12.2 Low Vision Aids

Figure 12.1. Classifying information access technologies

and touch. Sections 12.6–12.8 consider access to information and telecommunications technologies, with the focus on hardware in Section 12.6 and software and, in particular, the design of accessible web content and using web authoring tools, in Section 12.7. Accessible telecommunications for blind and visually impaired people are considered in Section 12.8. An overview of these information access technologies is presented in Figure 12.1.

12.2 Low Vision Aids

Low vision aids is the term used for a group of assistive technology devices designed for visually impaired and blind people with some useful vision. However, despite being an important group of assistive technology systems that could be of benefit to a large number of blind and visually impaired people, they are often overlooked. For instance, there are 1.7 million people in the UK who are unable to read standard print with any ease and some 36% of blind people can read large print (Gregory 1996). Thus, many people who are registered as blind have some useful visual faculties that could be augmented by an appropriate low vision aid to, for instance, give them access to print and mobility.

12.2.1 Basic Principles

The technology of optical low vision aids is well developed. However, there may be other technological approaches that could be developed to extend the range of applications and/or users. The aim of this section is to provide an overview of currently available low vision assistive technology systems. The interested reader is referred to Dickinson’s book (Dickinson 1998) for a comprehensive and detailed presentation of low vision technology. Dickinson’s taxonomy of low vision aids will be followed in this section.

Currently available low vision assistive technology is based on optical systems using magnification. Figure 12.2 illustrates the impact of a low vision aid by showing the image on the retina with and without the aid that increases the image size on the retina.

Since the retinal image size depends on the angle θ subtended at the nodal point of the eye, the magnification obtained by a low vision aid is given by

388 12 Accessible Information: An Overview

Figure 12.2. Low vision aids: generic analysis

Useful formulae are obtained by making the following approximations for small angles θ1 and θ2 (see Figure 12.2):

where the symbols θ, h and d denote the angle subtended by the retinal image at the nodal point of the eye, the size of the object and the viewing distance respectively. Subscripts ‘1’ and ‘2’ denote without aid and with aid respectively. This gives the following expression for the magnification, M:

The following four different cases, illustrated in Figure 12.3 and which lead to different types of assistive technology, are distinguished by Dickinson (1998):

1.Increasing the object size, but keeping the viewing distance constant

2.Decreasing the viewing distance while keeping the object size constant

3.Creating a magnified real image

Figure 12.3. Optical geometries for low vision aid methods

4.Telescopic magnification, which makes an object appear larger, without changing the object size or viewing distance

The optical geometries for the four different methods are illustrated in Figure 12.3.

Method (i) Increasing the object size while keeping the viewing distance constant

This method is illustrated in Figure 12.3 (Method (i)), which shows the retention of the same viewing distance (implies d2 = d1), and the increase in object size, giving h2 > h1. Therefore,

390 12 Accessible Information: An Overview

The common low vision aid that uses this method is large print, such as large print books, large print newspapers and a range of other large print documents, including bills, accounts, terms and conditions and official letters.

The magnification that can be achieved in large print books is limited by upper limits on the size and weight of a book before it becomes unwieldy rather than technical factors. Typically magnification of 2.5× can be achieved before the book becomes excessively large. However, restrictions of this type do not affect short documents, such as official letters.

Method (ii) Decreasing the viewing distance while keeping the object size constant

This is shown in Figure 12.3 (Method (ii)), where the unchanged object size implies that h2 = h1, whereas the reduced viewing distance gives d2 < d1, hence,

This method is most commonly implemented by plus-lens magnification. This gives the user an increased retinal image size by bringing an object closer to the eye without requiring the accommodation normally required to view close objects. The magnification formula for a plus lens assistive system is obtained by assuming that the user has a reading distance of 25 cm or 0.25 m. Since the power of a lens in dioptres is equal to the reciprocal of its focal length in metres, 0.25 m is the focal length of a lens with power 1/0.25 = 4 D. Therefore, the old viewing distance is the focal length of a lens of power 4 D. The new viewing distance is the focal length of the magnifier lens. Therefore substituting these facts in the magnification formula gives

(12.6)

where this expression has been obtained by using the fact that the power of a lens is the reciprocal of its focal length.

In practice, the user’s reading distance may not be approximately equal to 25 cm. Since the magnification is proportional to the reading distance, users with greater reading distances will obtain higher magnifications and users with shorter reading distances will obtain less magnification. Therefore, users who already hold reading matter very close to their eyes would require a lens with exceedingly short focal length and very high power to obtain any benefit from this approach. Consequently, this method is of most benefit to users who have moderate or high reading distances.

Method (iii) Creating an enlarged image at a constant viewing distance

This method is similar to Method (i). The difference is that an enlarged image rather than an enlarged object is produced at the constant viewing distance, as