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.
If you want to learn more, please visit our website.
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.
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.
WHAT&#;S STRABISMUS? A small percentage of the population experiences vision problems due to strabismus. Strabismus is the failure of both eyes to simultaneously direct their gaze at the same object in space due to an imbalance of the extraocular muscles. When eye muscles don't move together in perfect harmony, blurred or double vision can occur. Diplopia is double vision caused by a lack of fusion and is usually associated with an imbalance of the extraocular muscles.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.
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.
OPTICAL CENTER Rays of light striking a corrective lens at an oblique angle are refracted, but rays that enter the lens straight on are not. This small area of a lens, where no refraction takes place and where there is no prism, is called the optical center (OC). It corresponds to the thinnest point of a minus lens and the thickest point of a plus lens.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 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.
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
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.
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.
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.
By Andrew S. Bruce, LDO, ABOM, NCLEC
Release Date: April 1,
Expiration Date: July 21,
Learning Objectives:
Upon completion of this program, the participant should be able to:
Faculty/Editorial Board:
Andrew S. Bruce, LDO, ABOM, NCLEC
Credit Statement:
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). General Knowledge. Course STWJHI305-2 ABO
Let&#;s be honest, how many of us feel the urge to run and hide when presented with a prescription that includes prism? It&#;s OK&#;you&#;re not alone. Most of us rarely see a prescription with prism; even seasoned opticians have little practical experience working with prescribed prism unless they work in a practice that specializes in prism. We can easily become intimidated, over think and over complicate the subject. Take a deep breath and follow along with me as we take a step-by-step approach to prism creation (unwanted or prescribed) in a lens, along with its function and visual impact. Once you understand the basic effects and uses of prism in eyeglasses, you will find much of your anxiety driven fear of prism alleviated. The purpose of this threepart course is to provide an introduction and explanation of prism and its uses, moving on to more advanced discussions and calculations.
What is a prism? Prism is a transparent triangular refracting medium with a base and apex (Fig. 1). Its apical (prism) angle determines its dioptric power. A prism of one prism diopter power (&#;D) produces a 1 cm apparent displacement of an object located one meter away. Light entering the prism will deviate toward its base&#;however, the apparent image shifts (is displaced) toward the prism apex (Fig. 2). When used to correct for eye deviations, prism displaces the image of the object to align with the eye&#;s deviated visual axis. The image formed on the retina of the deviated eye will now be similar to that formed on the retina of the nondeviated eye so that binocular fusion occurs, and the brain creates one single focused image by fusing the left and right retinal images.
yanggu supply professional and honest service.
A prisms&#; ability to deflect light is measured in prism diopters, denoted as &#;D, the Greek capital letter delta symbol. A 1D prism will displace an image 1 cm at a distance of 1 meter. Note: A prism diopter and lens diopter is different. Prism diopters change the path of light. A lens diopter is a measure of focal power (vergence), its ability to converge or diverge light to shorten or lengthen the focal length. As you already know, lens dioptric power is the inverse of its focal length in meters. (1/f=D) But this is another lesson for another course; today, we are dealing with prism diopter.
There are two methods used for specifying prism called rectangular coordinates (Fig. 3) and polar coordinates. The prescriber&#;s method uses rectangular coordinates of horizontal and vertical measurements. The laboratory reference system uses a polar coordinate system of specifying the direction in degrees. Prescribing doctors and opticians in the U.S. typically use rectangular, while optical labs and European countries use polar coordinates.
Base In (BI), Base Out (BO), Base UP (BU) and Base Down (BD) or a combination of vertical and horizontal base directions.
Rules for horizontal or lateral prism: Compounding effect (prismatic effects of each eye are additive)&#;The prism bases must be in the same direction OU. For example, BI OU. Canceling Effect (prismatic effects are subtractive)&#;The prism bases must be in opposite directions OU. For example, BI and BO.
Rules for vertical prism: Compounding effect&#;The prism bases must be in opposite directions OU. For example, BU and BD. Canceling effect&#;The prism bases must be in the same direction OU. For example, BU OU.
Oblique prism: Prisms are rarely just up, down, in or out. Most are oblique, which requires that a horizontal and vertical base direction be specified. Using rectangular coordinates will look like this example: OS 2D BI and 3D BU.
Rectangular coordinates can be converted to polar coordinates to determine the resultant prism. Labs typically use the 360 or 180 degree reference system to determine prism base direction. This means that the multiple base directions of the rectangular system are not used. Instead, the prism is resolved into the net amount of prism and degree of placement. What follows are the steps for converting rectangular coordinates to polar coordinates using the 360 degree lab reference system. Example: OS 5BO 2BU (Fig. 4)
1. Create a grid showing 0 degree to 360
degrees. (Fig. 4)
2. Visualize which eye from the patient&#;s perspective.
3. Mark a point on the grid at 5BO and
another at 2BU.
4. Draw a vertical line from the 5BO point
and a horizontal line from the 2BU point.
5. The point of intersection of these two lines
is the resultant of the two combined.
6. Now draw a line from this point to the
center of the grid.
7. Using a mathematical equation known as
The Pythagorean Theorem, the length of
this resultant line can be calculated:
Resultant^2 = 5^2+2^2
Resultant = &#; 29 = 5.39
8. Now, using geometry, the angle between the resultant and the horizontal (xaxis) can be calculated:
Sin angle = 2 &#; 5.39 = 0.371
Sin angle = 21.8 degrees
Resultant polar coordinates of 5BO and 2BU OS = 5.39&#; @ 21.8 degrees
Don&#;t be concerned if this is too much math for comfort, or if you&#;re wondering what on earth is &#;Sin angle,&#; this is advanced optics. It will be handy when you sit for your Masters Certification, you will need to know how to determine resultant prism and how to resolve prism, and you will need an algebraic calculator. But for now, it is mentioned to illustrate the difference between the prescriber&#;s method and the lab method. Both are correct. 5BO and 2BU for the left eye, equals 5.39&#; @ 21.8 &#;. Although prism can be ordered in either polar coordinates or rectangular coordinates, your prism notation on your order should be precisely the same as the prescriber&#;s notation on the prescription.
How does the brain handle prism? The eyes work as a team to produce binocular vision. When prismatic correction is placed in front of one eye, it affects both, the brain applies the effect binocularly. For this reason, the prism can be applied in just one eye or split between both, leading us to the topic of splitting prism. Note: Splitting prism should be done with the prescriber&#;s permission. Always check with the prescriber before splitting prism. Why do we want or need to split prism, and how is this beneficial? Answer: to balance the added weight and thickness resulting from prism, between the two lenses.
Let&#;s look at the following prescription example:
OD: 4.00 DS 8&#;BD
OS: 2.00 DS
First, before even considering the prism, the right eye is going to be thicker at both the upper and lower edges. It&#;s twice the power of the left! Now, consider prism construction, it has a wide base. Prism will always add thickness in the direction of the base. Considering this, return to the above example, 8&#;BD is going to add additional thickness to the lower edge of the right lens. The thickness disparity between the lenses will degrade lens cosmetics and increase the potential for distortion in the lens periphery.
Here is where splitting prism power using the rules of compounding can be beneficial. It allows us to &#;split&#; the prescribed prism power between the two lenses instead of having one thick heavy lens with a visible imbalance in thickness between the right and left lenses. Splitting the 8&#;BD by applying 4&#;BD in the right eye and 4&#;BU in the left eye will &#;share&#; the thickness of the prism between the two lenses. This is more cosmetically appealing than one thick lens with prism and one thin lens without. The prismatic effect of splitting the prisms using the rules of compounding has the same effect as if all of the prism is applied to the right eye.
&#; Always get permission from the prescriber.
&#; Always adhere to compounding rules.
&#; Always make sure the direction of the
prism base as prescribed for the original
eye remains the same.
For example, OD: 16BD OS: PL
OK to split as OD: 8BD and OS: 8BU NOT
OD: 8BU and OS: 8BD
What are some of the conditions for which prism is prescribed? First, what does prism do? It shifts the image in the direction of the apex. Prism is prescribed for various reasons, with the most common reason being muscle imbalance and eye alignment issues from strabismus. It is also prescribed for convergence issues, hemianopia and other conditions. The purpose of the prism is to alter the path of light from the object so that the images viewed by both left and right eyes correctly correspond to the visual axis of the eyes. As an example: If a right eye has esophoria (tendency to turn inward), then base out prism will bring the path of light from an object &#;inward&#; to be inline with the eye&#;s deviated visual axis. This allows the right eye to see the same object as the left eye and form an image on its retina, closer in similarity.
Strabismus refers to misalignment, or deviation of the gaze or abnormal turning of the eyes, typically due to a muscular imbalance. The extraocular muscles of our eyes need to be able to maintain parallel alignment of each eye; both eyes need to be looking at the same thing in space and time. They control eye movement, in tandem, to ensure that any disparity between right and left retinal images is tolerable. This enables the brain to combine, or fuse, the separate images from two eyes into a single image&#;a process called Binocular Fusion.
The most common use of prescribed prism is to compensate for strabismus, a condition where the extraocular muscles cannot maintain a balanced alignment of the two eyes. Strabismus is a broad medical term describing eye deviations that can be broken down into: phorias&#;a tendency for eye turn, deviation; and tropias&#;a definite eye turn, deviation.
Eye deviations fall into two main categories:
1. Comitant&#;The most common in children. The deviation is constant, regardless of the direction of gaze, and
2. Incomitant&#;Deviation is always changing, depending on the direction of gaze.
The prefix of a phoria or tropia indicates the direction of eye deviation.
Eso = In (Esotropia or esophoria)
Exo = Out (Exotropia or exophoria)
Hyper = Up (Hypertropia or hyperphoria)
Hypo = Down (Hypotropia or hypophoria)
Strabismus can cause diplopia (Fig. 5). As stated earlier, extraocular muscles need to collaborate to make binocular fusion possible. If a muscular imbalance is present, resulting in strabismus, there can be an excessive disparity between the images each eye is sending to the brain, preventing binocular fusion. Ultimately, the brain &#;sees&#; and &#;reports&#; two separate images; hence, diplopia (double vision). When this happens during the years of eye development and is left untreated, the brain will often suppress (turn off) the weaker eye. The correctly functioning eye takes over, and input from the other eye is suppressed, a condition called amblyopia or lazy eye. In the past, it was thought that strabismus had to be treated roughly before age 7 to prevent permanent amblyopia. Now even adult amblyopes benefit from treatment.
To review, to add prism to a lens has the effect of shifting the image of an object being viewed in the direction of the prism apex; thus, with eye deviations, it reduces the disparity of images formed on the left and right retinas. This enables the binocular fusion of the two retinal images in the brain. The primary use of prescribed prism is to aid binocular fusion, NOT to fix muscular alignment issues.
Prescribed prism can also help improve quality of life for patients experiencing vision field loss due to neurological diseases such as a stroke or brain injury. These blind spots are known as Scotoma. Brain injuries or aneurysms, sometimes impact the occipital lobe of the brain, the area responsible for visual processing. These can result in visual field defects in specific quadrants, depending on the specific location of the injury.
Fig. 6 illustrates the visual field defect a patient experienced after a stroke. As you see, the patient has lost vision in their left hemisphere of both eyes. When prisms are used to treat such conditions, the conventional rules for compounding and canceling prism are no longer employed. The objective is to &#;shift&#; the patient&#;s gaze in a direction to attempt to &#;look around&#; the defect and perceivably &#;widen&#; their field of vision, improving their quality of life.
Example: In Fig. 6, the following prism may
be prescribed:
OD: BO
OS: BI
Generally, to accomplish the desired outcome, very high prism powers are required, which creates not only cosmetic problems but also a confusing visual environment in which objects unexpectedly appear and disappear from view.
Aesthetics and the Impact of Frame Selection with Prism
As a professional optician, always try to visualize the end product prior to fabrication. In a similar way to how lens thickness can be affected by PD, OC placement, frame dimensions and cylinder axis orientation, the base direction of the prescribed prism will also add thickness to the finished lens.
For example, presented with the following
prescription:
OD: PL sph 5BIOS: PL sph 5BI.
The thickest part of the lens will always be in the direction of the base. The base is the thickest part&#;makes sense? In this example, the lenses will be thickest at the nasal edge (BI OU).
Knowing this helps with frame selection:
&#; Keep frame PD as close as practically possible to the patient&#;s anatomical PD to minimize the necessary horizontal decentration.
&#; Fitting the patient in a zyl frame without nosepads will not only cover up some of the lens edge, improving the cosmetics but also eliminate the need to adjust the nosepads around the thick nasal edges.
&#; If anatomical features necessitate nosepads, the application of an edge roll at the nasal edge might help remedy the adjustment dilemma while also improving cosmetics.
Occasionally, I&#;m faced with a moderate to high prescription and a patient electing to go &#;bigger&#; in frame size, despite my begging and pleading. In this case, all I can do is reiterate my recommendations and forewarn them of the distortion they may experience in the periphery. I hope that this helps prepare them and avoid a negative first impression of their new eyewear. Aspheric designs can help reduce the effect due to a flatter angle formed as the eye rotates away from the optical center of the lens.
Antireflective coating: As stated earlier, prisms deviate light. It&#;s their job! Our real world presents the eyeglass wearer with omnidirectional light entering their lenses. Light entering the lens at oblique angles in an uncoated lens is going to scatter and reflect more from both the surface and internally, resulting in halos and ghost images that will be particularly apparent at night when driving into oncoming headlights. This effect will be exacerbated in the presence of a prescribed prism. The application of an antireflective treatment will minimize reflections and scatter to reduce eyestrain and fatigue, enhancing clarity and acuity&#;our primary objective.
Nonprescribed prismatic effect of eyeglass lenses with centration errors (Fig.10): Prism occurs whenever there is a difference in lens thickness between two points on the lens. Because lenses with power always have a variation in lens thickness, prescription eyeglass lenses produce prismatic effects away from the optical center of the lens. At any point away from the optical center of the lens, a minus lens, which is thicker at the edge and thinner at the center, produces a prismatic effect with the prism base pointed away from the optical center. Conversely, a plus lens, which is thicker at the center and thinner at the edge, produces a prismatic effect with the prism base pointed in toward the optical center. The amount of prism at any point on a lens is directly proportional to the power of the lens and the distance from the optical center. Prentice rule is used to calculate the amount of prism present at any point in a lens.
Prism = Decentration (distance) x Power ÷ 10 (Example: +6.00 D x 5 mm = 30 ÷ 10 = 3 &#;D. This formula gives you the amount of prism, but we need to know the base direction. If this is a right lens, then the amount of prism 5 mm below the OC is 3D D BU in a plus lens. If we want to know the amount of prism in this same lens, 5 mm out from the OC it is 3&#; D BI.
A plus lens (Fig. 8) is constructed of base to base prisms, and because light deviates toward the base, it emerges from a plus lens converging: plus power (vergence). Plus lenses are a base in configuration.
A minus lens (Fig. 9) is constructed of prisms arranged apex to apex, causing the light to bend toward the edges and emerge diverging: minus power (vergence). Minus lenses are a base out configuration. If you move a plus lens in front of your eye, you will see &#;against motion.&#; Do the same with a minus lens; you will see &#;with motion.&#;
What does prism do for our patients? As discussed earlier, it can be both beneficial if prescribed, but it can be detrimental if mistakenly induced in the patients&#; ophthalmic lenses. How can prism be mistakenly induced?
Prismatic effect from horizontal decentration errors (OCs and PDs not aligned): Through the optical center of the lens, rays of light do not deviate and therefore produce no prism. (OC = ZERO prism). As the distance from the optical center increases, rays of light will be deviated by increasing amounts.
As illustrated in Fig. 10, misaligned PDs or OCs can induce unwanted prism, which can potentially cause vision problems and discomfort. These problems can present as distortion, headaches, a pulling sensation, eyestrain and fatigue and in severe cases, diplopia (double vision).
ANSI standards indicate the following prism tolerance limits: horizontal prism < 2/3 &#; D and vertical prism < 1/3 &#; D
In Part 2, we will address vertical prism imbalance in multifocal lenses and the effects of antimetropia or anisometropia.
Now the job comes back from the lab, and it&#;s time to verify. You are verifying the correct prism amount and base direction, or you are determining the prism error in a lens. Take a deep breath, and here we go.
&#; Mark the lens OC.
&#; Viewing the lens through a lensometer, verify the prism through the OC, starting with the highest powered lens (most plus) first.
&#; Very important: Set the platform position to check the highest powered lens and DO NOT move it.
&#; Record vertical and horizontal prism present at this point in the first lens&#;the point of intersection of the middle line of the lensometer mires in each meridian.
&#; Switch to the opposite lens, keeping lensometer platform height unchanged.
&#; Verify prism.
&#; Record vertical and horizontal prism.
&#; Calculate net prism using rules for compounding and canceling prism and direction.
Note: The above procedure works for single vision and lined multifocal designs.
In a PAL, the prism can be verified only at the Prism Reference Point (PRP)&#;the point below the fitting cross and horizontally centered 17 mm in from the two engraved circles. This makes verifying prism in a PAL much easier than other lens design&#;it is definitive!
1. Locate engraved circles and mark them.
2. Position lens over PAL cutout template (or locate midpoint between the engraved circles).
3. Mark the PRP position (the dot below the fitting cross and centered between engraving marks).
4. Read vertical and horizontal prism through this point (PRP) for both lenses and calculate net error/imbalance or verify the prescribed prism amount.
NOTE: Do not confuse the fitting cross with the PRP. Verify prism at the PRP and verify distance power at the distance reference point above the fitting cross.
Do not be surprised if you find vertical prism ground at the PRP, despite not being prescribed. This is likely due to prism thinning. Prism thinning is a technique utilizing equal amounts of vertical prism in the same direction in each eye, usually base down, to create additional thickness in the lower portion of a PAL to accommodate the steepening base curves necessary to provide the increase in add power. It can also be used to reduce edge thickness in a high minus PAL. Since it is equal in both magnitude and direction and relatively minimal, it does not adversely affect a patient&#;s acuity. Ground prism resulting in no net effect is known as yoked prism and will be discussed in greater detail in Part 3.
Congratulations, you have completed this basic introduction to the world of prism. We will build on this foundation in Parts 2 and 3. I hope you are feeling less intimidated, and for those of you who can&#;t get enough, I hope I&#;ve whetted your appetite, and you are eagerly anticipating more. Either way, I hope you will all join me for the upcoming Parts 2 and 3 of The Spectrum of Prism Optics.
If you want to learn more, please visit our website Optical Glass Prisms.