Focimeter

1. Beenish Latif
Optometrist
COAVS/ KEMU
13-04-2020

2.  The introduction of the lensmeter, also
called a lensometer, focimeter, or
vertometer changed the way the power(s) of
lenses were neutralized.
 The lensmeter is essentially a centered
telescopic optical system aligned with a
standard optical lens and a rotatable target
illuminated by a light source.
 It measures the focal length of a lens and
converts it into diopters on a circular number
line called a Power Drum.

4.  The eyepiece — mounted in a screw-type
focusing mechanism, the eyepiece plays an
important role in the accuracy of your
readings. It is essential that the eyepiece be
focused to the individual eye of each user.
 Lens Holder Handle — Used to hold a lens in
place against the aperture.

5. The Reticle
The Reticle is a permanently
series of concentric rings used
to measure and locate prism
base direction, it also contains
orientation lines for each lens
meridian and a protractor
scale.

6. The Target
The target is a cross hair, with the thin,
closely spaced lines representing the
sphere component of lens power and the
thicker, widely spaced lines representing
the cylinder power.
The lensmeter actually does not read the
prescription of a lens, but rather the
powers of the lens in each meridian. In
the case of a spherical lens all of the lines
of the target will focus at the same time,
while in the case of a sphero-cylinder
lens, the lines will focus separately at
different Power Drum Readings.

7.  Spectacle Table Lever — Used to raise, or
lower the level of the Spectacle Table.
 Spectacle Table — The resting place for the
frame when neutralizing finished eyewear.
 Power Drum — Hand wheel with numbered
scale readings between +20 and -20 Diopters.
 Locking Lever — Used to elevate or depress
the position of the instrument for individual’s
height or posture.

8.  On-off Switch — Power switch.
 Lens Stop — Aperture against which the lens
rests.
 Cylinder Axis Wheel — Used to orient or
neutralize cylinder axis.
 Prism Axis Scale — Used for orientation of
prism axis

9.  To measure lens power
 An illuminated target is at the first focal point
of
a collimating lens and viewed sharply through
a telescope
 The lens to be measured is put at the second
focal point of the collimating lens
 The target is moved until it appears sharp again
 The target displacement is a measure of the
 strength of the lens

12.  It is also used for:
a) check the power of Spherical lenses
b) Cylindrical lenses (axis as well)
c) Prisms
d) locating and marking the lens Optical Center,
and orienting the axis of a sphero-cylinder
lens;
e) locating and “spotting” or “marking” the Prism
Reference Point, or PRP (formerly called the
Major Reference Point, or MRP), which is the
point where the amount of prism prescribed is
equal to the amount of prism found;
f) verifying lens “add” power in bifocals, trifocals
and progressives;

13.  Focus the eyepiece, in order to compensate for
small uncorrected visual errors.
 It is important to eliminate the normal
accommodation (focusing ability) of the eye.
 keep both eyes open when using the instrument.
 Turn the eyepiece counter-clockwise (into the
plus numbers) and look into the eyepiece.
 Slowly turn the eyepiece in a clockwise direction
until the concentric circles of the Reticle come
into focus.
 The Power Drum Reading should be at 0.00 or
Plano.

14. 1. Focus the eyepiece and check the calibration.
2. Retract the Lens Holder Handle and place the lens
between it, and the Lens Stop with the convex (front )
surface facing you. This will allow you to read the lens
for Back Vertex Power (BVP)
3. Turn the Power Drum top toward you until you reach a
number +10.00 or greater.
4. While looking into the eyepiece, turn the Power Drum
top away from you until the image of the target begins
to become sharply imaged.
5. Once you have brought the both the spherical lines
(three thin, closely spaced) and the cylinder lines (three
thick, widely spaced) of the target into clear focus, lift
the Lens Holder Handle, and reposition the lens until the
cross hairs of the target are centered on the Reticle
rings. It is important to lift the Lens Holder to avoid
scratching the lens surface.

15. 6. You have now located the lens Optical
Center, and should note the Power Drum
Reading.
7. If you want to spot or mark the lens Optical
Center, depress the Marking Device Control
and press it forward against the lens surface.
This will leave three marks, two that are
guides to axis orientation on sphero-cylinder
lenses and the center dot that represents the
Optical Center or Prism Reference Point
(PRP).

16. Focused Mires
Unfocused Mires

18.  Focus and calibrate.
 Place lens in position and set the Power
Drum at about +10.00 D.
 Set the Cylinder Axis Wheel to the desired
cylinder axis.
 Turn the Power Drum top away from you and
look for the sphere lines (the thin, closely
spaced ones) first.
 As you are moving the Power Drum in the
minus direction, you want the sphere lines
to come into focus first.

19.  If you find the thicker cylinder lines coming
in first, rotate the lens 90 degrees.
 If the central area of the sphere lines are not
straight and continuous (indicating that you
have not properly aligned the axis).
 adjust the lens until the lines are clear,
straight and continuous.
 Turn the Power Drum top away from you until
you now see the thicker cylinder lines
clearly, note the position of the Power Drum
and center the lines on the Reticle.

20.  lensmeter is actually measuring the total
powers of the principle meridians, rather
than the actual prescription,
 it is necessary to do a conversion, using the
following steps.

21. 1. The sphere reading (PDR #1) will be your
sphere power in the Rx.
2. The sign of the cylinder is determined by the
direction that you have moved the Power Drum
from the sphere reading (PDR #1) to the
cylinder reading (PDR #2). In this case it will be
minus due to the procedure that we used.
3. The amount of the cylinder is determined by
the “distance” traveled from PDR #1 to PDR
#2. This distance is in fact the total number of
diopters (or fractions of diopters) that you
have moved on the number line attached on
the Power Drum.
4. The Axis is determined by simply noting the
position of the Axis Wheel.

22.  Examples of Converting Power Drum Readings
to an Rx
Example #1
PDR #1 = -2.00 (sphere)
PDR #2 = -3.50 (cylinder)
Axis Wheel Reading = 180 degrees
 The sphere power is -2.00, the direction you
have moved from PDR#1 to PDR#2 is minus,
making the sign of the cylinder minus and
you have moved a “distance” of 1.50
diopters.

23.  Example #1 =
-2.00 -1.50 X 180

24.  Example #2
PDR #1 = +1.00 (sphere)
PDR #2 = -1.25 (cylinder)
Axis Wheel Reading = 90 degrees
The sphere power is +1.00, the direction you
moved from PDR #1 to PDR #2 is minus,
making the sign of the cylinder minus and you
have moved a “distance” of 2.25 diopters.
In this case it is important to note that you have
moved from a plus number (+1.00) to a minus
number (-1.25). You must take care to count the
total distance moved. From +1.00 to Plano is
1.00 and from Plano to -1.25 is an additional
1.25 diopters, making the total 2.25 diopters.

25.  Example #2 =
+1.00 -2.25 X 90

26.  Adjust glasses on platform by turning glasses so
that temples are toward you and bottom of
glasses are positioned for viewing to determine
add power.
 Rule of thumb: For determining add power, turn
glasses around to read from front vertex.
 Bifocal power is determined by rotating the
power drum until the thin lines are in focus
again.
 The difference between this number and sphere
reading power is the amount of “add” to the
lenses.