optical prism decentration

OPTOM FASLU MUHAMMED

The optical center (OC) of a lens is the point at which light rays can pass with no deviation.

It is important that a lens is ground so that its optical center is directly in front of the patient’s pupils to allow optimum vision through the lens.

The patient’s PD and an optical center height will determine the placement of the lens within the frame

Hold the lens as in step 2 above: that is, with the cross lines of the image perpendicular to each other.  

Now move the lens up-and-down or side-to-side to ensure that the lines of the cross image exactly overlap with the lines of the object cross.

Held in this position, the point at which the cross lines intersect on the lens is the optical centre of the lens.  

Mark the optical centre with a felt tip pen. Neutralisation of the lens should occur at the optical centre.

When light goes through the optical center (OC) of the lens, it goes straight through. It is not bent.

When light goes through any other point on a lens, the ray of light is bent.

When the optical center of a lens is moved away from its expected position in front of the eye, that lens now causes a prismatic effect.

The farther the lens is moved or decentered from its original position the greater the amount of resulting prism.

At the exact OC of a lens, front and back lens surfaces are parallel to each other. The line that passes through the OC of a lens is known as the optical axis.

If the optic axis of a lens passes through the center of the pupil, the lens is centered in front of the eye. If the lens is moved so that it does not coincide with the line of sight of the eye (for our purposes at the center of the pupil), it is said to be decentered.

What happens when the lens is moved so that the center of the lens is no longer in front of the center of the eye?

When the wearer looks right through the center of the lens, the object is not displaced from its actual location.

But when a plus or minus lens is moved off-center in relationship to the location of the eye, the object appears displaced . This means that a decentered lens causes a PRISMATIC EFFECT.

Δ = cF is commonly known as

Prentice’s

c = image displacement in cm F =lens focal length

When a lens is decentered, a prismatic effect is created. With decentration, both prism power and prism base direction are manifested.

The power of the prism depends on the amount of lens decentration and the refractive power of the lens being decentered.

The prism base orientation depends on the direction of decentration and whether the lens is positive or negative.

Q

Q: If a lens having a power of +3.00 D is decentered 5 mm away from the center of the eye, how much prismatic effect will this cause?

To find the prismatic effect, simply multiply the distance in centimeters that the lens has been displaced by the power of the lens. Since 5 mm equals 0.5 cm,

Prism dioptres = 0.5 (3.00) Δ = 1.5

When a sphere lens is decentered both horizontally and vertically, the most straightforward solution for finding the prismatic effect is to consider each component by itself.

Q:If a +3.50 D sphere is decentered 4 mm in and 5 mm down, what is the resulting prismatic effect?

In this situation, the two decentrations may be handled independently. The horizontal decentration results in: Δ = (0.4)(3.50) = 1.40 or 1.40Δ BI

The vertical decentration gives:

Δ = (0.5)(3.50) = 1.75

or 1.75Δ Base down

Cylinders produce varying prismatic effects when decentered. These prismatic effects depend not only on the power of the cylinder but also on the orientation of the cylinder axis.

If the axis of a Plano cylinder is oriented in the direction of decent ration, there will be no prismatic effect induced regardless of the amount of decentration - no power in the axis meridian of a Plano cylinder.

If, however, the cylinder axis is at right angles to the direction of decent ration, the amount of prism induced varies according to Prentice’s rule.

1. Calculate for the sphere and cylinder separately and combine the results.

2. Transpose the prescription to crossed cylinder form. Each cylinder may then be worked independentlyand the results combined.

3. Use higher mathematical computations