Refractive Errors: Dr. Leow Thye Yng
Refractive Errors: Dr. Leow Thye Yng
Refractive Errors: Dr. Leow Thye Yng
POSITIVE or CONVERGING
NEGATIVE or DIVERGING
positive negative
The POWER of a lens , P
The “ DIOPTRE (D ) “
A positive lens of one dioptre (+1.00D)
converge parallel light rays to a ‘real’ focal
point one metre from (after) the lens.
A negative lens of one dioptre (-
1.00D) diverge parallel light rays as if they
are coming from a ‘virtual’ point one metre
infront of the lens.
ONE DIOPTRE LENS
P = +1.00 D
f = +1.0 m or
+100 cm
P = - 1.00 D
f = -1.0 m or
-100 cm
The DIOPTRE (D)
1
Power, P (D) = -----------------------
----
Focal Length, f (metre)
n2 - n1
P (dioptre)=
---------------------
r (metre)
wherer = radius
of curvature of the refractive surface in
metres
Refractive Power of the
anterior corneal surface
Power of cornea ~ 40 D
Power of the crystalline lens ~ 20D
Refractive Power of the ave. eye~ 60 D
Assuming n = 1.33 for the eye,
ave. length = n / power = 22.22mm
The axial length of most eyes fall between 22
to 24mm (ultrasound scan).
Key Words
EMMETROPIA
AMETROPIA
- Myopia or ‘Short-
- Hypermetropia (Hyperopia )
sightedness’
or ‘Long-
sightedness’ - Astigmatism
ACCOMMODATION
PRESBYOPIA
Anisometropia , Amblyopia
EMMETROPIA
D (D
distance m. this letter subtend 5 min. -
eg. 60, 36, 24, 18, 12, 9, 6, 5 metres)
Snellen Acuity
Recording Visual Acuity
(Snellen Acuity)
Test Distance (m.) Snellen
Acuity = -------------------------
-----
Distance (m.) at
which the smallest visible letter subtend
5 min. of arc
Test6/18,
eg. 6/5, 6/6, 6/9, 6/12, Distance
6/24,is 6/36,
usually at 6; m.
6/60 5/60, 4/60, 3/60, 2/60, 1/60 ; CF
(Counting fingers), HM (Hand movements), PL
(Perception of light),
Determination of
Refractive Errors