3. Equivalent Viewing Power (EVP)
Evp is a measure of the power of the lens or
optical system .
Evp is a term use to describe the magnifying
effect of a lens which is equal to power
resulting from the combination of a
magnifier(assumed to be a thin lens)and a
near addition(or the accomodation exerted)
Evp is the reciprocal of equivalent viewing
distance.
4. Methods to determine EVP.
There are multiple ways to determine EVP:-
A) RDM METHOD :-
we use RDM to determine where the patient
must hold the object. Because the object is at
F, it will be one focal length from the eye, and
the EVP is just the reciprocal of the focal
length.
We first calculate the relative distance using
x = tested distance/M letters read
Then get Evp using D=1/f
5. Methods to determine EVP.
For example :
A patient has a best corrected acuity of 3M
measured at 30 cm, and the patient wants to
read 1M print . calculate EVP?
Sol:
30/3=x/1M x=10 cm.
As
D=1/f =100/10 =10 D.
Any aid that has an EVP of +10.00 D can be
used by this patient to read 1M print, regardless
of the vertex distance.
6. Methods to determine EVP.
B) Add power method :
Calculations of EVP or EVD based upon the
add power need to be based upon where the
patient is actually holding the print.
We can calculate by using following formula:
F =ratio of current VA denominator/Desire VA
denominator X 2.50
7. Methods to determine EVP.
As an example, suppose that a patient has a
best corrected visual acuity at 40 cm of
20/80, and the patient wants to read 20/20
print . Considering presbyopia . Calculate the
power need?
Sol:
80/20 X2.50 =4 x 2.50 =10.00D
8. Methods to determine EVP.
If the patient was a presbyope, at 40 cm the
patient would need a +2.50 D add to see the
20/80 print.To read 20/20 print, the patient
would just need 4 times as much add or a
+10.00 D lens. This is the same as moving
the print from 40 cm to 10 cm, with a 10 cm
distance corresponding to a +10.00 power
add. Working with add powers is just the
reciprocal of working with distances.
9. Methods to determine EVP
C) Reference distance method :
We can find out require power by using :
Magnification =dF
‘ d’ is expressed in meters, and ‘F’ is in
diopters.
If the reference distance is 40 cm and the
patient needs 4X magnification.Solve for the
power?
Sol: 4x= 4F F= +10 D
10. EQUIVALENT POWER:-
The EVP of an Low vision aid is its equivalent
power.
It is given as :-
Fe =F1+F2-(d)F1F2
F1= power of device
F2= accomodation or addition use
d= distance b/w F1 & F2
11. EQUIVALENT FORMULAS OF
SOME DEVICES :
A) SPECTACLES:-
Fe=F1+F2
B) HANDHELD MAGNIFIERS:-
Fe=F1+F2-(d)F1F2
C) STAND MAGNIFIERS:-
Fe=(transverse magnification) (F2)
12. EQUIVALENT FORMULAS OF
SOME DEVICES :
D) TELEMICROSCOPES:-
Fe=[F(reading cap)] x [MagTS]
E)CCTV AND ELECTRONIC DEVICES :-
Fe= (F2) x image size/object size
13. Equivalent Viewing Distance
EVD is the distance from the eye that the patient
has to hold the object so that it subtends the
same angle as the smallest letter size that the
patient can read at the reference distance.
OR
EVD is the distance from the eye at which the
smallest letter size as viewed by the patient at
the reference distance subtends the same angle
as the image formed by a near aid.
14. Equivalent Viewing Distance
A measure of the patient’s working distance
that is related to visual acuity
(equivalent viewing distance, which is the
reciprocal of the equivalent viewing power)
Example: A patient has a best visual acuity of
3M at 30 cm and wants to read 1M Print?
calculate the value of EVD?
15. Equivalent Viewing Distance
ANS: using formula
Req. EVD= Required TPS(THRESHOLD
print size)/current TPS x Current EVD.
x=1o cm.
We state that the EVD is 10 cm.
If we position the 1M print 10 cm from the eye
we will get 3X relative distance magnification
(RDM) and the patient will just be able to
read the print.
16. EVD & EVP IS RELIABLE THAN THE
MAGNIFICATION
The most obvious problem with using
magnification to specify a near vision aid is that
the magnification of a lens of given power is not
a constant because of its dependence up on
reference distance.
For example,
A lens of power +10.00 D, can have many
different magnifications depending upon the
reference distance used for measuring visual
acuity.
17. EVD & EVP IS RELIABLE THAN THE
MAGNIFICATION
If the reference distance is 40 cm, then the
magnification is 4X. If the reference distance
is 25 cm, then the magnification is 2.5X. The
solution, as previously mentioned, is to
specify low vision near aids by either their
power or by an equivalent viewing distance.
18. EVD & EVP IS RELIABLE THAN
THE MAGNIFICATION
However, most low vision aid suppliers still
specify their visual aids by a magnification in
terms of “X”, although the values specified
may not be all that useful. For this reason we
cannot avoid the use of magnification
completely.
19. REFERENCES
Bennett and Rabbetts' clinical visual optics
intranet.opt.uh.edu
A guide to low vision practitioners
(American Academy)
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