Lecture: Low Vision Devices: Providing Functional Vision for Patients

This live webinar discusses the range of low vision devices now available and how they can be adapted to be most useful for patients with various low vision concerns.

Lecturer: Dr. Nicole Ross, OD, MSc, FAAO


(To translate please select your language to the right of this page)

DR ROSS: Hi there! I’m Dr. Nicole Ross, and today we’re going to be talking about low vision devices. And so for today’s lecture, which is an extension on what we discussed last time, we’re really going to focus on clinical optics and the optics of prescribing low vision devices. And really the devices we’re going to discuss today represent categories of devices, and the first category we’re going to discuss are devices prescribed for reading. And so in the last lecture, we talked a lot about finding the equivalent power required for someone to read 1M print, and the different methods we might use to come up with that measurement. We’ve got really five main ways of doing it, based on the patient’s history, based on Kestenbaum’s rule, the acuity reserve rule, the inverse of the near acuity, or the inverse of the critical print size. And depending on the situation and the patient and the requirements, you might use any of these rules to come up with sort of an estimate for the equivalent power that might be required to read 1M print. Today we’re going to look at the other side of the column here and talk about the equivalent power of the prescribed low vision device, whether that’s spectacles, loupes, simple handheld magnifiers, stand magnifiers, a near telescope system, or an electronic system. And ultimately, the equivalent power of the low vision device must be generally slightly greater than or equal to your measurement that you came up with as your estimate, in order for that to be successful for the patient. So as I mentioned, we have sort of the following rules that we can use to come up with the equivalent power required for a patient to read 1M print, which is equivalent to about 20/50 or 8-point. My favorite method is the last method on this list, which is the inverse of the critical print size. Now, this method is effective if the patient’s goal is continuous reading. And it is also effective if, in this case, we’re using Minnesota reading card, developed by Gordon Legge, and that particular card is at a third grade reading level. So the patient’s reading level has to be at that level. And what we’re doing — we’re trying to find a critical print size, and then take the inverse of that, to come up with equivalent power estimates. We’re asking the patient to wear some reading correction. Generally if they’re presbyopic or older. Just so that they’re in focus for the test. We’re asking them to read starting from very large print sizes of 8M or 32 point, and then we’re asking them to continue reading, and we’re trying to pick up on that point where they start to slow down. And that last print size that they read fluently at their maximum speed — you don’t have to time it. You can just even sort of qualitatively listen for that. We want to know, at that print level, that print level where they last read at their maximum speed is their critical print size. And then we might also want to know their threshold, just to see what that range is, but that critical print size becomes our metric for our estimate. So in this example, if we have a patient wearing a +3 near addition over the trial refraction for the exam, they’re holding a card at 30 centimeters, and the critical print size is 2.5M, the last print they read at their maximum speed, we take the inverse of this critical print size acuity, so 2.5M, divided by that 30 centimeter test distance, and we get an estimate that this patient requires around 8 diopters to read 1M print. And so now we can decide: What do we want to do? And what do we want to prescribe for the patient? We know we need 8 diopters, but should we put 8 diopters in a near addition for a near vision only spectacle? Should we put that 8 diopters into a hand magnifier or should we think about a stand magnifier, and thinking about some system of a stand magnifier and spectacles that together would be our 8 diopters? So we talked a lot last lecture about prescribing near vision only spectacles, very strong spectacles for reading. And the reason sort of to point over and go over this again is that this is a normal approach to use spectacles for reading, but when we’re prescribing near additions, we’re not doing it where we’re going much higher. In our last example, we did +8. Here there’s a much more limited depth of field, a limited range where the patient is in focus. But this does provide a very wide field of view for the patient, because we’re putting the magnification required at the spectacle plane. Loupes are another approach, sort of similar to a near vision only spectacle. With a loupe, this is sort of clipped onto the glasses, and there’s some distance between the spectacle plane and the loupe. Again, you would prescribe a loupe based on your FEQ measurement. So if our FEQ estimated measurement is 8 diopters from our reading assessment, we would want to prescribe an 8 diopter loupe. This here is a type of loupe that’s used often here in the US for hobbies. The Donegan OptiVISOR. And as you can see, this flips down over the spectacles. The advantage of a loupe has always been that you gain some working distance, so you can hold the page slightly further away than you would be able to if you had prescribed a near vision only spectacle. However, the distance that you gain is simply the distance between the spectacle plane and the loupe. Because we are still limited by the fact that, to be in focus, our page in this case for an 8-diopter loupe, that page needs to be 12 centimeters from the loupe plane. Hand magnifiers are another approach. Most hand magnifiers now are labeled in diopters. Because the magnification or X label on magnifiers is not uniform between countries, and it is not uniform between manufacturers even within countries. So most hand magnifier distributors for low vision devices will indicate the diopteric value on the hand magnifier. So in our previous example, where the patient required 8 diopters to read 1M, we could prescribe a hand magnifier. One question we often get that requires some thought into clinical optics is whether or not the patient should be looking through the distance portion of their glasses, or if they’re a bifocal wearer, should they be looking through the near addition of the bifocal, while they’re using their hand magnifier. And the answer is it depends. So when we think about a 2-lens system, which this now is, when we’re thinking about a near addition and a hand magnifier, the equivalent power of the system together is defined by this formula here. F1 plus F2 minus C(F1)(F2). And these are the diopteric power of the lenses. In this case, the power of our near addition and the power of our hand magnifier. And C is the separation in meters between those two lens planes. So some interesting things happen with arithmetic when using this formula. First of all, we find, when C is zero, meaning the patient is holding the magnifier right up against the spectacle plane, we find that our formula ends up being that the equivalent power is F1 plus F2. So we actually get more equivalent power — you get more power out of the system when you use your bifocal and the magnifier together. At a close distance, with no separation between the two. So when might this be useful? Say the patient is using their hand magnifier, and they tell you… Well, I can’t quite make out some smaller print. Like my medicine bottles and things, with this hand magnifier. Could you prescribe a stronger one? Before prescribing a stronger hand magnifier, it might be worthwhile to ask the patient to hold the magnifier right up against the glasses, bring the medicine bottle close, and see if they can’t achieve this goal. Because you are getting more equivalent power by operating this way. In our second scenario, where the separation between the spectacle plane and the hand magnifier is not zero, but it’s less than the focal length of the hand magnifier, so, for example, if we were using a 10-diopter hand magnifier, the focal length of a 10-diopter hand magnifier is 10 centimeters. So if the distance between the spectacles and the hand magnifier is some number less than 10 centimeters, the equivalent power of using a bifocal and the hand magnifier together is gonna be higher than just using the distance prescription and the hand magnifier alone. So you will, again, get more power out of this system. When we had this situation in the third bullet, where the distance between the spectacle plane and the hand magnifier was equal, in the example of a 10-diopter hand magnifier, if that’s 10 centimeters, then the equivalent power is actually the same, whether or not we’re using the bifocal or the distance prescription. The arithmetic works out, such that equivalent power is 10 diopters in each case. So it doesn’t matter what the patient is doing in that scenario. They will achieve only 10 diopters. Now, if we bring that magnifier at a much further distance away from the spectacle plane, much further than 10 centimeters, then it starts working out that the equivalent power that we get out of this system, when we’re using our bifocal, is actually less than the equivalent power of the system if the patient was just using their distance correction and looking through the hand magnifier. So at further distances, away from the spectacle plane, the patient is better off, and will get more power out of the system by using the distance portion of their glasses. And this latter point is also an important training point for the patient. If they’re looking through their bifocal when they’re using their hand magnifier, and the hand magnifier is quite a ways away from the spectacle plane, they’re really not getting all of the power out of that hand magnifier that you prescribed. They’re getting a much lower value. So that is an important teaching point and training point for the patient. So to sort of prove this theory, let’s go through some examples. So here we have a 20-diopter handheld magnifier, and the patient is wearing some bifocals with a +4 near addition. So if the patient is looking through the bifocal and looking through the hand magnifier, the total equivalent power that they’re going to get out of this system, if they’re holding the hand magnifier right up against the glasses, is gonna be 24 diopters. So they’re gonna get a bit more. Whereas if they looked through the distance-only portion of their glasses, in this scenario, they would only achieve the power of the hand magnifier, which is 20 diopters. So if we look through the bifocal, and the separation between the hand magnifier and the glasses is zero, we’ll get 24. If we look through the distance, we’ll only get 20. If C is 5 centimeters, this is the focal length of a 20-diopter hand magnifier. You can see that if we look through the bifocal, the equivalent power is 20 diopters. And this is the same as if the patient looked through their distance prescription and used the 20-diopter hand magnifier. They would get 20 diopters. In either scenario, the patient is achieving the same metric. Now, if we have C=10 centimeters, that’s greater than the focal length of the hand magnifier. We see that when they’re looking through the bifocal, the equivalent power that they’re getting through the system actually is now dropped. It’s only 16 diopters. Whereas if the patient just looked through their distance prescription, and used that hand magnifier, they would achieve 20. So again, it sort of illustrates that point. The other sort of training piece and point worth going over in terms of clinical optics of the hand magnifiers is field of view. So as the power of a hand magnifier increases, the usable field of view decreases. Obviously because the magnification is higher. But also, as the distance of the hand magnifier from the spectacle plane is decreased, the field of view is also decreased. So there’s two factors that affect field of view. One is the power that you’re prescribing. We don’t want to overprescribe magnification, because the patient will lose some field of view. But we also lose field of view as we’re holding that magnifier away from the spectacle plane. And so this is the formula that we use to determine the linear field of view of a hand magnifier. So the linear field of view is related to… Distance that that hand magnifier is from the eye and the spectacle plane. So in this example, if we have an 8-diopter handheld magnifier, with a 20 millimeter lens diameter, that’s held about 6.25 centimeters from the eye, our linear field of view is about 4 centimeters. So again, you really see these two factors at play. The power of the hand magnifier, and the distance that that hand magnifier is held from the eye, affecting the field of view, in this equation. So now I have a question for you. What is the linear field of view of a patient’s 24-diopter hand magnifier? If it has a 35 millimeter lens diameter, held 6.25 centimeters from the eye? Excellent. So it looks like everybody got that question right. The answer would be 3.25 centimeters. So with this relationship, obviously there’s an inverse relationship here. So if the magnifier is held one focal length from the eye, the field of view is equal to the diameter of the magnifier. The linear field of view, that is. If the magnifier is held two magnifier focal lengths from the eye, the field of view will be equal to half the diameter of the magnifier lens. Again, because of that inverse relationship. So hand magnifiers, when we’re prescribing these, there’s a few nuances we have to be aware of. Do we want the patient to look through the bifocal, or to look through the distance portion? That might be a different answer in different situations. It’s a generally familiar approach. The distance that the hand magnifier is held from the spectacle plane is flexible, but that will affect the equivalent power. They’re small, they’re portable, but some training is needed, and they aren’t hands-free. So it does require fairly good hand control. In a situation where you have a patient with poor hand control, or a tremor, you might want to consider a stand magnifier, if you want to consider an optical system. This is a high plus lens with a housing placed around it, which sets the object distance less than the focal length available, and creates a magnification by creating a virtual, erect, and divergent image. So the stand magnifier is placed directly on the reading material. It may be illuminated or not illuminated depending on the lighting needs of your patient. And the location of the image that is created, that virtual, erect image, because that image is not at infinity — it’s at a finite distance — that image distance is going to determine what spectacles you need to prescribe to go with your stand magnifier. So in younger patients, like the Perkins School for the Blind, they can use that accommodation to focus on distance. But for my seniors and elders, they don’t have accommodations. I need to prescribe a near addition for them to see the image on the stand magnifier. So this is courtesy of one of my colleagues at the American Schools of Optometry. As pictured here, we have our spectacle plane. We have some distance between the spectacle plane and our stand magnifier. Within the stand magnifier, there’s a lens, the object is placed at a certain distance, and the virtual image is somewhere down here, beyond the plane of the object. The working distance here is really the distance that that patient has to be in focus in order to see the image. And it’s for this working distance that we need to prescribe a near addition for a presbyopic patient. Stand magnifiers aren’t as simple as locating the add, like we did with the hand magnifier. For a stand magnifier, we have to consider the enlargement ratio, which is the difference between the emerging vergence and the power of the lens over the emerging vergence. The equivalent power of the stand magnifier considers this enlargement ratio and the near addition or the accommodation demand for the patient. So the equivalent power of the system, when we prescribe a stand magnifier, is really a combination between the near addition in the glasses and the enlargement ratio provided by the stand magnifier. So in this case, if we know the lens power, of our stand magnifier is 15 diopters, and we know our image distance of 5 centimeters, we can figure out what the enlargement ratio is. When you’re verifying different devices, if you don’t have the advantage of a lookup table to find out what the image distance and what the enlargement ratio is, you can actually use a telescope to locate the image distance, and I’m happy to go over that with anybody who is interested, and I also have a series of lookup tables that I’ve worked on with my colleagues here at the University of California, Berkeley, on all the different magnifiers, to verify the enlargement ratios. Once we have the enlargement ratio, we then can consider our near addition. In this case, we have a near addition of 3 diopters we’d like to prescribe. And then the combination together provides an equivalent power of 12 diopters. Now, the one thing — point that I do want to go over here — is that when we’re prescribing a near addition, we can really be creative with our stand magnifiers, because we really can split the amount of magnification we’re prescribing in the spectacle plane, versus in the stand magnifier itself. And we can kind of really play with those numbers quite a bit. Depending on the patient’s needs. But the one thing that we’re sort of bound by is this image distance. So for a given stand magnifier, I can never really prescribe a near addition stronger than the image distance provided by this stand magnifier. Because this really represents the strongest near addition I can prescribe. Because it requires that the patient’s glasses and spectacle plane be right smashed up against that stand magnifier. To see it. So some areas where different clinicians get in trouble with stand magnifiers is if they’re prescribing near additions that are too high, that the patient cannot resolve the image of that stand magnifier. So I have a question for you. So using sort of the data below, if we have a lens power of a stand magnifier of 16 diopters, and the emerging versions out of that stand magnifier is -8.75 diopters, and if a patient is using a +4 near addition with this system, what is the equivalent power that they’re getting in total? So this question really does require a couple of steps. First, we’re sort of calculating what that enlargement ratio is. And then we have to consider the near addition. So the first step is really to calculate the enlargement ratio. So with the enlargement ratio, one sort of important thing to consider is that, because divergent light is always coming from a stand magnifier, it’s divergent light, the emerging vergence value is always going to be negative in our formula. And so with that, we get an emerging version, about 2.8 times. If we then multiply this with what the patient is using, we get an equivalent power of about 11 diopters. So that is sort of an important thing to remember from our sort of basic optic principles, is that emerging vergence of divergent light makes a negative vergence value. So the next question is: Where would the image for this stand magnifier be located? So for the image distance, the image distance is a sort of given away here. And let me see if I can draw with my pen. The image distance is really the inverse of this value here. In this case, -8.75 diopters. So in terms of the image distance, it would be the inverse of that value. Now, to prescribe what is the maximum near addition I can prescribe with this magnifier, it is going to be the inverse of that. So it will be +8 diopters. Let me illustrate an example. So here we have our stand magnifier. It has a lens with some housing. We’ve got a piece of paper, and that represents where our object is. And then we have sort of an image distance here. And so the maximum spectacle prescription that I can use with this stand magnifier is when my eye is really right — and this is sort of my lens here — when that’s right up against the stand magnifier. So when that distance is very close, the distance of the maximum add that I can prescribe is really driven by this image distance. Which is equivalent in power, but opposite in sign, to the emerging vergence here. So for this next question, and our last problem, I’m asking: Is the magnifier appropriate for this patient? So we do a reading assessment with this patient. We ask her to read from large print sizes, and we find that her critical print size is about 4M, at 30 centimeters. And so if we prescribe a stand magnifier, with her current bifocals, with the +3 near addition, and the lens power is 16, and our image distance is 25, would that be appropriate for this patient? So here what the question is asking is whether or not the equivalent power is needed for this patient that’s based on the reading assessment is less than or equal to the equivalent power of the device. Because we want the device to have a bit more power, or be equal in power to our estimate. And so the way that we sort of work this through is to consider, well, what is the estimate of the equivalent power required for this patient to read at her peak reading speed? So we take our critical print size, we invert it, and we get 13 diopters that that patient needs and requires. So now what is the equivalent power of the low vision device that’s proposed? Well, to find the equivalent power of a stand magnifier, I need to know my enlargement ratio, and I need to know the near addition. I know the near addition. The patient wants to use her own bifocals with the +3 near addition. So I can use the data here provided in the equation. I know the lens power. And I know the image distance. And these two elements you can commonly verify for any stand magnifier, with a very, very simple optical bench system. So I have those two values, and I can calculate my enlargement ratio, which is 5X. I then take that enlargement ratio, and multiply it by my +3 near addition, and I have 15 diopters. And so in this case, the equivalent power of the device is slightly greater than or equal to the equivalent power of my estimate, and so yes, this magnifier is definitely appropriate for the patient, and can be prescribed. It takes a little bit of thought process to get there, and it definitely is appropriate, so the majority of the audience is correct. I do want to mention a special class of stand magnifiers. There are many different special classes of stand magnifiers, but one in particular that we often use in children are the paperweight or dome magnifiers. And why are these commonly prescribed in kids? Well, they have some very special properties. Number one, the enlargement ratio is going to be equal to the index of the refraction of the material, because it’s a hemisphere. Because it’s a hemisphere, it has a very, very short image distance. In fact, the image distance is about the same distance as the object distance. And so for children who like to accommodate, they can use a lot of accommodation, or you can use a very high add with a dome magnifier, because that image distance is so short. And this is really typically not the case with our other stand magnifiers, that often have image distance in the order of 20 to 25 centimeters, for many manufacturers. So with that image almost being in the plane of the object, it allows a pediatric patient to sort of hold and have a magnified view at sort of their habitual distances, using the accommodation. Because kids tend to really like to hold things close and use that relative distance magnification. And so a dome gives them sort of the appropriate enhancement. In that scenario. So for stand magnifiers, you can see it’s a little bit more complicated than prescribing the other devices. You know, we had our equivalent power estimate from our reading assessment. We could just grab that 8-diopter hand magnifier, and it was really very quick. With the stand magnifier, there’s a few more steps to consider, because we have to consider: Well, how much do I want to do in the spectacle plane in the near addition, and how much do I want to do for the stand magnifier? The lighting in stand magnifiers, because the housing is nice and tightly controlled, is really fantastic. And for a patient with poor hand control and tremors, it is great. But in terms of portable and spot reading situations, this may not be the way to go, because it can be quite bulky, and it can be also terribly difficult for any sort of writing task. Another system to consider is obviously an electronic stand magnifier, or a desktop closed circuit television system. And so here we have a distance camera that is focused on the reading material. And it’s electronically providing a magnified view on the screen. And so electronic magnifiers can provide some contrast enhancement, and they can provide much larger fields of view than optics can typically produce. And there’s many portable versions of these as well. That are readily available, that, again, allow for that contrast enhancement, a larger field of view, and also to be there in more portable situations. In this case, reading a menu. I now want to sort of shift our attention for our last half hour from reading to distance magnification, because there was a request at the last lecture to really go over telescopes and distance optical aids. So let’s talk a little bit first about what are indications for prescribing a telescope. So there can be distance viewing situations, and also near or intermediate viewing situations, where you might prescribe a telescope. The common instances where you’re prescribing for distance would be to maybe see a traffic light, see a street sign, aisle signs in the grocery store, distant menu boards or business names, building directions, or maybe other needs, like looking at a chalkboard, looking at museum exhibits, or maybe in a situation such as this, looking at a slide presentation. For near/intermediate indications for telescopes, we don’t often prescribe telescopes for near, but we can, and typically, we’re doing it because we require quite a high equivalent power for the near task. But we aren’t able to accomplish this task at a close working distance required, as if we prescribed just spectacles. So in order to get a greater working distance, we then have to go to a 2-lens telescope system. So for a lot of the near telescopes I prescribe, oftentimes it’s for a specific task where a greater working business is required. Like knitting, model building. Even surgical loupes really fit into this model of a near telescope, or the near telescope that I prescribe for dentists. Very occasionally, my patient will use it for reading a book. A near telescope system. But most of the time, it’s for a specific task and a hobby that has to occur at some other intermediate viewing distance. And so there are really two general classes of telescopes that we think about in low vision rehabilitation. One are handheld telescopes, monoculars or binoculars. And there are spectacle-based systems. We can put the telescope in the center of the glasses, we can put it in a bioptic position, or we can put it in a reading or surgical position. And so to satisfy the request of the audience about going over clinical optics, I want to go over the optics of a telescope. Telescopes generally are afocal for distance viewing. So you have parallel rays coming into the telescope, parallel rays coming out of the telescope. The telescope itself has no diopteric power, but it does create angular magnification. And so we can’t really confuse that lack of diopteric power with magnification. And to provide that magnification in a distance system, really we have two lens elements. We have an objective lens, and we have an ocular lens or an eyepiece lens. And so the objective lens is the lens that’s furthest from the eye. It’s always a converging element. It is always a plus lens. And it must have a focal length that is longer, compared to the ocular lens. The ocular lens is the one closest to the eye. And this can be positive, which is the case for Keplerian telescope, or it can be negative, which is the case for a Galilean telescope. And the ocular lens and the power of that lens must be positioned so the image formed by the objective lens is the primary focal point for our ocular lens. So as the light is coming into the telescope, it’s going through the objective lens first. It’s forming an image, and then that image has to be captured by our ocular lens. And so the ocular lens often has a focal length that is much shorter, compared to the objective. So here is our Keplerian telescope. And as you can see, it’s a little bit easier to illustrate here. So we have light coming into the objective lens. This is a converging element. And an image is formed here. This image now becomes the object for the next lens in the system. And then that again converges the diverging rays, and we get parallel light coming out of the telescope. And so for a Keplerian system, we’ve got two positive lenses for our objective and our ocular lens. As you can see, the two focal points are inside the telescope itself. Because of this, the image is inverted, and so often we actually have to put some mirrors in the plane here, to revert the image. And the magnification that we’re getting from the telescope is really a ratio between the angles here and the angles here. If we now look at a Galilean system, this is a similar sort of concept, so we have parallel rays entering the objective lens. That’s a converging element. It converges those rays. This then becomes a virtual image now for my ocular lens. And then parallel rays are out of the telescope. And so as you can see in this ray diagram, the focal points really come together for both lens pieces, outside of the telescope. The image in a Galilean telescope is always erect, so we don’t need any mirrors or systems to invert it. And again, just like a Keplerian system, it’s really the ratio between these angles here and here that gives us our magnification. Our angular magnification. So how do you tell a Keplerian and a Galilean system apart? There’s a very practical way to do it. One is just to look at the eyepiece. So we know for a Keplerian system, the exit pupil is actually sort of floating outside the telescope, and then the two focal lengths are coming together inside the telescope. So you can really see sort of this floating exit pupil that you observe in a Keplerian telescope, versus this Galilean system here. So we can really spot them. The other thing is, because of the optical principles we’re using, the Galilean systems are sort of limited in the magnification provided. So it’s really hard to make a Galilean telescope for a patient as a low vision device at higher powers than 4X. Whereas Keplerian systems, we can go much higher than 4X, quite easily, with that system. So that’s also another thing to know. Is sort of those powers of the systems have different capabilities. So there are different telescope formulas that we have to keep in mind. One is just from our ray diagrams here. We can see that the tube length of the telescope, meaning the distance between the two lenses, is really the sum of the focal lengths of the two lenses. And then the magnification is the resultant of the two angles created by those lenses, or also can be really a relationship and a ratio between the powers of those two lenses. Now, if you had… A patient brought you a telescope and said to you: I need a replacement telescope that is almost exactly the same magnification and power as this one… You know, it’s not as easy to verify the lenses in a telescope as it is in a stand magnifier. But what you can do is, knowing that there is really a ratio that is happening, there is a ratio in lens powers, we can also use the ratio of the entrance and exit pupil of the telescope to come up with the magnification, and we can physically see this and measure it as I illustrate in the previous picture. And so the magnification of the telescope is a relationship between the entrance pupil size and the exit pupil size. Now, there’s one other very important clinical optic point with telescopes and refractive error. And that is that most patients like to use monocular handheld telescopes, especially, without their habitual spectacle prescription. And this really applies to if the telescope is focusable, and you can alter the tube length. And the reason that patients like to do this is because the closer that they get that telescope to the eye, just like any other magnification system, the better the field of view that they will have. So in terms of how we prescribe and account for this, this will, by removing the refractive error, this will affect the power and the magnification that we get out of the telescope, and how the patient is adjusting the tube length. And so some general principles here is that for Keplerian systems that are focusable, a patient with a myopic refractive error will make the tube length shorter, and they will actually get more magnification out of this system with the glasses off than they would with the glasses on. Whereas with a hyperope, they’re sort of the opposite situation. They’re making the tube length longer, and they’re getting less magnification with the glasses on. Sorry, with the glasses off, versus on. Galilean telescopes, the patient with the myopic refractive error is gonna make the tube length shorter, but they’re gonna get less magnification out of that Galilean system, whereas the hyperope, always making the tube length longer, with the Galilean telescope, they’ll actually get more power. So when we’re prescribing telescopes and thinking about handheld versus spectacle, we really have to think about three considerations. One is: Do we want the patient to be monocular or binocular? And this may have several factors, in terms of their scotoma and so on. What is the required task? If it’s a hands-free task, then we’re sort of bound to do a spectacle-based system. And sort of some examples here… We have some spectacle-based systems. Here this is a full diameter telescope, where you’re really looking straight through the telescope. These systems up at the top are bioptics, where the telescope is at the top, and the patient has to tilt their head down to look at the telescope, but they can go back and forth between glasses and telescope, and these are examples of handheld systems that the patient can hold. The last thing we have to think about is: What is the required magnification that we need in our telescope to prescribe? And the required magnification, again, is gonna be a ratio of what we need and what the demand for the task is. So in this case, if we have a 20/200 patient, but they need to read a sign that demands a 20/60 acuity, 200/60 gives us a need for a 3X telescope. So in general, I would say that spectacle-based systems are really helpful for continuous use tasks. Some spotting tasks, if you want to put it just at the top of the glasses in a bioptic position. But really for hands-free tasks that require a spectacle-mounted system. And these can get heavy, the higher in power you go with your telescope. Whereas a handheld telescope, these are really great for spotting tasks, for brief reading of a sign or a quick thing, and can be more inconspicuous. But if the patient has a tremor and can’t keep that telescope still, that is gonna limit their ability to use it. So in terms of the issues of refractive error and so on, some things to consider is: If you do want to do sort of a spectacle-based system, and you’re sort of worried about not being able to have it focusable, and you want to do an afocal telescope, you can get these various clip-on systems that can be helpful as well. So I want to sort of actually end there, so that we have time for questions. You know, I did have a few tips about near telescope systems as well. But in general, I would say that in telescope systems, really if you’re going with a spectacle-based system, alignment is critical. In a handheld system, really, good coordination is critical for alignment. And we have to consider sort of weight and cosmesis when prescribing these things as well. So there was one question up there, that I noticed, sort of about the basic things that I recommend to have on hand for a low vision clinic, when you’re just starting out. And this is sort of our list that Dr. Melkin and I have come up with, in terms of some handheld magnifiers and some stand magnifiers to begin with, when you’re thinking of this. So with that, we’ll get to the questions in the panel here. How do I choose the correct or best low vision devices with so many limitations in my country? In this case, the country is Indonesia. So actually, in your region, there are suppliers in Asian countries that make low vision devices at a lower cost for non-profit organizations. So I can give you some information there. In terms of prescribing the best device, I think it really depends on the patient’s needs. And then really those optical requirements of what is the equivalent power really required for the task. And then sort of playing with… Well, how can I get that equivalent power with limited resources? Do I want to do stand magnifier and glasses, because a hand magnifier power isn’t in availability? So you can sort of be creative there, once you know those key factors. Next question. Generally, a patient with macular disease needs eccentric fixation. How do we manage them with magnifiers and telescopes? That is a really excellent question. So for magnifiers and telescopes, you want to encourage them to use their eccentric fixation. And in order to do that, and to maintain that, oftentimes you want them to keep their eye fairly stable, and then have flexibility to move the system. So you don’t want to sort of move too many things at a time. And you generally want to keep fairly close working distances. So that the field of view is quite large. Because if you have to eccentrically fixate, and you’re holding the system further away, and your field of view is small, your patient is going to have less success. So kind of encouraging some closer working distances are going to help that patient who eccentrically fixates. So the next question is: For intermediate correction, would you consider prescribing bifocals and trifocals? The answer is yes. I often do this. I do this quite a lot, actually. Now, for the intermediate task, the correction you’re going to prescribe is based on the working distance of the task. If the patient wants to see a computer screen, for example, at 50 centimeters, you’re sort of bound by that distance, and you can’t go really higher than a 2-diopter add, unless the patient gets closer. Now, having said that, you could do a near telescope-type system, but the field of view is going to be less. So there is a give and take there in terms of… For intermediate tasks, you’re really limited by that working distance, as to how much magnification you’re going to get with a single-lens system. And then if you need more magnification, you’re gonna have to go with a two-lens system. One question here: Is there any restriction of age for prescribing low vision aids? That’s a really interesting question. So I see a lot of pediatric low vision. So the maturity level of the child is one thing. But there’s also an importance of age in terms of their maturity and development. So I find that there’s really a fine line here, in that you don’t want to prescribe too many devices at too young of an age, and it becomes sort of more of a show and tell piece in the classroom that gets passed around more than it’s used. But, on the other hand, I think prescribing devices early is really important for a patient’s social development. I find that if I wait to prescribe devices in children beyond the age of eight, children who I think could really use and benefit from low vision aids, and I wait too long, that self-awareness piece kicks in. The adolescent angst kicks in. And really the ability of the patient to adapt and accept low vision aids and the concerns that they have about how their peers view them… Is a lot more strenuous at older ages. Whereas, if we can introduce low vision devices at a younger age, and the patient’s friends and community and family become aware that they required additional assistive equipment to see, and it becomes a very much accepted piece, that patient has to have a lot more success. So I would say that if we were going to make an error about when to prescribe, I would rather make the error on the side of prescribing too young than waiting too late. A follow-up question is: Based on my explanation, which device is good for certain ages and occupations? It depends, but many of the children I see have unique demands and hobbies that they do, that require different aids, but generally speaking, if I was going to pick a boilerplate answer, I would say that a dome magnifier is a first step. It’s a great device for a child. And a monocular handheld telescope. Those are the two things that I would probably uniformly always prescribe as a first step. And then once we have success with those two things go to other devices. Now, certainly, there are always cases where those devices might not be appropriate, because of the really poor contrast or field loss or whatever the case may be for a particular patient, but I’m saying generally, for most kids, a dome magnifier and a monocular telescope are gonna be really helpful. The next question is about prescribing a near telescope system, and whether it must be angled in towards the near focal point. So if I go back to my slides, and I apologize for running out of time on the near telescope system, but to answer your question, let’s go over it here. So for a near telescope system, we do have to sort of consider this same Feq equation. Demand for the near task or estimated task has to be comparable or slightly less than the equivalent power of the device for the telescope. So when we’re prescribing a near telescope, we have to have a cap power that’s on the objective lens. And so, because for a near system, infinity and parallel light is not entering the telescope anymore, it’s going to be — divergent rays are now entering the telescope. So we need an additional cap over the objective to converge those rays. And the cap that you choose is going to depend as you put it on the near point or the working distance of the patient’s needs. So if a patient is playing cards, for example, and the cards are 50 centimeters from the spectacles, then I need a 2-diopter cap on my telescope. And then the equivalent power that I will get out of the system will be the power of the cap, multiplied by the magnification of the telescope system I’m using. So again, you can play with — the distance is really what’s going to determine the cap, and then that power of the telescope and the cap together are gonna provide you your magnification to prescribe. Your point about alignment is a good one. On another piece, not just prescribing the cap, but if you’re doing a binocular near telescope, you do have to turn the two telescopes in, so that they can properly converge on the page. So the alignment has to be right. Because you want the exit pupil of the telescope to enter the pupil of the eye appropriately. So you do need that near PD for the task. In terms of prescribing telescopes, kind of going into a bit more detail, so in terms of prescribing telescopes, let me go back to my slides here. So first step is to determine: Do you want the patient to use both eyes or one eye? And that could depend on a number of factors. You might prescribe a telescope — and we often do — for the better-seeing eye. But if that better-seeing eye also has a large blind spot in it that the other eye doesn’t have, the patient might be better off doing the task with both eyes together. So that’s your first decision in prescribing. Do you want a binocular system or a monocular system? The next question you have to decide is really: What is the task? And the demand involved in the task? You know, is it a task that requires hands-free? Is it a task that does require use of the hands, that we really need a spectacle-based system? So what is the task that the patient is trying to do? And then the third thing is: Once we’ve identified what it is the patient is trying to do, the question is: How much magnification do you need to get there? And you can figure that out either by thinking about it in an acuity level, which I did here in this example, or you can think about it in terms of a ratio of distances and objects. You could think about it in terms of a ratio of distances. The patient might tell you: I want to be able to see the grooves in my key. I can see it here, but I want to see it further away. That ratio will give you the magnification required as well. So those are really the three questions you have to answer when prescribing telescopes. Two eyes or one eye? Number two, what is the task? And number three, what is the magnification required for the task? And by answering those three questions, you can prescribe pretty much any telescope system. The next question: Can we attach a Galilean/Keplerian system to the spectacles? And the answer is yes. I often do that. These are Galilean 2x telescopes on a slider that’s attached to a clip here, that’s then on spectacles. This system that I’m pointing to here — this system is a Keplerian telescope that actually… We drill a hole in the lens, and we insert that telescope and glue it in. And so you can actually just mount a telescope right in the glasses. When you do that, though, you have to sort of consider what position you want to put it in. You could put the telescope right in the middle of the frame, sort of as a full diameter mount. You could put it just at the top here, so the patient can look through the telescope or not through the telescope and go back and forth. Or you can put the telescope down at the bottom of the lens, almost like where you put a bifocal for a reading task. So you do have those options. One question is: How often would I prescribe a binocular, compared to a monocular telescope, for distance? This is a difficult question, because… So much is task-dependent. I would say that, for things like TV watching, I often prescribe a binocular spectacle-based system, such as this one here. I would say that for spotting tasks, I often prescribe a handheld telescope, just because it’s easier to hold it up and take it down really quick. So I would say it really depends on the task. Another person asks for a patient with low contrast sensitivity and low vision what is the best low vision aid? I would say that if the contrast sensitivity is very low, like less than 1 log unit, then it’s going to be hard in general to use optical systems, because unless you’re using very brightly illuminated ones — the best device for a patient with low contrast sensitivity would be an electronic aid. So the electronic magnifiers, desktop magnifiers, et cetera. One question is: Are devices sometimes given on a trial and error basis more than formula calculations? I would say that my prescribing habits are not. It’s really to prescribe based on formula. Most of the time. But with that in mind, you may want to give the patient two options to try at home with a particular task. Especially if the situation is unique. You might pick two powers that you sort of narrowed it down to. And you’re finding out which is convenient for the patient. Or you’ve picked the power that you need, and you’re really deciding between what is gonna be more practical for the patient. So you’re trying to decide between a hand magnifier, versus a stand magnifier system and glasses. Both have the same equivalent power, but you’re trying to find out what’s more convenient for the patient to use. So I think there is some training and some further valuation that’s sometimes often needed after your sort of clinical assessment, that a rehab therapist can do or a staff member can do, beyond the exam room with the patient, or have the patient as you say take it home and try it. Well, there’s a lot of really great questions, and I’m really happy to answer all of them, and I’m more than happy to answer further questions if you email. So please do send your questions in, and I’m really happy to answer them. One area that we didn’t touch on today was a lot of the electronic head-mounted systems that are really exciting, that are really — a large part of my research. And I’m happy to share thoughts on those as well. And those are really just starting to come to the forefront. So thank you again for your time, and please don’t hesitate to email me with the further questions that we didn’t get to today.

Download Slides


February 22, 2018

Last Updated: October 31, 2022

2 thoughts on “Lecture: Low Vision Devices: Providing Functional Vision for Patients”

Leave a Comment