- Email: [email protected]
diffractive bifocal contact lens
The Holographic/Diffractive Bifocal Contact Lens David S. Loshin, OD, PhD
Segment Bifocal Contact Lens
ABSTRACT Presently, practitioners are somewhat limited in the available options for correction of presbyopia with contact lenses. Neither the form of contact lens design (segmented, aspheric, or concentric) or fitting scheme (such as monovision) have high enough success to dominate the market. To meet this growing demand, a new holographic/diffractive bifocal contact lens is being introduced by several manufacturers 1'2,3,4,s. Although different lens materials and slightly modified designs have been proposed, these lenses all form images based upon a similar concept. This paper attempts to simplify the principles behind "how the holographic bifocal contact lens works." This will include a review of diffraction and refraction as well as the application of these concepts in the diffractive lens design. Comparisons between the holographic and conventional bifocal contact lenses along with advantages and disadvantages will also be presented.
distance vision
near vision
zones which contain the distance and near corrections. The lens is fit so that both zones fall within the pupUlary zone and images of distance and near objects are superimposed on the retina. In theory when attending distance objects, one concentrates on the focused distance image and suppresses the blurred near image; when attending near objects, the oppo-
INTRODUCTION Bifocal Contact Lenses Currently there are several types of bifocal contact lenses on the market. These lenses may be classified based upon two related criteria: 1) near and distance vision zone location and 2) how the lens provides the two images. The conventional bifocal contact lens or segmented bifocal has a near zone similar in relative shapes and position to that found in spectacle lenses. In straight ahead gaze, the lens is usually centered on the eye with the near portion below the pupil. This position is maintained by truncation or prism ballast. Upon downward gaze the lens pushes against the lower lid; the eye moves relative to the lens, and one is able to view through the near portion. With experience and proper patient selection, this type of presbyopic correction can be quite successful; however, fitting is somewhat difficult and time consuming, which makes this option undesirable by many practitioners. Simultaneous vision lenses have two concentric
Volume 16, Number 3 March 1989
Simultaneous Vision Concept focused distance
neat" uTtage
focused
near object
blurred distance image
77
site scheme is used. With this type of correction, pupil diameter (for given lens parameters) alters the balance between distance and near image intensity. With a small pupil, light reaching the retina could be limited to the central zone. If the pupil is large, the contrast of the "in focus" image is reduced by the blurred "out of focus" image. In addition, if the area of the two zones are not equally represented, one image may be significantly brighter than the other. These types of lenses do not require truncation or prism ballast for proper positioning; however, they are most successful when the bottom lid stops the lens in downward gaze permitting translation of the lens on the eye. Aspheric and concentric bifocal contact lenses fall in this catagory. The aspheric bifocal contact lens has a progressive change in addition power symmetrically placed from the center to the periphery of the lens. This type of lens should provide corrections for intermediate disASPHERIC CONTACT LENS DISTANCE IMAGE 1.0
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image at the expense of near image quality. Larger pupils provide more addition power (area of the lens) and slight improvement in near images; however, this occurs in conjunction with a reduction in distance image contrast. A compromise between the distance and near image quality exists for one pupil size. We have demonstrated this change in image quality as a function of pupil size through the measurements of the modulation transfer function (MTF). The details of these measurements are given elsewhere 6,7. In this paper I will only point out that higher values of modulation will yield superior image quality. For the distance images (curves on left), the 2mm pupil yields a far superior image as compared to the 4mm or 6mm pupil. The near image MTFs (curves on right) all show approximately equally poor image quality for all pupil sizes. The concentric bifocal has a circular central zone surrounded by an annular outer zone. The lens is manufactured either with a central distance-annular
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tances when regions between the distance and near correction are exposed. The image formed will always be somewhat degraded, since the lens area falling within the pupil contains a continuum of powers. The size of pupil will determine the degree of image degradation, with smaller pupils yielding a better distance 78
near correction or a central near-annular distance correction. Image quality of the concentric bifocal can also be demonstrated through MTF measurements. The lens we used for these measurements had a near addition in the center with the distance correction in the periphery. With a 2mm pupil diameter, rays could only pass through the central zone, and the best near image was found. Note also no distance image is found for the 2mm diameter pupil. For larger pupils diameters, the near image quality decreased and the distance image quality increased. The 6mm pupil diameter yielded the best distance image quality and the worst near image quality. The concentric bifocal may also be used to demonstrate the dependence of pupil size on the image illumination. In our lab, we measured the relative intensity of the distance and near images formed by a concentric bifocal contact lens (with a 2mm optical zone) with a 3mm and a 4mm pupil diameter. As shown in the graph below, the 3mm and 4mm pupils form a distance image (right peak) with approximate relative amplitude of 0.4 and 0.3 respectively. For the near image (left peak) the 3mm pupil has a relative amplitude (0.4) that is two times that of the 4mm pupil (0.2). These findings may be explained based upon the difference in relative areas of the near and disICLC
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tance portions of the lens as the pupil diameter increases. For example, for an increase in pupil diameter from 3mm to 4mm, the area of the near portion of the lens increases approximately 1.5 times resulting in an increase of near image relative amplitude of approximately 1.8 times. THROUGHTHE LENS FOCUS Concentric Bifocal Contact Lens 0.5
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One other form of bifocal correction that may be considered to yield simultaneous vision is monovision. This technique of correcting one eye for distance and one eye for near vision has somewhat different mech-
40
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anisms for suppression of blurred images. In monovision, one eye is used for distance vision and the other eye is used for near vision. In each case the corresponding eye is suppressed. This differs from the other simultaneous lenses described which require suppression of one image in each eye. Some practitioners use simultaneous vision lenses in monovision fitting. For example, a concentric bifocal lens with different zone diameters in each eye has been found to more equally balance image intensities. Although the monovision technique reduces binocularity, many practitioners have great success with the fitting scheme. What about the new holographic bifocal contact lens? When one hears the term hologram, one thinks of lasers and three dimensional images. A hologram is actually a photograph of an interference pattern formed by two beams, one directly from a laser (reference beam) and the other from laser light reflected off of an object (object beam). After processing, the hologram is placed back in the path of the reference beam. Light is diffracted by the hologram into wavefronts which are exactly the same as those that were previously leaving the object. The holographic bifocal contact lens is really a diffractive lens. The lens actually forms images by both refraction and diffraction. At this point a review of
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Volume 16, Number 3 March 1989
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79
refraction and diffraction is in order. REFRACTION The concept of refraction should be familiar to practitioners since they use the principles for all conventional corrections. Refraction or the change in velocity of light in an optical media as compared to that of vacuum, usually results in a change of direction, or bending of emergent rays. For refraction by a prism, the amount of bending is called the deviation: the angle between the incident ray and the emergent ray. Deviation is a function of index of refraction and the apical angle of the prism. For ophthalmic prisms, deviation may be expressed as:
One type of refracting element that employs this concept of deviation directly is a Fresnel lens. The lens consists of concentric prisms with increasing apical angle from the center to the periphery. Since all the rays which are refracted by the lens are focused to a point, the peripheral prisms deviate the light more than the ones closer to the center. When compared to a conventional lens, the Fresnel lens has reduced image quality due to the scatter at the facets between concentric prisms, the discrete steps of constant deviation and possibly the lack of any phase relationship from one prism to the next. Spherical lenses are a special case of this prismatic deviation concept of refraction. Here the lens has an L
8=(n-1)~ where:
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8 is the deviation
n is the index of the prism ~, is the apical or refracting angle If the index of refraction is constant, the deviation increases as the apical angle increases. This is easily demonstrated by imaging through a bar prism.
infinite number of prisms with smoothly increasing apical angles from the center to the periphery of the lens. The deviation smoothly increases to refract rays towards a single image point. DIFFRACTION
REFRACTION THROUGH A PRISM
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The concept of diffraction is not quite as familiar to practitioners as refraction. Diffraction can be considered to be the interaction of light waves with obstacles. This interaction often results in a change in direction of travel that can be controlled by proper configuration of the obstacle. Unlike refraction, diffraction produces multiple images called orders. These orders are symmetrically displaced spatially and are
D I F F R A C T I O N ORDERS
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labeled as 0, -+ 1, _+2, _+3, etc. Typically, as the order of diffraction increases, the images formed in each order decrease in intensity. Here again the distribution of image intensity (i.e. the intensity of each order) can be controlled by selection of the obstacle. Diffraction is easily demonstrated with a diffraction grating which consists of narrowly separated dark bars. For a specific wavelength, the amount of diffraction (or spatial displacement of the orders) is a function of the separation of the bars i.e. the smaller the separation, the greater the diffraction and resulting spatial separation of the orders. For example, a grating with 2000 lines per inch diffracts light less than a grating with 7000 lines per inch and the orders are spatially closer together. ICLC
DIFFRACTIONTHROUGHA GRATING GratingSpacingWillSpatially Alter Order Location
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Paralleling the extention of refraction by a prism to a refractive imaging element (Fresnel lens), the concept of diffraction can be extended to a diffractive imaging element; the Fresnel zone plate. A Fresnel zone plate consists of alternating dark and light rings of varying width and separation. The spacing of the rings varies
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rrn = where: rm = the radius of the mth annulus m = the annulus number f = focal length of the Fresnel zone plate X. = the wavelength of light Higher power zone plates have central zones with shorter radii and for a given plate size more annuli are required. This concept will be important when determining the add power of the diffractive bifocal lens. Another principle that must be discussed in order to explain the action of the zone plate is that of the phase relationship between waves. Electromagnetic Volume 16, Number 3 March 1989
I
radiation which includes visible light may be represented as sinewaves. These waves have maximum
-
from the center to the periphery of the plate, with the closer spacing in the periphery for greater diffraction. This is similar to the Fresnel lens which has larger apical angle prisms in the periphery for greater deviation. The focal length of the zone plate is a function of the radius of the annuli and wavelength:
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(peak) and minimum (tough) displacements, the magnitude of which is the amplitude of the wave. The distance between successive peaks or troughs is the wavelength. Phase is the spatial relationship between the peaks and troughs of two or more waves. When waves are added a new wave results. The phase between the waves determines the amplitude of the resulting wave. When the waves are in phase peaks add to peaks and troughs to troughs and the net has a greater amplitude. When waves are out of phase the peaks and troughs of one wave do not correspond to the peaks and troughs of the other waves. If the waves are ,rr out of phase (or one half wavelength in path), the peaks of one wave correspond to the troughs of the other wave, and the result is a wave with a reduced amplitude. I will call waves that are in phase: additive (constructive interference) and waves that are ~r out of phase: subtractive (destructive interference). Regions of the incident light can be considered to consist of waves that are both in phase and out of phase with each other. The zone plate is designed to permit waves that are in phase to pass through (clear portion) and waves which are out of phase to be blocked (dark portion). The resulting image is composed of all waves that are in phase or are additive. If properly configured, a large percentage of the transmitted light will be diffracted into the desired order or 81
IMAGING WITH A STANDARD FRESNEL ZONE PLATE
.Imaige formed with m phase waves. Out of phase waves are blocked and do not contribute to image. I out of phase subtractive
image location. The Fresnel zone plate, however, is not considered to be an efficient imager since a portion of the incident light is lost due to the blocking of the out of phase waves. One can increase the efficiency of the zone plate by replacing the opaque zones with a different index of refraction. This will allow more of the incident light to be transmitted through the plate. Since higher indices retard the velocity of waves, by using the proper index and depth, the portion of the out of phase waves that strike the blocked zone may be retarded by a multiple of half-wavelengths. This will change the out of phase
waves into in phase waves and all the waves diffracted through the plate will become additive. Alternately, if part of the material in the blocked zones is cut away (with the proper depth), the waves which fall in these regions will increase in velocity. This will result in a phase shift by a multiple of half-wavelengths with the same result. A combination of the two methods (i.e. cut away material and replace it with a different index) will provide more phase control. All these methods form an image that is much brighter than that formed by a zone plate. This type of imager is called a phase
plate.
IMAGING WITH A MODIFIED FRESNEL ZONE PLATE: A PHASE P L A T E clear zone with gradient index of refraction
Image formed with all waves since all are in phase. Out of phase waves are shift to in phase by changing the index o f the opaque zones.
out of phase subtractive
DIFFRACTIVE BIFOCAL The diffractive bifocal forms images by both diffraction and refraction. Diffraction is accomplished by cutting circular annular grooves on the back surface of a standard contact lens. Tears fill these regions of the lens providing a difference in index as with the phase plate. As with the zone plate, the radii and spacing of 82
these annuli determine the addition power. The depth and shape of the cuts of the annuli grooves control the intensity and distribution of images formed in each order of diffraction. In order to have optimum performance, the majority of the light must be concentrated equally into two orders which represent the distance and near images. Klein and Ho e have shown how the ICLC
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positive first order of diffraction. Thus there is even distribution of energy into two orders: a case that could be (and may be) used for the diffractive bifocal lens. Note that 20% of the incident light is lost to higher orders of diffraction, therefore, even for the "best" case, the two equally bright images formed represent only 80% of the incident light. Now that we have discussed how two equally bright
distribution of light intensity in each order may be altered by changing the surface profile of the grooves. For example, with the alternating profile with zero phase, the majority of the energy is unevenly distributed over three orders (0, +1, +2) and therefore is unacceptable for the bifocal lens application. The nonalternating profile divides the image intensity equally with approximately 40% in the zero and 40% in the
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images can be formed by diffraction, we must go one step further to explain the role refraction plays in the holographic lens. As previously stated, the zero order of diffraction does not change direction, i.e. an object
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at an infinite distance would be diffracted to an infinite image location. This would be the case if the grooves were cut into a flat plate, however, the contact lens has curved surfaces which have dioptric power. Light in the
DIFFRACTIVE BIFOCAL CONTACT LENS
contact lens distance
object
Near image formed by diffraction. Distance image formed by refraction.
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tear layer
Volume 16, Number 3 March 1989
83
zero order of diffraction is therefore refracted to an image location depending upon the total power of the lens. This refraction is similar to that of a conventional single vision contact lens used to correct distance vision. The diffractive bifocal contact lens forms the near image by diffraction and the distance image, for the most part, by refraction. The resulting images have a slight reduction in intensity. In addition, just as with the other simultaneous lenses, the images have a reduction in contrast, i.e. a blurred image is superimposed over the focused image. An analogy to this situation could be made using two slides projectors. Start with the exact same slide in both projectors, with the two projected images superimposed on a screen. The net image would be bright. Now defocus one image or put another slide in one projector and defocus. Superimposed upon the original image is a veiling luminance which results in a contrast reduction. This can result in visual problems when the original objects viewed have reduced contrast to start. One example is reading a menu in a dimly lit restaurant. Since the distribution of the blurred diffracted image differs from that of the projectors, the in-focus diffracted image has less reduction in contrast than the image formed by the projectors.
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ADVANTAGES pupil independance. equal image intensity. simutaneous vision diffracledblur blur ease of fitting. success quickly known
and no annular zones fall within the pupil. This eliminates or reduces the near image intensity. Churms et al. 9 and Stone 1° have reported some clinical results with the Diffrax lens. The initial visual adaptation difficulties reported include: a. b. c. d. e.
Slight loss of contrast at distance and near Ghost image especially at near Print appears to "standout" from the page Patients had difficulty finding the near image Objects appear to have a colored fringe
The reduction in image contrast is the major disadvantage of the diffractive lens. This should not be a problem in bright, high contrast conditions, however, with evening or dimly lit environment or task, it may present a problem. The diffracted blur image has a different intensity distribution than a refracted blurred image. This, in theory, results in less contrast reduc84
We are currently measuring the MTFs of several types of the diffractive bifocal lenses. Preliminary results indicate that image quality is indeed somewhat independent of pupil size. The distance image (for the lenses measured to date) has slightly better image quality when compared to the diffracted near image. The relative intensities (as a function of pupil size) also conforms with the theory of equal intensities for the two images. CLINICAL IMPLICATION OF THE DIFFRACTIVE BIFOCAL CONTACT LENS PrO and Cons Listed in the table are the advantages and disadvantages for the diffractive bifocal contact lens. A major advantage of the diffractive lens is pupil independence, i.e. the distribution of image intensity is independent of pupil size. If the incident light is diffracted by just some of the annuli, the two images will have approximately equal intensity. This contrasts the other simultaneous vision lens as shown by the variation in image intensity as a function of pupil size for the concentric bifocal contact lens. One potential problem is low add powers where the first diffraction annuli has a fairly large radius (=3mm)
DISADVANTANGES o reduced illumination o reduced contrast o decentration ghost images
tion than with other types of simultaneous lenses. Decentration of the lens may result in ghost images, especially in night driving and reading. This can be annoying to patients and may require some adaptation. One may expect chromatic aberration or dispersion to be a problem since the diffraction is dependent upon wavelength of light. In the design of the lens, the positive first order of diffraction compensates for the chromatic aberration of the eye. This results in few severe complaints about the dispersion. Stone also reported that selection of the patient is extremely important. Such factors as corneal astigmatism, ocular refraction, add power, previous lens wearers, distance and near visual acuity, and occupation must be considered before fitting the lens. Clinical "hearsay" from several conversations with investigators indicates that the lens is fairly easy to fit. Proper centering is essential for the lens to work properly. Patients usually reject or accept their resultICLC
ing vision through the lens very quickly oven in the fitting session with the trial lenses. This could lead to a high success rate. Verification of the Lens Since the diffraction power is designed with tears filling the grooves, the best way to verify the power of this lens is in a wet cell. This will alter the refraction power just as with any soft or hard lens surrounded by a media other than air. If the lens material is known, however, it is a simple manner to compensate for power readings from published tables. The procedure is simple. The lens in a wet cell filled with saline is placed on a standard lensometer. The target will focus in two positions. One position will represent the power of the distance correction and the other the near correction or add. For example, a + 1.00DS/ + 2.00D add diffractive lens will have power readings of +0.25 for distance and +2.25D for the near diffractive correction. The add power is the difference between the near and distance readings: + 2.25D - (+ 0.25D) = + 2.00D add If the lens material is known, for example Polymacon, the + 0.25D reading converts to a + 1.00D distance correction by simply compensating for the saline (rather than air) as the surrounding material. These values are available in tables which come with many wet cells. The lens could also be measured in air, where the
distance correction would be read correctly. For this case, the near image will be distributed into a different order which would be a function of the material and the type and depth of the grooves. This would depend on the lens design and may vary from manufacturer to manufacturer. Lens Hygiene One would expect the diffractive lens to be more prone to deposit build up due to the irregular back surface. Preliminary clinical results indicate that there is a slight reduction in vision after about two to three days of wear, although no further reduction is shown after three to six weeks with proper care. The manufacturers recommend that the diffractive lens be cared for just as any conventional soft or hard lens. Further studies after long term use of the lens may indicate a special cleaning regime. Diffractive Intraocular Lens This same diffractive lens principal has been extended to the development of a diffractive intraocular lens by 3M Corporation. Grooves cut into a standard IOL are filled with fluid and diffract light to form the near image. The distance image is created by refraction. Published preliminary results from clinical diffractive o IOL studies are optimistic. 4 These studies show 42'/o of all patients did not require a corrective add after the 4th visit. The criteria for these results are listed in the table. This lens has not been approved by the FDA to date.
3M DIFFRACTIVE MULTIFOCAL IOL** Visit 3 (4-8 Weeks) . 18 eyes (32.1%) at visit 3 meet the following criteria: A. Near acuity (Jl to J3). B. No additional add required to achieve J1 to J3 near acuity. C. Distance acuity (better than 20/40) with spherical equivalence of -1 to +1. 2. 38 eyes (67.9%) at visit 3 did not meet at least one of the criteria stated above. ** As reported at the symposium, "Innovations in Multifocal Lens Designs" Copenhagen 1988.
Manufacturers of Diffractive Bifocal Lens There are two manufacturers of the diffractive bifocal contact lens, one in the United States (Allergan Hydron) and the other in the United Kingdom (Pilkington). The Hydron diffractive bifocal contact lens has been apManufacturer AUergan Hydron
Pilkington
3M Corporation
Volume 16, Number 3 March 1989
Name Holographic Bifocal Contact Lens Diffrax Lens
Diffractive IOL
proved by the FDA and should be available in the United States in early 1989. The Pilkington diffractive bifocal contact lens has been available in Europe for approximately a-year. The 3M diffractive bifocal IOL should be on the market in the near future. Material Polymacon Soft Lens
Polycon II Gas Permeable Hard *
Distribution
Comments
United States Early 1989
FDA approved
Europe currently available *
UK Patent
FDA pending
85
CONCLUSION The diffractive lens will be a viable option for patients who wish a presbyopic contact lens correction. When the lens comes on the market in the United States in early 1989, you should consider the diffractive lens as part of your fitting schemes. Because of the ease of fitting, the diffractive lens may interest
References 1. Cohen, A. (1980) Multifocal Zone Plate. U.S. Patent 4,210,391. 2. Cohen, A. (1978) Phase Shift MultifocalZone Plate. U.S. Patent Ser. No. 970,751. 3. Freeman, M.H. (1986) UK Patents GB 2, 101,764B: GB 2, 127, 988B; GB 2, 129, 157B assigned to Pilkington PE ltd. 4. Freeman, M.H. and Stone J. The new diffractive bifocal contact lens. Transaction BCLA Conference, 15-22, 1987. 5. Innovationsin Multifocal Lens Design Symposium published in Ocular Surgery News Supplement, D. Sanders, editor-in chief; November 1988, 3-15. 6. Loshin, D.S. and Hug, T. Image quality of contact lenses. Presented at the American Academy of Optometry Annual
86
more practitioners in fitting contact lenses for their presbyopic patients.
Acknowledgement I would like to thank Allen Cohen and Stanley Klein for their consultation on the diffraction lens, Meeting, 1986, Toronto Canada. 7. Loshin, D.S., Kuether, C. and Hug, T. The evaluation of the progressive bifocal spectacle lens through measurement of the MTF (submitted 1988). 6. Klein, S.A. and Ho, Z.Y. Multizone bifocal contact tens designs. SPIE Vol 679. Current Developments in.Optica Engineering and Diffraction Phenomena 25-35, 1986. 9. Churms P.W., Freeman M.H., Melling J., Stone J., and Walker P.J.C. The development and clinical performance of a new diffractive bifocal contact lens. Optometry Today 27:22, 721-724, 1987. 10. Stone J. Experience with the Diffrax Lens. Optician, March 4, 21-36, 1988.
ICLC
Segment Bifocal Contact Lens
ABSTRACT Presently, practitioners are somewhat limited in the available options for correction of presbyopia with contact lenses. Neither the form of contact lens design (segmented, aspheric, or concentric) or fitting scheme (such as monovision) have high enough success to dominate the market. To meet this growing demand, a new holographic/diffractive bifocal contact lens is being introduced by several manufacturers 1'2,3,4,s. Although different lens materials and slightly modified designs have been proposed, these lenses all form images based upon a similar concept. This paper attempts to simplify the principles behind "how the holographic bifocal contact lens works." This will include a review of diffraction and refraction as well as the application of these concepts in the diffractive lens design. Comparisons between the holographic and conventional bifocal contact lenses along with advantages and disadvantages will also be presented.
distance vision
near vision
zones which contain the distance and near corrections. The lens is fit so that both zones fall within the pupUlary zone and images of distance and near objects are superimposed on the retina. In theory when attending distance objects, one concentrates on the focused distance image and suppresses the blurred near image; when attending near objects, the oppo-
INTRODUCTION Bifocal Contact Lenses Currently there are several types of bifocal contact lenses on the market. These lenses may be classified based upon two related criteria: 1) near and distance vision zone location and 2) how the lens provides the two images. The conventional bifocal contact lens or segmented bifocal has a near zone similar in relative shapes and position to that found in spectacle lenses. In straight ahead gaze, the lens is usually centered on the eye with the near portion below the pupil. This position is maintained by truncation or prism ballast. Upon downward gaze the lens pushes against the lower lid; the eye moves relative to the lens, and one is able to view through the near portion. With experience and proper patient selection, this type of presbyopic correction can be quite successful; however, fitting is somewhat difficult and time consuming, which makes this option undesirable by many practitioners. Simultaneous vision lenses have two concentric
Volume 16, Number 3 March 1989
Simultaneous Vision Concept focused distance
neat" uTtage
focused
near object
blurred distance image
77
site scheme is used. With this type of correction, pupil diameter (for given lens parameters) alters the balance between distance and near image intensity. With a small pupil, light reaching the retina could be limited to the central zone. If the pupil is large, the contrast of the "in focus" image is reduced by the blurred "out of focus" image. In addition, if the area of the two zones are not equally represented, one image may be significantly brighter than the other. These types of lenses do not require truncation or prism ballast for proper positioning; however, they are most successful when the bottom lid stops the lens in downward gaze permitting translation of the lens on the eye. Aspheric and concentric bifocal contact lenses fall in this catagory. The aspheric bifocal contact lens has a progressive change in addition power symmetrically placed from the center to the periphery of the lens. This type of lens should provide corrections for intermediate disASPHERIC CONTACT LENS DISTANCE IMAGE 1.0
Jt •
0.8o i
image at the expense of near image quality. Larger pupils provide more addition power (area of the lens) and slight improvement in near images; however, this occurs in conjunction with a reduction in distance image contrast. A compromise between the distance and near image quality exists for one pupil size. We have demonstrated this change in image quality as a function of pupil size through the measurements of the modulation transfer function (MTF). The details of these measurements are given elsewhere 6,7. In this paper I will only point out that higher values of modulation will yield superior image quality. For the distance images (curves on left), the 2mm pupil yields a far superior image as compared to the 4mm or 6mm pupil. The near image MTFs (curves on right) all show approximately equally poor image quality for all pupil sizes. The concentric bifocal has a circular central zone surrounded by an annular outer zone. The lens is manufactured either with a central distance-annular
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tances when regions between the distance and near correction are exposed. The image formed will always be somewhat degraded, since the lens area falling within the pupil contains a continuum of powers. The size of pupil will determine the degree of image degradation, with smaller pupils yielding a better distance 78
near correction or a central near-annular distance correction. Image quality of the concentric bifocal can also be demonstrated through MTF measurements. The lens we used for these measurements had a near addition in the center with the distance correction in the periphery. With a 2mm pupil diameter, rays could only pass through the central zone, and the best near image was found. Note also no distance image is found for the 2mm diameter pupil. For larger pupils diameters, the near image quality decreased and the distance image quality increased. The 6mm pupil diameter yielded the best distance image quality and the worst near image quality. The concentric bifocal may also be used to demonstrate the dependence of pupil size on the image illumination. In our lab, we measured the relative intensity of the distance and near images formed by a concentric bifocal contact lens (with a 2mm optical zone) with a 3mm and a 4mm pupil diameter. As shown in the graph below, the 3mm and 4mm pupils form a distance image (right peak) with approximate relative amplitude of 0.4 and 0.3 respectively. For the near image (left peak) the 3mm pupil has a relative amplitude (0.4) that is two times that of the 4mm pupil (0.2). These findings may be explained based upon the difference in relative areas of the near and disICLC
CONCENTRIC BIFOCAL CONTACT LENS DISTANCE IMAGE
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CONCENTRIC BIFOCAL CONTACT LENS NEAR IMAGE
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tance portions of the lens as the pupil diameter increases. For example, for an increase in pupil diameter from 3mm to 4mm, the area of the near portion of the lens increases approximately 1.5 times resulting in an increase of near image relative amplitude of approximately 1.8 times. THROUGHTHE LENS FOCUS Concentric Bifocal Contact Lens 0.5
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-2.75 OS / +1.50 acid 2 r a m optical zone
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Image L o c a t i o n ( m m )
One other form of bifocal correction that may be considered to yield simultaneous vision is monovision. This technique of correcting one eye for distance and one eye for near vision has somewhat different mech-
40
so
anisms for suppression of blurred images. In monovision, one eye is used for distance vision and the other eye is used for near vision. In each case the corresponding eye is suppressed. This differs from the other simultaneous lenses described which require suppression of one image in each eye. Some practitioners use simultaneous vision lenses in monovision fitting. For example, a concentric bifocal lens with different zone diameters in each eye has been found to more equally balance image intensities. Although the monovision technique reduces binocularity, many practitioners have great success with the fitting scheme. What about the new holographic bifocal contact lens? When one hears the term hologram, one thinks of lasers and three dimensional images. A hologram is actually a photograph of an interference pattern formed by two beams, one directly from a laser (reference beam) and the other from laser light reflected off of an object (object beam). After processing, the hologram is placed back in the path of the reference beam. Light is diffracted by the hologram into wavefronts which are exactly the same as those that were previously leaving the object. The holographic bifocal contact lens is really a diffractive lens. The lens actually forms images by both refraction and diffraction. At this point a review of
Reconstructionof Hologram
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Volume 16, Number 3 March 1989
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79
refraction and diffraction is in order. REFRACTION The concept of refraction should be familiar to practitioners since they use the principles for all conventional corrections. Refraction or the change in velocity of light in an optical media as compared to that of vacuum, usually results in a change of direction, or bending of emergent rays. For refraction by a prism, the amount of bending is called the deviation: the angle between the incident ray and the emergent ray. Deviation is a function of index of refraction and the apical angle of the prism. For ophthalmic prisms, deviation may be expressed as:
One type of refracting element that employs this concept of deviation directly is a Fresnel lens. The lens consists of concentric prisms with increasing apical angle from the center to the periphery. Since all the rays which are refracted by the lens are focused to a point, the peripheral prisms deviate the light more than the ones closer to the center. When compared to a conventional lens, the Fresnel lens has reduced image quality due to the scatter at the facets between concentric prisms, the discrete steps of constant deviation and possibly the lack of any phase relationship from one prism to the next. Spherical lenses are a special case of this prismatic deviation concept of refraction. Here the lens has an L
8=(n-1)~ where:
V
8 is the deviation
n is the index of the prism ~, is the apical or refracting angle If the index of refraction is constant, the deviation increases as the apical angle increases. This is easily demonstrated by imaging through a bar prism.
infinite number of prisms with smoothly increasing apical angles from the center to the periphery of the lens. The deviation smoothly increases to refract rays towards a single image point. DIFFRACTION
REFRACTION THROUGH A PRISM
~
apical angle (~) ~
apical angle (y) % .
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The concept of diffraction is not quite as familiar to practitioners as refraction. Diffraction can be considered to be the interaction of light waves with obstacles. This interaction often results in a change in direction of travel that can be controlled by proper configuration of the obstacle. Unlike refraction, diffraction produces multiple images called orders. These orders are symmetrically displaced spatially and are
D I F F R A C T I O N ORDERS
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labeled as 0, -+ 1, _+2, _+3, etc. Typically, as the order of diffraction increases, the images formed in each order decrease in intensity. Here again the distribution of image intensity (i.e. the intensity of each order) can be controlled by selection of the obstacle. Diffraction is easily demonstrated with a diffraction grating which consists of narrowly separated dark bars. For a specific wavelength, the amount of diffraction (or spatial displacement of the orders) is a function of the separation of the bars i.e. the smaller the separation, the greater the diffraction and resulting spatial separation of the orders. For example, a grating with 2000 lines per inch diffracts light less than a grating with 7000 lines per inch and the orders are spatially closer together. ICLC
DIFFRACTIONTHROUGHA GRATING GratingSpacingWillSpatially Alter Order Location
~
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Paralleling the extention of refraction by a prism to a refractive imaging element (Fresnel lens), the concept of diffraction can be extended to a diffractive imaging element; the Fresnel zone plate. A Fresnel zone plate consists of alternating dark and light rings of varying width and separation. The spacing of the rings varies
FresnelZone Plate
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rrn = where: rm = the radius of the mth annulus m = the annulus number f = focal length of the Fresnel zone plate X. = the wavelength of light Higher power zone plates have central zones with shorter radii and for a given plate size more annuli are required. This concept will be important when determining the add power of the diffractive bifocal lens. Another principle that must be discussed in order to explain the action of the zone plate is that of the phase relationship between waves. Electromagnetic Volume 16, Number 3 March 1989
I
radiation which includes visible light may be represented as sinewaves. These waves have maximum
-
from the center to the periphery of the plate, with the closer spacing in the periphery for greater diffraction. This is similar to the Fresnel lens which has larger apical angle prisms in the periphery for greater deviation. The focal length of the zone plate is a function of the radius of the annuli and wavelength:
•
trough
(peak) and minimum (tough) displacements, the magnitude of which is the amplitude of the wave. The distance between successive peaks or troughs is the wavelength. Phase is the spatial relationship between the peaks and troughs of two or more waves. When waves are added a new wave results. The phase between the waves determines the amplitude of the resulting wave. When the waves are in phase peaks add to peaks and troughs to troughs and the net has a greater amplitude. When waves are out of phase the peaks and troughs of one wave do not correspond to the peaks and troughs of the other waves. If the waves are ,rr out of phase (or one half wavelength in path), the peaks of one wave correspond to the troughs of the other wave, and the result is a wave with a reduced amplitude. I will call waves that are in phase: additive (constructive interference) and waves that are ~r out of phase: subtractive (destructive interference). Regions of the incident light can be considered to consist of waves that are both in phase and out of phase with each other. The zone plate is designed to permit waves that are in phase to pass through (clear portion) and waves which are out of phase to be blocked (dark portion). The resulting image is composed of all waves that are in phase or are additive. If properly configured, a large percentage of the transmitted light will be diffracted into the desired order or 81
IMAGING WITH A STANDARD FRESNEL ZONE PLATE
.Imaige formed with m phase waves. Out of phase waves are blocked and do not contribute to image. I out of phase subtractive
image location. The Fresnel zone plate, however, is not considered to be an efficient imager since a portion of the incident light is lost due to the blocking of the out of phase waves. One can increase the efficiency of the zone plate by replacing the opaque zones with a different index of refraction. This will allow more of the incident light to be transmitted through the plate. Since higher indices retard the velocity of waves, by using the proper index and depth, the portion of the out of phase waves that strike the blocked zone may be retarded by a multiple of half-wavelengths. This will change the out of phase
waves into in phase waves and all the waves diffracted through the plate will become additive. Alternately, if part of the material in the blocked zones is cut away (with the proper depth), the waves which fall in these regions will increase in velocity. This will result in a phase shift by a multiple of half-wavelengths with the same result. A combination of the two methods (i.e. cut away material and replace it with a different index) will provide more phase control. All these methods form an image that is much brighter than that formed by a zone plate. This type of imager is called a phase
plate.
IMAGING WITH A MODIFIED FRESNEL ZONE PLATE: A PHASE P L A T E clear zone with gradient index of refraction
Image formed with all waves since all are in phase. Out of phase waves are shift to in phase by changing the index o f the opaque zones.
out of phase subtractive
DIFFRACTIVE BIFOCAL The diffractive bifocal forms images by both diffraction and refraction. Diffraction is accomplished by cutting circular annular grooves on the back surface of a standard contact lens. Tears fill these regions of the lens providing a difference in index as with the phase plate. As with the zone plate, the radii and spacing of 82
these annuli determine the addition power. The depth and shape of the cuts of the annuli grooves control the intensity and distribution of images formed in each order of diffraction. In order to have optimum performance, the majority of the light must be concentrated equally into two orders which represent the distance and near images. Klein and Ho e have shown how the ICLC
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positive first order of diffraction. Thus there is even distribution of energy into two orders: a case that could be (and may be) used for the diffractive bifocal lens. Note that 20% of the incident light is lost to higher orders of diffraction, therefore, even for the "best" case, the two equally bright images formed represent only 80% of the incident light. Now that we have discussed how two equally bright
distribution of light intensity in each order may be altered by changing the surface profile of the grooves. For example, with the alternating profile with zero phase, the majority of the energy is unevenly distributed over three orders (0, +1, +2) and therefore is unacceptable for the bifocal lens application. The nonalternating profile divides the image intensity equally with approximately 40% in the zero and 40% in the
RELATIVE INTENSITY OF ZONE PLATE DIFFRACTION order m =
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images can be formed by diffraction, we must go one step further to explain the role refraction plays in the holographic lens. As previously stated, the zero order of diffraction does not change direction, i.e. an object
.0018 .0288
at an infinite distance would be diffracted to an infinite image location. This would be the case if the grooves were cut into a flat plate, however, the contact lens has curved surfaces which have dioptric power. Light in the
DIFFRACTIVE BIFOCAL CONTACT LENS
contact lens distance
object
Near image formed by diffraction. Distance image formed by refraction.
near object
tear layer
Volume 16, Number 3 March 1989
83
zero order of diffraction is therefore refracted to an image location depending upon the total power of the lens. This refraction is similar to that of a conventional single vision contact lens used to correct distance vision. The diffractive bifocal contact lens forms the near image by diffraction and the distance image, for the most part, by refraction. The resulting images have a slight reduction in intensity. In addition, just as with the other simultaneous lenses, the images have a reduction in contrast, i.e. a blurred image is superimposed over the focused image. An analogy to this situation could be made using two slides projectors. Start with the exact same slide in both projectors, with the two projected images superimposed on a screen. The net image would be bright. Now defocus one image or put another slide in one projector and defocus. Superimposed upon the original image is a veiling luminance which results in a contrast reduction. This can result in visual problems when the original objects viewed have reduced contrast to start. One example is reading a menu in a dimly lit restaurant. Since the distribution of the blurred diffracted image differs from that of the projectors, the in-focus diffracted image has less reduction in contrast than the image formed by the projectors.
o o o
o o
ADVANTAGES pupil independance. equal image intensity. simutaneous vision diffracledblur blur ease of fitting. success quickly known
and no annular zones fall within the pupil. This eliminates or reduces the near image intensity. Churms et al. 9 and Stone 1° have reported some clinical results with the Diffrax lens. The initial visual adaptation difficulties reported include: a. b. c. d. e.
Slight loss of contrast at distance and near Ghost image especially at near Print appears to "standout" from the page Patients had difficulty finding the near image Objects appear to have a colored fringe
The reduction in image contrast is the major disadvantage of the diffractive lens. This should not be a problem in bright, high contrast conditions, however, with evening or dimly lit environment or task, it may present a problem. The diffracted blur image has a different intensity distribution than a refracted blurred image. This, in theory, results in less contrast reduc84
We are currently measuring the MTFs of several types of the diffractive bifocal lenses. Preliminary results indicate that image quality is indeed somewhat independent of pupil size. The distance image (for the lenses measured to date) has slightly better image quality when compared to the diffracted near image. The relative intensities (as a function of pupil size) also conforms with the theory of equal intensities for the two images. CLINICAL IMPLICATION OF THE DIFFRACTIVE BIFOCAL CONTACT LENS PrO and Cons Listed in the table are the advantages and disadvantages for the diffractive bifocal contact lens. A major advantage of the diffractive lens is pupil independence, i.e. the distribution of image intensity is independent of pupil size. If the incident light is diffracted by just some of the annuli, the two images will have approximately equal intensity. This contrasts the other simultaneous vision lens as shown by the variation in image intensity as a function of pupil size for the concentric bifocal contact lens. One potential problem is low add powers where the first diffraction annuli has a fairly large radius (=3mm)
DISADVANTANGES o reduced illumination o reduced contrast o decentration ghost images
tion than with other types of simultaneous lenses. Decentration of the lens may result in ghost images, especially in night driving and reading. This can be annoying to patients and may require some adaptation. One may expect chromatic aberration or dispersion to be a problem since the diffraction is dependent upon wavelength of light. In the design of the lens, the positive first order of diffraction compensates for the chromatic aberration of the eye. This results in few severe complaints about the dispersion. Stone also reported that selection of the patient is extremely important. Such factors as corneal astigmatism, ocular refraction, add power, previous lens wearers, distance and near visual acuity, and occupation must be considered before fitting the lens. Clinical "hearsay" from several conversations with investigators indicates that the lens is fairly easy to fit. Proper centering is essential for the lens to work properly. Patients usually reject or accept their resultICLC
ing vision through the lens very quickly oven in the fitting session with the trial lenses. This could lead to a high success rate. Verification of the Lens Since the diffraction power is designed with tears filling the grooves, the best way to verify the power of this lens is in a wet cell. This will alter the refraction power just as with any soft or hard lens surrounded by a media other than air. If the lens material is known, however, it is a simple manner to compensate for power readings from published tables. The procedure is simple. The lens in a wet cell filled with saline is placed on a standard lensometer. The target will focus in two positions. One position will represent the power of the distance correction and the other the near correction or add. For example, a + 1.00DS/ + 2.00D add diffractive lens will have power readings of +0.25 for distance and +2.25D for the near diffractive correction. The add power is the difference between the near and distance readings: + 2.25D - (+ 0.25D) = + 2.00D add If the lens material is known, for example Polymacon, the + 0.25D reading converts to a + 1.00D distance correction by simply compensating for the saline (rather than air) as the surrounding material. These values are available in tables which come with many wet cells. The lens could also be measured in air, where the
distance correction would be read correctly. For this case, the near image will be distributed into a different order which would be a function of the material and the type and depth of the grooves. This would depend on the lens design and may vary from manufacturer to manufacturer. Lens Hygiene One would expect the diffractive lens to be more prone to deposit build up due to the irregular back surface. Preliminary clinical results indicate that there is a slight reduction in vision after about two to three days of wear, although no further reduction is shown after three to six weeks with proper care. The manufacturers recommend that the diffractive lens be cared for just as any conventional soft or hard lens. Further studies after long term use of the lens may indicate a special cleaning regime. Diffractive Intraocular Lens This same diffractive lens principal has been extended to the development of a diffractive intraocular lens by 3M Corporation. Grooves cut into a standard IOL are filled with fluid and diffract light to form the near image. The distance image is created by refraction. Published preliminary results from clinical diffractive o IOL studies are optimistic. 4 These studies show 42'/o of all patients did not require a corrective add after the 4th visit. The criteria for these results are listed in the table. This lens has not been approved by the FDA to date.
3M DIFFRACTIVE MULTIFOCAL IOL** Visit 3 (4-8 Weeks) . 18 eyes (32.1%) at visit 3 meet the following criteria: A. Near acuity (Jl to J3). B. No additional add required to achieve J1 to J3 near acuity. C. Distance acuity (better than 20/40) with spherical equivalence of -1 to +1. 2. 38 eyes (67.9%) at visit 3 did not meet at least one of the criteria stated above. ** As reported at the symposium, "Innovations in Multifocal Lens Designs" Copenhagen 1988.
Manufacturers of Diffractive Bifocal Lens There are two manufacturers of the diffractive bifocal contact lens, one in the United States (Allergan Hydron) and the other in the United Kingdom (Pilkington). The Hydron diffractive bifocal contact lens has been apManufacturer AUergan Hydron
Pilkington
3M Corporation
Volume 16, Number 3 March 1989
Name Holographic Bifocal Contact Lens Diffrax Lens
Diffractive IOL
proved by the FDA and should be available in the United States in early 1989. The Pilkington diffractive bifocal contact lens has been available in Europe for approximately a-year. The 3M diffractive bifocal IOL should be on the market in the near future. Material Polymacon Soft Lens
Polycon II Gas Permeable Hard *
Distribution
Comments
United States Early 1989
FDA approved
Europe currently available *
UK Patent
FDA pending
85
CONCLUSION The diffractive lens will be a viable option for patients who wish a presbyopic contact lens correction. When the lens comes on the market in the United States in early 1989, you should consider the diffractive lens as part of your fitting schemes. Because of the ease of fitting, the diffractive lens may interest
References 1. Cohen, A. (1980) Multifocal Zone Plate. U.S. Patent 4,210,391. 2. Cohen, A. (1978) Phase Shift MultifocalZone Plate. U.S. Patent Ser. No. 970,751. 3. Freeman, M.H. (1986) UK Patents GB 2, 101,764B: GB 2, 127, 988B; GB 2, 129, 157B assigned to Pilkington PE ltd. 4. Freeman, M.H. and Stone J. The new diffractive bifocal contact lens. Transaction BCLA Conference, 15-22, 1987. 5. Innovationsin Multifocal Lens Design Symposium published in Ocular Surgery News Supplement, D. Sanders, editor-in chief; November 1988, 3-15. 6. Loshin, D.S. and Hug, T. Image quality of contact lenses. Presented at the American Academy of Optometry Annual
86
more practitioners in fitting contact lenses for their presbyopic patients.
Acknowledgement I would like to thank Allen Cohen and Stanley Klein for their consultation on the diffraction lens, Meeting, 1986, Toronto Canada. 7. Loshin, D.S., Kuether, C. and Hug, T. The evaluation of the progressive bifocal spectacle lens through measurement of the MTF (submitted 1988). 6. Klein, S.A. and Ho, Z.Y. Multizone bifocal contact tens designs. SPIE Vol 679. Current Developments in.Optica Engineering and Diffraction Phenomena 25-35, 1986. 9. Churms P.W., Freeman M.H., Melling J., Stone J., and Walker P.J.C. The development and clinical performance of a new diffractive bifocal contact lens. Optometry Today 27:22, 721-724, 1987. 10. Stone J. Experience with the Diffrax Lens. Optician, March 4, 21-36, 1988.
ICLC