Optical and Low Tech Devices
Posted May 9, 2021
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This is Mark Wilkinson from the University of Iowa Department of Ophthalmology and Visual Sciences. In this presentation we will be discussing optical and low-tech devices to enhance the visual abilities of individuals who are visually impaired.
We will be discussing what you need to know to prescribe effective devices.
Options to read print. Optical magnification options for distance and near vision enhancement. Absorptive lenses, Non-optical devices and lighting for optimum visual functioning.
To Prescribe the Appropriate Low Vision Device you need to start with the patient's visual enhancement needs for distance, intermediate and near vision tasks. You will then need to determine the magnification required for your patient to accomplish these specific goals. To do that, you will need to review the results of the tests of visual functioning with specific attention paid to the patient's contrast needs. You will need to understand the optical properties of the systems being considered. Finally, you will need to discuss available lighting, particular task lighting which we will talk about in a few minutes.
You also want to know the cause of your patient's vision loss, what devices they have used or tired in the past. Any working distance requirements they have, in particular do they require a certain working distance for near vision tasks. Other considerations include the weight of the device, the cost of the device and any cosmetic concerns the patient has about the appearance of the devices that can enhance their vision.
It is important to review with your patients the different options that are available to them to read print. Those options include Relative Size Magnification, where the person is restricted to just reading enlarged print. There is Relative Distance Magnification, where the person holds the materials closer to the eye to make it appear bigger. They can use Angular Magnification by using a low vision device. We will discuss devices in more detail in a few minutes. Finally, they can use Electronic Magnification, which is a high-tech option which will be discussed in greater detail in a separate presentation about high tech options.
With Relative Size Magnification the object being view is physically enlarged to increase the size of the object on the retina. If you need 2x magnification, you simply increase the size of the material you want to read by 2x. This is the basis of large print materials.
With Relative Distance Magnification, you move the object of regard closer to enlarge the size of the image on the retina. If an object held at 40cm is moved to 10cm, the object will appear to be 4x bigger, because it is 4x closer.
When magnification of reading material is accomplished by moving the materials closer to the eyes until it is large enough to be readable, we have to consider the accommodative demands required by this closer working distance.
In order to focus on the image at this shorter distance, the person must accommodate or use a lens that focuses at that distance.
For a 10cm working distance, 1/10 = 10D
For a 20cm working distance, 1/20 = 5D
Angular Magnification is accomplished using optical devices. With an optical device, the ratio of the angle of subtense of the image formed by an optical instrument compared to the angle of the actual object gives the amount of magnification produced by the device.
Conventional Spectacles/Bifocal are the starting point for enhancing the vision of individuals who are visually impaired. In general, the glasses RX power is determined by a trial frame refraction. The spectacle correction is used for distance acuity improvement as well as for near or intermediate tasks improvement, where lower magnification is required.
Additionally, conventional RXs can be use optical devices. It is important to know that stand magnifiers require accommodation or a reading add. Also, stand magnifiers have a maximum add limitation.
Finally, electronic magnification devices most often require accommodation or an add to allow the person to see comfortable at the working distance of these devices.
Let's now review the different types of Glasses that may be prescribed.
First, we have conventional spectacles. The RX for conventional spectacles is typically determined by a trial frame refraction as mentioned before. Watch the trail frame refraction presentation for more information on how to refract with a trail frame and trial lenses.
Conventional spectacles come in a variety of different lens designs, based on the needs of your patient. You may prescribe a multifocal lens design in either a bifocal, trifocal or progressive addition style. A Franklin segment may be prescribed if you are prescribing a very high bifocal addition.
When taking care of individuals who are visually impaired, we often consider a +4.00D add as an intermediate distance add. The +4.00D add is helpful for individuals who require a 6D or greater add to read.
These individuals still need magnification for less detailed tasks such as signing their name, cutting their finger nails, seeing food on their plates, Cooking and reading larger print, such as headlines, etc.
Next are Reading Spectacles. With reading spectacles, the material to be read is held at the focal point of the lens. This makes the image appear to come from infinity.
When prescribing Reading Spectacles, you need to consider if patient is monocular or binocular. If they are monocular, no prism will be needed in their reading correction. That said, sometimes the poorer seeing eye will confuse the better seeing eye. In that case, you may need to fog or occlude the fellow eye if it interferes with the better eye.
If the visual acuities are similar at near, and if binocular acuity is better or the same as monocular acuity, base in prism will be needed for adds of +4.00 to +12.00D.
When prescribing prism power, you will take the add strength power + 2 prism diopters Base In in each eye. For example, with a +6.00D add, you will prescribe 8 prism diopters Base In, in each eye. This reduces the convergence requirements need to align the eyes for the closer working distance required with stronger reading RXs.
Here are some examples of prismatic spectacles. Powers are between +4.00 to +12.00D. They are available in half eye or full field lens designs. Hi-index lenses are available for a thinner lens profile. It is important to tell your patients that these lenses are purely for reading. They are not for use during ambulation.
Here you see High plus aspheric injection molded lenses. They are available in powers of +10.00 to +20.00D. They can be fabricated in regular and hi-index materials. They are available in half eye or full field designs. These higher-powered lenses are used with better seeing eye.
Another option in this category is the Noves lenses, which use diffractive optics to allow for a 5mm thick lens. They can only be used monocularly.
Next, we have microscopic spectacles. They come in 6x to 12x designs, with equivalent power of 24 to 48D. Simply divide the dioptric power by 4 to get the magnification. For example, a 24 diopter lens, divided by 4 = 6x.
The advantages of microscopic spectacle lenses is that they have reduced distortion, better edge to edge clarity and a larger field of view than non-microscopic lenses of similar power. Microscopic spectacles are available in bifocal or full diameter designs.
The Clear Image Lens design is a Doublet Microscopic Lens system. It comes in powers of 2x to 20x. The advantage of this design is an even greater reduction in distortion when compared to microscopic lenses. There is better Edge-to-edge clarity and a Larger field of view. The bottom picture shows the Clear Image Telephoto lens. With this lens design, the patient will hold the material they are reading right up to the lens. The extended cylinder on this lens sets the focal distance for the lens.
Here are a few images of different specialty Microscopic lenses. They are available in powers up to 80D. They are available in wide angle microscopic designs, hi add bifocal designs and press on adds.
Let's now switch gears to hand magnifiers. With hand magnifiers, the material being viewed is held at focal point of the lens. At that focal point, the image appears to come from infinity. Hand magnifiers are used with the patient's distance Rx.
Hand Magnifiers come in both Illuminated and non-illuminated designs. It is important to know, and tell your patients who typically want a large and very strong magnifier, that the larger the lens diameter, the weaker the lens power.
With stand Magnifiers, the material being read is positioned inside focal length of lens. This creates a virtual image that appears to come from a finite distance behind the lens. Because the image does not appear to be coming from infinity, as is the case with a hand magnifier, the patient will need to use a stand magnifier with a reading correction.
Stand Magnifiers come in both Illuminated and non-illuminated designs. Again, it is important to know, and tell your patients that the larger the lens diameter, the weaker the lens power.
Now let's talk about telescopes. Telescopes come in two designs. First, there are
Galilean telescopes. Galilean telescopes have a + objective and – eyepiece. Galilean telescopes come in lower powers, typically 2x – 4x. Galilean telescopes are lighter, less complex and less expensive and usually a little easier to use. They also have a larger exit pupil which contributes to their ease of use.
The other telescope design is the Keplerian design. The Keplerian design uses a + objective and + eyepiece. Another term for a Keplerian telescope is an astronomical telescope. To make a Keplerian telescope a terrestrial telescope, a prism is required to invert image to right side up. Keplerian telescopes are typically higher in power, usually 4x and above. These telescopes are heavier and more expensive because they have a longer tube length than a Galilean telescope and because of the need for an inverting prism in the design. Additionally, Keplerian telescopes are poorer at light gathering than a Galilean telescope.
Telescopes can be spectacle mounted in powers of 1.5x to 8x. Design options include bioptic, with the telescope mounted in the upper portion of the lenses. Other mounting options include full diameter, wide angle and expanded field. Auto focus systems are also available.
Telemicroscopes are also known as reading telescopes or surgical loupes. Telemicroscopes can be spectacle mounted in a full diameter, center mounting system or in a bioptic configuration. Telemicroscopes are available in both Galilean and Keplerian designs.
Optical magnification is very helpful for many of our patients who are visually impaired. That said, you should be aware that there are a number of problems with optical systems. Those problems include, the stronger the system, the smaller the field of view and the closer the working distance required to use the device. Additionally, the depth of field is more limited and contrast is decreased.
Lighting and glare can be an issue and higher-powered optical systems have more aberrations. As noted previously, higher powered systems are difficult to impossible to use binocularly.
The advantage of absorptive lenses is that the reduce glare and decrease photophobia. This enhances and increases contrast to allow for clearer vision.
It is important to know that the type of filter that is best for any given patient is determined by assessment and trial, not by the diagnosis of their condition. People vary a lot in what filter works best for them, which is why an individualize trial of filters is the only way to determine the best filter for any given individual.
Here you see a child with Pingelapese Achromatopsia who is clearly much more comfortable with a tinted lens that still allows you to see his eyes, but gives him significant relief of his photophobia by selectively absorbing the lower end of the visable light spectrum.
Non-Optical Devices come in a variety of different forms. Here you see a reading stand which is helpful for maintaining proper placement of reading materials. Clipboards can provide the same assistance in reducing postural fatigue. Additionally, reading stands and clip boards facilitate getting adequate lighting on the reading materials.
Typoscopes are another option that both reduces glare from glossy paper and minimizes figure ground confusion.
There are a wide variety of items to enhance the ability to do activities of daily living that have enhanced contrast. For example, items with white letters on a black background are often easier for individuals with vision loss to view.
Lighting is the single most important factor in enhancing visual functioning.
There are a number of different types of lighting available including Incandescent, Fluorescent, Halogen and the newer LED lights which have more or less replaced incandescent bulbs because of their energy efficiency. I do not recommend halogen lights, because they are too hot.
Probably as, if not more important that the light bulb is the light fixture. The light fixture must be flexible to allow for adjustment of distance to paper and angle of the light. Typically, having the light shine in from behind and to the side of the better seeing eye provides the best illumination, without causing glare on the material being read.
Now you know about optical and low-tech devices and options to enhance visual functioning.
Thanks for your attention. If you have any questions, you can contact me by email.