SPHERICAL EQUIVALENT Flashcards
We sometimes
We sometimes need to find the average of a spherocylindrical prescription. This is called the spherical equivalent.
The Spherical is used in?
The spherical equivalent is used in contact lenses, certain tests such as the visual field, and is also important for surgical patients. So, how does it work?
Recall
Spherical lenses create
spherocylindrical lenses
Recall when we learned about spherical and spherocylindrical lenses. Spherical lenses create a focal point because they have the same refractive power in all meridians. But spherocylindrical lenses have 2 different refractive powers in 2 different meridians. So instead of a focal point, a spherocylindrical lens creates 2 focal lines perpendicular to one another.
Half way
Half way in between the 2 focal lines a blurred circle is formed called the “circle of least confusion” and is represented by the spherical equivalent of the spherocylindrical lens. It is the least blurry image possible through a spherical lens. Sturm’s interval (or interval of Sturm) is the distance between the 2 focal lines. Ellipses are formed in the space between.
Though
Through a spherocylindrical lens, the 3 dimensional envelope of light rays between the 2 focal lines is called the Conoid of Sturm (Sturm’s conoid). It is a cone shape, hence the term conoid. Cross sections along the conoid are ellipses and you can also see the circle of least confusion here.
If we provide
If we provide correction using the spherical equivalent, the circle of least confusion will be focused on the retina. This provides a slightly blurred image but eliminates the distortion from the ellipses.
For minor amounts of astigmatism, a spherical SCL may be considered instead of a toric SCL because spherical lenses may provide acceptably good vision and their fitting does not have the potential for as many complications as toric SCL.
For minor amounts of astigmatism, a spherical SCL may be considered instead of a toric SCL because spherical lenses may provide acceptably good vision and their fitting does not have the potential for as many complications as toric SCL.
1,
The calculation for spherical equivalent is very simple… 1) Take ½ of the cylinder power and add it to the sphere 2) Drop the cylinder power 3) Drop the cylinder axis
problem
+2.00 +1.50 x 005 ½ (+1.50) = +0.75 +2.00 +0.75 +2.75 +2.75 is our new sphere power.
problem
+2.75 +1.50 x 005 Then we will drop the cylinder and the axis. +2.75 sph is the spherical equivalent.
problem
-5.00 +4.50 x 060 ½(+4.50) = +2.25 -5.00 +2.25 -2.75 -2.75 sph
it works
It works the same if you take a minus cylinder prescription.
plus
+3.50 -1.00 x 095 ½(-1.00) = -0.50 +3.50 +(-0.50) +3.00 +3.00 sph
pkus
+1.00 -3.00 x 080 ½(-3.00) = -1.50 +1.00 +(-1.50) -0.50 -0.50 sph
