately.
It is useful to draw an optical cross for recording your findings.
You have an unknown lens and you want to know where the optical centre is and what
its power is.
You rotate the lens and see scissors movement now you know the lens is a cylinder
or sphero-cylinder.
EXAMPLE
Optical centre
of the lens
FINDING THE
OPTICAL CENTRE
Figure 9.7: The lines of the cross inside and outside the lens are aligned,
so the centre of the cross is at the optical centre of the lens
Notice:
The cross is not in the centre of the circle.
This means that in this case the optical centre of the lens is not in
the middle of the lens.
You can also find the optical centre of a lens by looking at the two reflections of a light
source (like a light globe) on the front and back surfaces of the lens.
Tilt the lens until you get the smaller reflection in the middle of the larger one.
Large reflection
Figure 9.8: Finding the optical centre by reflection
You can mark the optical centre of the lens at the centre of the cross or where the two
reflections meet by using a marker or a felt tipped pen.
Move the lens up and down and see what type of movement it causes.
If you see with movement, its means the vertical meridian has minus power.
Direction
of lens
movement
Direction
of lens
THE POWER OF THE movement
VERTICAL MERIDIAN
4.50
Direction Direction
of lens of lens
movement movement
+2.25
Figure 9.12: Optical cross with vertical and horizontal meridian powers recorded
The axis is in the direction of the most plus powered (or least minus powered) meridian.
WHERE IS THE AXIS?
In this example, the most plus powered meridian is in the horizontal (180) meridian.
The amount of cylinder power in the lens is the difference between the powers of the
HOW WOULD YOU two principal meridians.
WRITE THE Looking at the optical cross (Figure 9.12), and knowing that the axis of the lens is at
PRESCRIPTION
180, you would write the prescription of this lens as:
OF THIS LENS?
+2.25 / 6.75 x 180.
VERTOMETRY (FOCIMETRY)
Vertometry is an accurate way to measure the power of spectacle lenses.
Vertometry is performed by using an instrument called a vertometer (also known as a focimeter or a lensmeter).
Lens rest
Axis
Wheel
Eyepiece
Graticule
Power wheel
Frame table
When you measure a lens on the vertometer, the edge of the lens rests on the frame table, so that the lens lies over
the lens rest. The lens is then clamped in place so that it does not move, and the power wheel is turned to measure
the power of the lens.
The eyepiece is the part of the vertometer that you look through to measure the lens power. When you look through
the eyepiece you see the black lines and circles of the graticule, and the brightly lit coloured target. The graticule will
be visible even when the vertometer is turned off, but you can only see the target when the vertometer is turned on.
The target is usually green.
VERTOMETER
There are two types of vertometer target:
TARGETS
Vertometers that use this sort of target need an axis wheel (as seen in Figure 9.13).
CROSSED LINE If a sphere is being measured, all the lines of the target can be equally focused at the
TARGET same time. If a sphero-cylinder is being measured, only the lines in one direction can be
focused at any one time:
If the single line is in focus, the three parallel lines will be blurred.
If the three parallel lines are in focus, the single line will be blurred.
- +2.00 - 0.00
- -
- -
- -
- +1.00 - 1.00
- -
- -
Vertometers that use this sort of target do not need an axis wheel.
90
180 0
90 90
180 0 180 0
- +2.00 - 0.00
- -
- -
- -
- +1.00 - 1.00
VERTOMETER METHOD
It is important to keep the height of the frame table the same for
measuring both the right and left lenses. This is so that you can look
for prism in the spectacles later.
Figure 9.18: The three parallel lines are clear, straight and unbroken.
The single line is also clear, straight and unbroken. This is a spherical lens
Blurry line
Figure 9.19: The three parallel lines are clear, straight and unbroken.
The single line is blurry. This is an astigmatic lens
Step 4: If you are measuring an astigmatic lens, the number on the power wheel will
tell you the power of the most positive meridian of the lens. This will be the
spherical power when you write the astigmatic lens prescription.
5
Blurry
MEASURING Another way of looking at Step 6 is to look at how far you have turned the
LENS POWER (cont.) power wheel and in which direction.
Step 7: Find the axis of the lens.
The axis of the cylinder is the direction of the second power reading (the least
positive power).
You measure the direction of this line by looking at the axis numbers on the
graticule inside the eyepiece.
-
-
- +2.00
-
-
-
- +1.00
-
-
-
- +0.00
-
Axis -
-
- 2.00
-
-
-
- 3.00
-
-
-
- 4.00
90
DOT TARGET
VERTOMETER –
SPHERICAL LENSES
180 0
90
DOT TARGET
VERTOMETER –
ASTIGMATIC LENSES
180 0
Step 3: Slowly turn the power wheel to decrease the power until the second set of
stretched dots (little lines) becomes clear. This time the dots will be stretched
in a direction 90 to those in Step 2. The number on the power wheel will now
tell you the power of the least positive meridian of the lens.
90
180 0
DOT TARGET
VERTOMETER –
ASTIGMATIC LENSES
(cont.)
Figure 9.25: Second power reading (least positive meridian). This time the ring
of stretched dots are stretched in a direction 90 to that of the first power reading.
Step 4: Find the cylindrical power of the lens.
Cylindrical power = second power reading (least positive power) first power
reading (most positive power).
Again, another way of looking at Step 4 is to look at how far you have turned
the power wheel and in which direction.
Step 5: Find the axis of the lens. The axis of the lens is the direction of the little lines of
the second power reading. You measure the direction of these lines by looking
at the axis numbers on the graticule inside the eyepiece.
EXAMPLE 2: 90
- +2.50
MEASURING AN -
ASTIGMATIC LENS -
180 0 -
- +1.00
-
-
Axis
90 - +2.00
-
-
-
180 0
- +1.00
-
-
EXAMPLE 2:
MEASURING AN
ASTIGMATIC LENS
(cont.)
Figure 9.27: Second reading
The cylinder power of this lens is (second power reading first power reading):
+1.00 (+2.50) = 1.50 DC.
If you have a sphero-cylindrical lens, it is best if you turn the power wheel until you are half way between the first
reading and the second reading. At this point the target will look like a ring (although it will be a bit blurry).
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.
If you have a pair of spectacles and you measure the distance between the optical centres of the
two lenses, this distance should be the same as the person’s PD.
If it is not the same, the spectacles have prism in them.
More often, if there is prism in a pair of spectacles, it means that the lenses were not put into the frame properly. An
error like this can cause the person to have asthenopia (eye strain) or even double vision when they wear their
spectacles.
Spectacles with unwanted prism in them cannot be dispensed to a person, and need to be remade.
Step 1: Clamp the right spectacle lens against the vertometer lens rest. The lens
should be clamped so that the optical centre of the lens is in the middle of the
graticule.
5 3 1
Figure 9.29: Crossed line target for left lens showing 1 base-down prism for the left eye
Figure 9.30: Target for left lens showing 1.5 base-in prism
The procedure described here is for checking front surface bifocals (which most bifocals
are).
Step 1: Check the distance powers as normal.
Step 2: Now turn the spectacles around so that the temples of the spectacles are pointing
towards you. Place the front surface of the right distance portion against the lens
rest (Figure 9.31).
Step 3: Focus the lines or dots of the target that are closest to the vertical direction and
determine the power.
Step 4: Move the spectacles up and place the front surface of the segment against the
lens stop (Figure 9.32).
PROCEDURE –
CHECKING THE
ADDITION IN
BIFOCALS
Distance Prism
Reference Circle Reference Point
PROGRESSIVES Micro-etching
LENSES
Temporal Nasal
Material
Add Power
Logo
Near
Reference Circle
RE LENS
HAND NEUTRALISATION
Plus lenses give against movement.
Minus lenses give with movement.
A lens can be neutralised by finding a lens of equal and opposite power.
A lens that has been neutralised will give no movement.
Use a cross to:
– find out whether you have a spherical or sphero-cylindrical lens;
– find the power of the principal meridians.
The principal meridians of an astigmatic lens must be neutralised separately.
VERTOMETRY
Focus the eyepiece.
Clamp the right spectacle lens:
– The front of the spectacles must face towards you.
– Move the spectacles until the target is in the centre of the black graticule.
– The spectacles must be horizontal on the frame table.
Measure the lens power (turning the wheel slowly from plus towards minus).
Find the optical centre of the lens.
Clamp the left spectacle lens.
Measure the lens power and find the optical centre of the left lens.
Check for and measure vertical prism.
Use the optical centre marks that you made and the person’s interpupillary distance to check for horizontal prism.
To check the addition of bifocals, turn the spectacles around and measure the distance and near powers again.
The difference is the add.
1. If you are holding a +4.00 lens against an unknown minus lens and you see “with” movement, is the
minus lens ... (tick appropriate box)
2. You must always _____________________ the eyepiece before using the vertometer.
3. What are the three steps for finding the power of an astigmatic lens on a vertometer?
a. _____________________________________________________________________________
b. _____________________________________________________________________________
c. _____________________________________________________________________________
4. If you are holding a 5.50 D lens against an unknown lens and you see no movement of the cross,
what is the power of the lens? (tick appropriate box)
(a) 4.00
(b) +6.00
(c) +5.50
(d) 5.50
- -
- +2.50 - +2.00
- -
- +2.25 - +1.75
- -
- +2.00 - +1.50
- -