Prescribing Optical Magnification
Impaired visual acuity is a common problem that many eye health specialists encounter, especially when working in low vision rehabilitation clinics (Scheiman et al. 2006). Vision rehabilitation focuses on getting patients to utilise their vision to the best of their capability. Rehabilitation programs incorporate prescribing optical and screen based magnification, contrast enhancement, lighting modification, eccentric viewing training, orientation and mobility training as well as assistance with daily activities and counselling (Lovie-Kitchin and Whittaker, 1999). However, the following paragraphs will be focusing on optical magnifiers and how they are prescribed to patients.
Most patients who experience low vision have trouble reading and require optical magnifiers for help. The optical devices that are prescribed to patients enhance a patient’s vision by magnifying what it is the patient is viewing, thus allowing for detail to be detected (Scheiman et al. 2006). Patients will require optical devices for many different activities such as reading, sewing, watching television and managing their medications to name a few. The way that these optical devices are prescribed is based on what the patient will need them for and this, along with the severity of their low vision, will determine the optical power of the device.
INDICATION FOR USE
Optical magnifications are devices that are used and prescribed in low vision clinics to the appropriate client. These types of magnifications are more commonly used and prescribed for patients who experience difficulty with their day-to-day activities hence are seeking for a way to improve their living (American Foundation for the Blind, 2015).
Optical magnifications indications for use include:
➢ Clients level of visual acuity
➢ The reading material they aim to access- spot reading for medication, text books for school, reading documents for work, or simply recreational reading
➢ The environment they will be using it in
➢ Cognitive or physical limitations
➢ The distance for use - near or far
➢ Any secondary disorders/disease
(Vision Australia, 2012)
All of the above factors need to be considered when prescribing optical magnifications to ensure the most appropriate and comfortable magnifier is offered to the client to ensure their satisfaction.
LIST OF DIFFERENT MAGNIFICATIONS
Optical magnifications can be categorized into three main groups.
➢ Hand magnifiers
➢ Stand magnifiers
➢ Illuminated magnifiers
|Non-Illuminated Hand Held Magnifier||The patient adjusts magnifier that is held in the hand and the height until print comes into focus.||Portable
Easy to use Flexible working distance Familiar Inexpensive
|Require use of one hand
Require good hand control Are generally only useful in low magnifications
|Illuminated Hand Held Magnifier||Patient holds the magnifier in hand and adjusts height until print in focus. Also has a light source for more clarity||Portable
Inexpensive Useful for when light source poor Easy to use
|Require use of one hand
Require good hand control
|Non-Illuminated Stand Magnifier||These magnifiers rest on the page and do not need to be focused.||Available in higher magnification. Do not require good hand control as it rests on the page. Object always in focus||Fixed posture required. Difficult to get close enough for better viewing. Need hard, flat surface to view. Difficult to use for extended reading.|
|Illuminated Stand Magnifier||Magnifier that rests on the page and does not need to be focused. Also has a light source powered by batteries.||Available in higher magnification. Do not require good hand control as it rests on page. Object is always in focus.||Fixed posture needed. Difficult to get close enough for better viewing. Need hard, flat surface. Difficult to use for extended reading.|
|Pocket Magnifier||A small magnifier that can be carried in handbags.||Easy to use and portable.||Only available in low magnifications.|
|Visulette and strip magnifier||Looks like a glass paperweight. A strip magnifier is like a stretched out visulette.||Easy to use. Have set focal distance. Sits on page.||Only available in low magnifications.|
|CCT (Portable and Stable)||Device utilising a camera and monitor to magnify a page or object. The display can be monochrome or full colour and the products available in a variety of screen sizes and magnification levels.||Good for reading books. High magnifications.||Not portable. Expensive.|
|Spectacle Magnifiers||Magnified lens mounted into spectacles.||Easy to use. Hands Free. Portable.||Short working distance due to limited level of magnification.|
|Portable Video Magnifier||Portable magnifying device with a self-contained camera and viewing screen.||Portable. Less expensive than CCT. Useful for near tasks.||Field of view is too small for reading books.|
Vision Australia, 2012 Statewide Vision Research Centre (SVRC), 2009
DETERMINING THE INDICATION OF USE
Optical low vision devices are task-specific and as such the type of device prescribed will be dependent on the task the patient intends to use it for. The orthoptist will consider for example, whether they want a device that allows them to watch TV, check their mail, read a book comfortably, or whether perhaps they wish to be able to see bus and street signs to allow them to travel independently. There are various types of optical aids available and which one is prescribed will be determined by the nature of the task and magnification level required (Lindsay & Jackson, 2008). The orthoptist may elect to start with one device and see how the patient manages, then provide additional devices as the patient becomes comfortable with using the device (VisionAware, 2015). Optical low vision devices can be classed into two categories: (1) Near devices are designed for near work activities such as reading, writing and sewing. (2) Distance devices on the other hand are for activities such as watching movies, a sporting event, or being able to identify street signs and bus/train numbers. Both these types of devices can be enhanced by altering illumination and contrast to reduce glare (VisionAware, 2015).
DETERMINING THE AMOUNT OF MAGNIFICATION
There are generally two methods for estimating the amount of magnification needed for a specific task.
Method 1: This works for both near and distance vision tasks. The patient’s current visual acuity (VA) is divided by the required VA, so: M= what patient can read/what patient wants to read. For example, if a patient has a near acuity of N48 and they want to read N8 size print, then a 6x magnification would be needed. The same formula applies for distance acuity. If the patient were to have distance acuity of 6/36 and they wished to see 6/9 print, then a 4x magnification would be trialed (Lindsay, & Jackson, 2008).
Method 2: You can increase the reading addition and reduce the working distance (i.e. bring the reading material closer) until the patient is able to read the task material with a level of fluency. By offering near additions and simple plus lens magnifiers, the patient is able to hold material in focus at a close viewing distance and thus project an enlarged retinal image (Lindsay, & Jackson, 2008; Lovie-Kitchin, & Whittaker, 2010).
When prescribing optical aids, it is also important that we consider whether the patient needs to accommodate as this will influence their working distance. The closer the material is, the greater the accommodation required. The formula used to calculate the amount of accommodation is: Accommodative Demand = 100/WD (cm) If for example, the patient holds their reading material at 50cm, they would need to do 2.00D of accommodation. So the further the patient holds the material, the less accommodative effort that is required (Vukicevic, 2014).
The field of view refers to “the amount of information that can be seen through a device at one time” (Zimmerman, Zebehazy & Moon, 2010, p.208). The field of view becomes smaller with increased lens strength (greater magnification, more dioptres). Hence, weaker magnifiers offer a greater field of view. The distance between the magnifier and the eye will also influence the field of view, with the field of view being greater when the magnifier is held closer to the eye (Lovie-Kitchin, & Whittaker, 2010; Vukicevic, 2014).
The focal distance is the distance between the device lens and the material being viewed and can be calculated when the dioptric power (D) of a lens is known by using the formula:
FD = 100cm/D
If for example the lens power is +20 dioptres, the focal distance will be 5cm. So as the power of a lens increases, the focal distance decreases which means the patient needs to hold the device closer to the material (Zimmerman, Zebehazy & Moon, 2010).
For those patients requiring optical aids for reading, it is important to identify their goal reading rate as the magnification required for this task depends not only on the print size, but also on the reading rate that is required to be able to read fluently. Tasks such as reading labels require lower reading rates and therefore lower magnification as opposed to reading a book even if the two have similar print sizes. Individuals with normal vision tend to read at a rate of approximately 160 words per minute (wpm), and a rate of 80wpm or more is often necessary to read long text satisfactorily. In contrast, spot reading such as reading food or price labels requires a rate of around 40wpm (Lovie-Kitchin, & Whittaker, 2010). Consequently, the orthoptist may need to prescribe different devices for different reading requirements.
Patients should always use the device with the lowest magnification that meets their needs. Those experiencing difficulty with higher magnifications may find it helpful to initially train using lower magnifications. The most favourable responses will be achieved when patients locate and maintain the focal distance between the device and materials and, in the case of hand-held magnifiers, it is best if the patient begins with the device against the material and then moves away to locate the focal distance (Zimmerman, Zebehazy & Moon, 2010, p.210).
TUTORIAL FOR PRESCRIBING LOW VISION MAGNIFICATION
Lindsay, J., & Jackson. J. (2008). Dispensing III – The Low Vision Patient. Continuing Education and Training, 14(38), 30-37. Retrieved from http://www.optometry.co.uk/uploads/articles/C-8737%20210308%20PAYL.pdf
Lovie-Kitchin, J.E., & Whittaker S.G. (2010). Prescribing Near Magnification for Low Vision Patients. Clinical and Experimental Optometry, 82(6), 214-224. Retrieved from http://onlinelibrary.wiley.com/doi/10.1111/j.1444-0938.1999.tb06651.x/pdf
Scheiman, M. & Scheiman, M. & Whittaker, S. (2006). Low vision rehabilitation : a practical guide for occupational therapists. Thorofare, N.J. SLACK Inc
Statewide Vision Research Centre. (2009). Magnification in your pocket-magnifiers? Retrieved from http://www.svrc.vic.edu.au/CUmagnifiers.pdf
VisionAware. (2015). What Are Low Vision Optical Devices? Retrieved from http://www.visionaware.org/info/your-eye-condition/eye-health/low-vision/low-vision-optical-devices/1235
VisionAware. (2015). Magnifiers and Reading Glasses. Retrieved from http://www.visionaware.org/info/everyday-living/essential-skills/reading-writing-and-vision-loss/magnifiers-and-reading-glasses/1234
Vision Australia. (2012). Magnifiers. Retrieved from http://www.visionaustralia.org/living-with-low-vision/learning-to-live-independently/reading-and-writing/magnifiers
Vukicevic, M. (2014, June 25). Optical low vision aids Part 1. Retrieved from https://www.youtube.com/watch?v=nXPJOYss31I
Zimmerman, G.J., Zebehazy, K.T., & Moon, M. L. (2010). Chapter 7: Optics and Low Vision Devices. In A.L. Corn & J.N. Erin (Eds.), Foundations of Low Vision: Clinical and Functional Perspectives (2nd ed., pp. 192-237). New York: American Foundation for the Blind.