Biomedical Engineering Reference
In-Depth Information
First, without doing any calculations, you should know that the power of the con-
tact lenses will be less than the spectacle lenses. Since the contact lenses are closer
to the eyes, they are farther from the far point and consequently have a longer focal
length and lower power.
8
To be specific, whereas the secondary focal length for the
spectacles is
−
11.11 cm (e.g., 100/
−
9.00 D
=
−
11.11 cm), the secondary focal length
for the contact lenses is
−
12.61 cm [e.g., (
−
11.11 cm)
+
(
−
1.5 cm)
=
−
12.61 cm].
A
−
7.93 D contact lens has this focal length (e.g., 100/
−
12.61 cm
=
−
7.93 D).
Just to be sure we have these important concepts down, let's do one more example.
If we perform a trial-lens refraction at a distance of 12 mm and find
+
12.00 D,
what power contact lens should be prescribed?
Before we do any calculations, we know that the contact lens must have a shorter
focal length than the spectacle lens, which means it has more power. Rather than
a focal length of 8.33 cm (for the spectacle lens), the contact lens focal length is
1.2 cm shorter, or 7.13 cm. Such a contact lens has a dioptric power of
14.02 D.
Lens effectivity can also be calculated using equations. Although some people
prefer to use equations, I think it's better to solve these problems without them.
It's all too easy to forget equations, but once you understand the basic concepts
of lens effectivity, they should stay with you over the course of your professional
career.
+
CORRECTION OF AMETROPIA WITH LASER
AND SURGICAL PROCEDURES
Ametropia is most often corrected with an ophthalmic lens positioned in the spec-
tacle plane or a contact lens in the corneal plane. In recent years, it has become
commonplace to compensate (or partially compensate) for refractive errors with
surgical and laser procedures.
The first widely utilized surgical procedure was
radial keratotomy
(RK). This pro-
cedure was used primarily to compensate for myopia. Radial incisions are made in
the cornea, leading to the flattening of this tissue and a resultant decrease in the
eye's refractive power (Fig. 7-12).
Photorefractive procedures that utilize excimer laser technology have supplanted
radial keratotomy. In
photorefractive keratectomy
(PRK), the corneal epithelium is
removed prior to using an excimer laser to sculpt the underlying stroma. With
laser-assisted in situ keratomileusis
(LASIK), a microtome or laser is used to create a
corneal flap (Fig. 7-13). The exposed stroma is sculpted with the goal of adjusting
8. When we say that one corrective lens is stronger (or more powerful) than another, we are typically
referring to the absolute power of the lenses. That is, a
−
10.00 D lens is said to be stronger than a
−
5.00 D lens.
