Part 1: Understanding PRISM

A look at how prisms deviate light and are the basis for all corrective lenses

By Jenean Carlton, ABOC, NCLC

Prism correction, and how it applies to ophthalmic lenses, can be a difficult topic to comprehend. However, gaining an understanding of prism is made easier by learning how prisms deviate light, and how they are the basis for all corrective lenses.

Prisms are triangulated shapes of glass or plastic that 1) disperse light into its component spectral colors; and 2) change the direction of light passing through. Because a prism has the ability to change the direction of light passing through, an object viewed through a prism is perceived to be in a different location than it would appear if seen with the naked eye.

Ophthalmic prisms are frequently illustrated in the shape of a triangle that shows the difference in thickness between its base and apex; the base is the thickest part of a prism and the apex is the thinnest. Unless the degree of prism correction is somewhat strong though, prisms incorporated in ophthalmic lenses are difficult to distinguish without the use of a lensmeter.

BENDING LIGHT RAYS

A prism has no focusing power but changes the direction of light rays passing through. Rays of light enter a prism and bend toward the base, while objects viewed through a prism are displaced toward the apex.

This phenomenon of how a prism displaces light toward its base, and then effectively displaces objects towards its apex, is a constant rule in physics. The amount of displacement is dependent on the size of the angle of the apex. The greater that angle, the thicker the prism will be. Thicker prisms equate to a stronger amount of correction and displace objects to a greater degree; thinner prisms have a weaker amount of correction and displace objects to a lesser degree.

Prism power is measured in prism diopters and is denoted by the Greek letter delta (˘). One prism diopter displaces an object one centimeter at a distance of one meter from the eye.

An easy way to understand the basic principles of plus- and minus-powered lenses is to think of them as a combination of prisms.

PLUS-POWERED LENS BASICS

Plus-powered lenses are convex, with one or both surfaces curving outward. Convex lenses can be thought of as two prisms placed base-to-base, with the middle corners smoothed into a curved shape. Convex lenses converge light, meaning that rays of light passing through a plus-powered lens come together at a focal point behind the lens.

With plus lenses, the base of the prism will always be toward the optical center.

Plus-powered lenses correct hyperopia, also known as far-sightedness, as well as presbyopia.

■ HYPEROPIA. This is a refractive error where light rays come into focus behind the retina. Hyperopes are “far-sighted” because they see objects at a distance more easily than up close. The condition is attributed to the length of the eye being too short for light to come to a focus on the retina. For good vision, it is imperative that light enters the eye and then comes to a focus on the macula, a small, yellowish-colored indentation on the retina.

A vascular structure at the back of the eye, the retina houses millions of photoreceptor cells called rod and cone cells. Rods are employed for motion and vision in low-light conditions and are located mostly outside of the fovea centralis—the central-most area of the macula. Cone cells are primarily within the fovea centralis and provide sharp visual acuity and color vision.

■ PRESBYOPIA. The crystalline lens is the natural lens inside the eye that focuses light onto the retina and has about 16 diopters of power. The accommodative power of that lens diminishes with age, and that's when presbyopia occurs.

Accommodation is the process of the ciliary muscles to change the shape, and therefore power, of the crystalline lens so that distant and near objects focus clearly on the retina. Presbyopia is the inability of the eye to focus sharply on near objects due to the loss of elasticity of the crystalline lens.

MINUS-POWERED LENS BASICS

These lenses are concave, meaning one or both surfaces of the lens curve inward. Concave lenses can be thought of as two prisms placed apex-to-apex, with the bases being toward the thicker edges of the lenses. Since the base of a prism is the thickest part, the base of the prism will always be away from the optical center with minus lenses.

Concave lenses diverge light. Therefore, when light rays enter a minus lens, they spread apart. To visualize the path of light through a minus lens, the direction of the rays can be extended backward and drawn in an imaginary or virtual point of focus in front of the lens.

■ MYOPIA. Minus-powered lenses correct vision for patients with myopia. Myopia is a condition of the eye where rays of light come to a focus in front of the retina. Myopia is attributed to the length of the eye; the myopic eye is too long for images to come to a focus on the retina. Patients with myopia are said to be “near-sighted” meaning they see near objects better than those at a distance. Corrective lenses for myopia bring light to a focus onto the retina.

BEYOND PLUS AND MINUS

There are many other basics about corrective lenses and prism that go beyond whether the lenses are plus- or minus-powered.

■ PRESCRIBED PRISM. The human eye has six extraocular muscles that control eye movement: the medial, lateral, superior, and inferior rectus muscles, and the superior and inferior oblique muscles. The medial rectus muscle is positioned along the side of the eye near the nose and adducts the eye, moving it toward the nose. The lateral rectus muscle is responsible for horizontal eye movement and abducts the eye, thereby moving it toward the temple.

The superior rectus muscle lies across the top of the eye and raises, adducts, and intorts the eye. The inferior rectus muscle is underneath the eye and adducts, depresses, and extorts the eye. The superior oblique muscle is positioned across the top of the eye and depresses, abducts, and intorts the eye. The inferior oblique muscle is underneath the eye and elevates, abducts, and extorts the eye.

Extraocular muscles are innervated, or stimulated, by cranial nerves in the brain. The movement impulses from the cranial nerves produce contractions in the ocular muscles that result in movements of the eye.

Top: Plus lenses can be thought of as two prisms placed base-to-base, and minus lenses as two prisms placed apex-to-apex; Bottom: A. Light is deviated by the prism toward its base. B. An observer views an object through the prism and the object appears displaced toward its apex

■ BINOCULAR VISION. For most people, both eyes work together by projecting to the same point in space, with each eye capturing a slightly dissimilar image. We perceive one image because our brain combines the two images, one from each eye, into one.

One image is achieved because the extraocular muscles that control eye movement work in tandem so that each eye captures an image that is only slightly dissimilar from the opposing eye.

The ability of our eyes to take two slightly dissimilar images, one from each eye, and fuse them into a single image is called binocular vision. Good vision occurs when the eyes are parallel while looking straight ahead and are able to maintain this parallel alignment when gazing in other positions.

This is the key to binocular vision and is termed fixation.

PRISM CORRECTION

Prism correction is prescribed as base-out, base-in, base-up, or basedown, depending on the phoria or tropia, and degree of correction needed. A tropia is a deviated or turned eye that, for the most part, can't be controlled by the patient. Conversely, a phoria is a tendency for the eye to drift in or out of alignment and can be controlled by the patient to some degree.

Objects viewed through a basedown prism will appear to be displaced upward; objects appear to shift downward when viewed through a base-up prism. Objects viewed through a base-out prism appear to shift inward and those viewed through a base-in prism appear to shift outward.

If a patient has only one eye that requires prism correction, the refractionist may decide to split the correction between both eyes to balance the weight and cosmetic appearance of the lenses. When prism is split in this way, the lenses effectively displace objects viewed through both lenses so the patient is able to appreciate one fused image.

When a prescription includes prism, it must be decided whether decentering an uncut stock lens, in the case of single vision lenses, will provide the prescribed amount of prism or if grinding prism into the lenses is necessary.

PRESS-ON LENSES

When a strong amount of prism is needed to achieve binocular vision, a Fresnel or press-on prism is often used. Fresnel prisms are commonly called “press-on” prism and are made of thin plastic. They are available in up to 40.00 prism diopters and are easily cut to the shape of a spectacle lens and pressed onto the lens surface. Since they aren't permanently attached, Fresnel press-on prisms aren't ideal. However, when compared with the thickness and weight of lenses with a high degree of prism correction, they may be cosmetically preferred by patients. One drawback is that they have a pattern of fine lines embedded into the plastic, which patients may consider unattractive.

Prisms have the ability to change the direction of light passing through. Because of this, objects viewed through a prism are displaced toward the apex.

Whether plus or minus, corrective lenses can be thought of as a combination of prisms. Plus lenses can be viewed as two prisms placed base-to-base and minus lenses as two prisms positioned apex-to-apex.

Understanding the role of prism is an important basic in the most effective prescribing and dispensing of vision correction. EB

Understanding Prism Part 1

This fun crossword puzzle explores prism and how it pertains to vision correction. The answers to this puzzle will be listed in the November issue of Eyecare Business and online at eyecarebusiness.com. Look for clues on our Facebook page, facebook.com/eyecarebusiness.

across

4 Double vision.

8 The process of the ciliary muscles to change the shape and power of the crystalline lens.

10 A prism has the ability to __________ the direction of light passing through.

12 The human eye has six ________ muscles.

13 For good vision, light needs to enter the eye and come to a focus on the _________.

14 Optical lenses are a combination of ________.

15 Extraocular muscle are _________ by cranial nerves in the brain.

16 Natural lens inside the eye. (Two words)

19 The goal for treating strabismus is to establish _________. (Two words)

22 Light is deviated by a prism toward its _______.

23 Objects viewed through a prism are _______ to be in a different location.

25 Patients with this refractive error are “far-sighted.”

28 Minus powered lenses are ________.

30 The thickest part of a prism.

31 Concave lenses _________ light.

32 Photoreceptive cells responsible for movement and night vision.

down

1 The vascular structure at the back of the eye.

2 The area of a lens where no refraction takes place. (Two words)

3 The ability of our eyes to take two images and fuse them into one. (2 words)

5 A prism has no ___________ power.

6 The central most area of the macula. (Two words)

7 The thinnest part of a prism.

9 Convex lenses _________ light.

11 The rotation of the eye toward the nose.

14 The inability of the eye to focus sharply on near objects due to the loss of elasticity of the crystalline lens.

17 The rotation of the eye toward the temple.

18 Press-on prism.

20 Objects viewed through a prism appear to be displaced toward its _______.

21 The failure of both eyes to simultaneously direct their gaze at the same object in space due to an imbalance of the extraocular muscles.

24 Prism power is measured in ________ .

26 Patients with this refractive error are “near-sighted.”

27 Plus-powered lenses are ________.

29 Photoreceptive cells responsible for color vision.