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•Reversing the polarity of the screen image can be used to give the option of viewing white text on a black background in addition to the standard black on white. This is useful for people with opacities of the optical media, for instance due to cataract, and many CCTV users prefer this mode of viewing.
•CCTV provides greater contrast than optical systems.
•Some CCTVs have the facility for a split screen presentation, allowing different tasks to be viewed simultaneously, for instance in taking notes from a blackboard.
However, CCTVs also have the following disadvantages:
•Acquiring proficiency in CCTV use requires more practice than an optical aid.
•CCTVs are expensive and require regular maintenance and servicing. They are also bulky and, other than miniaturized systems, are not easy to transport.
•Persistence of the image can lead to blurring, particularly with white on black images and limit the maximum reading speed.
12.3.3 Video Magnifiers
The video magnifier is another implementation of the real image or transverse magnification principle. A typical product, the Pocketviewer, is shown in Figure 12.6a and, as can be seen, it is a hand-held video magnifier that is held over the text to be viewed. The device shown in the figure is a colour viewer but there is also a monochrome version available. A magnified image is displayed on the built-in flat screen. A typical unit comprises a 10-cm flat panel housed in a unit of dimensions 3.5 × 8.6 × 14.25 cm. A miniature video camera captures the text on the page and transfers the image to the screen with 7× magnification. The Pocketviewer uses a re-chargeable 1.5 V battery source or can be powered by an a.c. adaptor.
Figure 12.6b shows an internal view of the Pocketviewer that reveals the advanced miniaturisation used in this product. As shown in this photograph, the device has a flat outer case that opens up into two parts with circuit boards in each part. The lid or top part of the case holds a single circuit board that drives the flat screen assembly. (This circuit board is the top one with a small square label in its bottom right hand corner.) The bottom part of the Pocketviewer case contains three components. On the left is the raised camera board with the video camera underneath this board. This small board is easily found since it has the 5 VDC label on the top. The main board in the remainder of the lower part of the case does the signal processing, control, power supply and drives the camera and display tasks. The power supply is on the far right hand side of the main board and in the bottom right corner, the location of the power adaptor socket can be seen.
Two versions are available: the mono version with high contrast black-on-white or white-on-black and a full colour version. The mono-pocketviewer is considered appropriate for reading labels, price tags, credit card slips, restaurant menus, programmes, timetables and similar tasks, as well as browsing through magazines,
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Figure 12.6a. Pocketviewer. b Internal view of PocketViewer construction (photographs reproduced by kind courtesy of Humanware Group)
books or newspapers. The colour version will generally be required for maps, photographs, illustrations and three-dimensional objects. It can also carry out all the tasks that can be performed by the mono version.
12.3.4 Telescopic Assistive Systems
Telescopic assistive systems give users an enlarged retinal image while remaining at their normal viewing distance. They can be used for near (writing, handicrafts), intermediate (TV, music, playing cards) and distant (blackboard, street signs, bus numbers) tasks, but have a very restricted field of view. There are special designs for when the user is mobile, but they have the disadvantage of magnifying the object’s apparent speed as well as its size. The optical principles of telescopic assistive systems are afocal, with parallel rays of light entering the telescope from a distant object and parallel rays of light leaving the telescope to form an image at infinity.
There are two main types of low vision telescope:
•Astronomical or Keplerian Telescope (shown in Figure 12.7a). The image is inverted, as rays from the bottom of the object form the top of the image. The image must then be inverted, so it is the right way round. One possibility is the use of two right-angled (Porro) prisms, as shown in Figure 12.8. The use of
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Figure 12.7a,b. Optical geometries for astronomical and Galilean telescopes: a astronomical telescope; b Galilean telescope
prisms also allows the optical path length between the objective and eyepiece to be ‘folded’, thereby reducing the overall length of the telescope. The objective lens FO is convex and forms an image of the distant object at its second focal point. Diverging light from this point is then refracted by the convergent eyepiece lens, FE. The lenses are positioned so that the first focal point of the eyepiece lens is coincident with the second focal point of the objective lens, so that parallel light will emerge from the telescope system.
•Galilean Telescope (shown in Figure 12.7b). The eyepiece lens is positioned so that its first focal point is coincident with the second focal point of the objective lens. Rays of light converging to the second focal point of the objective lens are intercepted by the eyepiece lens and emerge parallel from the system. The image is the right way up, as rays of light from the top of the object go to the top of the image.
The total length, t, of each telescope is equal to the separation of the objective and eyepiece lenses, that is:
t = fO + fE |
(12.10) |
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where fE and fO are respectively the first and second focal lengths of the eyepiece and objective lenses. Using the fact that the focal length of the eyepiece lens is negative and assuming that it is thin, it can then be shown that
ta = fa.O + fa.E |
(12.11a) |
tg = fg.O + fg.E |
(12.11b) |
where the subscripts ‘a’ and ‘g’ refer to the astronomical and Galilean telescopes respectively and fE’ is the second focal length of the eyepiece lens. It is negative for the Galilean telescope and positive for the astronomical telescope and the objective focal lengths are both positive. Therefore, a Galilean telescope will be shorter than an astronomical telescope with the same magnification, though the difference can be reduced by using prisms to fold the light-path as described above and shown in Figure 12.8.
It can be shown (Dickinson 1998) that the magnification for both types of telescope is give by the negative ratio of the powers of the eyepiece and objective lenses, i.e.
= −FE
M (12.12)
FO
Since both powers are negative for the astronomical telescope, the magnification is negative for this telescope. This is consistent with the fact that the image is inverted. Since the eyepiece power is negative and the objective power is positive for the Galilean telescope, the magnification is positive for this telescope. This is consistent with the fact that the image is erect.
One of the disadvantages of a telescope is its very restricted field of view. Consequently, the user can only see part of the task at a time and needs to scan across the area to obtain a complete view. The maximum field of view is obtained by making the objective lens as large as possible, whereas the eyepiece lens can be smaller, as long as it does not cut off peripheral exiting rays.
People with myopia or hyperopia (without astigmatism) can compensate by changing the telescope length, adding their full lens prescription to the eyepiece lens or adding a partial lens prescription to the objective lens. Adding the full prescription to the eyepiece lens can be achieved by holding or clipping the telescope
Figure 12.8. Using two right-angled (Porro) prisms to accomplish image inversion and light-path folding
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to the prescription glasses or attaching a small auxiliary lens behind the eyepiece lens. Modifying the objective lens is never used in practice. People with myopia would need to shorten the telescope and people with hyperopia to extend it. This is a useful strategy for people with low to moderate myopia or hyperopia and is generally not affected by up to 2 D of astigmatism.
12.3.4.1 Using Telescopic Systems to View Near and Intermediate Distance Objects
Analogous principles can be used to allow near or intermediate distance objects to be viewed. In this case, the full correction for the viewing distance is added to the objective lens or an increased correction is added to the viewing distance to the eyepiece. The telescope can also be focused by changing its length. Changing the length of the telescope by increasing the separation of the objective and eyepiece is often used to adapt a telescope for finite (near or intermediate) working distances. The only restriction is the need to have a tube length of a reasonable size. Astronomical telescopes generally allow a greater range of focus than Galilean ones.
Focusing for near or intermediate distances by changing the telescope length can be considered equivalent to ‘borrowing’ some of the objective lens power. Consider Galilean and astronomical telescopes with objective and eyepiece powers of 15 D and 45 D (−45 D for the Galilean telescope) respectively. They both have magnification of 45/15 = 3×, namely, +3× for the Galilean telescope and −3× for the astronomical telescope. Using Equation 12.11 and the fact that the focal length in metres is the reciprocal of the power in dioptres gives
ta = fa.O |
+ fa.E = 1/15 + 1/45 = 0.067 + 0.022 = 0.089 m or 8.9 cm |
tg = fg.O |
+ fg.E = 1/15 − 1/45 = 0.067 − 0.022 = 0.045 m or 4.5 cm |
When the telescopes are focused on an object at a distance of 50 cm (0.5 m), the vergence of the light reaching the objective lens is 1/0.5 = 2 D. This requires a power of −2 D to neutralise it, which could be notionally ‘borrowed’ from the objective lens. Then the power of the objective lens of both telescopes becomes equal to 15 − 2 = 13 D. The magnification is now 45/13 = 3.46×, namely, +3.46× for the Galilean telescope and −3.46× for the astronomical telescope. Using Equation 12.11, the lengths of the two telescopes are now increased to
ta = 1/13 + 1/45 = 0.077 + 0.022 = 0.099 m or 9.9 cm tg = 1/13 − 1/45 = 0.077 − 0.022 = 0.055 m or 5.5 cm
Therefore, in both cases the increase in length is 1 cm.
More generally, if an object is viewed at x m, then the power of the objective lens needs to be reduced by (−1/x) D. Therefore, using Equation 12.12, the magnification
is increased to |
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M = |
−FE |
(12.13) |
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FO − 1/x
As x increases, 1/x tends to zero, giving the formula at Equation 12.12 for the magnification of an object at ‘infinity’. For small values of x, the reduction in the
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power of the objective lens and, consequently, the increase in magnification are significant. Since the focal length, fO of the objective lens is equal to the inverse of the power FO, Equation 12.13 shows that an object cannot be seen with the telescope at a distance closer than the focal length of its objective lens.
The lengths of the telescopes are now given by
ta = fg.O + fa.E |
(12.14a) |
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tg = fg.O + fg.E |
(12.14b) |
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where |
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fO = |
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1 |
(12.14c) |
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FO |
+ 1/x |
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A telescope can also be modified to view near objectives by adding a plus-lens reading cap to make a telemicroscope. The magnification obtained is then the product of the magnification of the telescope and that of the reading cap, so that using Equations 12.6 and 12.12 the total magnification is given by
Mtotal = Mtelescope × Mreading cap |
(12.15) |
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= |
−FT.E FRC |
(12.16) |
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FT.O 4 |
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where the subscripts ‘T’ and ‘RC’ refer to the telescope and reading cap respectively.
12.3.4.2 Telescopes for Mobility
The telescopes described in the above sections are used for sedentary or ‘spotting’ tasks, as they have a very limited field of view. However, a telescopic device that could be mounted in front of the eye and used to magnify at long distances would be useful for mobility. The approaches used include bioptic and contact lens telescopes.
In bioptic telescopes, the user’s prescription lens is a carrier lens with the compact telescope mounted in its upper part. This allows the user to look through the prescription lens as usual and obtain a magnified view of distant objects by lowering their head. There are both lower powered (about 2×) devices, as well as systems with a wider field of view (Bailey 1982). Careful fitting and intensive and structured training are required for successful and safe use (Feinbloom 1977; Kelleher 1979). However, although bioptic devices are used to support mobility and some states in the USA allow driving using them, they are intended for occasional rather than constant use.
A contact lens telescope uses a negative contact lens as the eyepiece and a positive spectacle lens as the objective of a Galilean telescope (Bettman and McNair 1939). The length of the telescope is equal to the separation of the lenses. This is the only type of magnifying system that would probably be allowed to be used for driving in the UK. Contact lens telescopes have limited magnification and they have been found to be particularly useful to people with congenital nystagmus, who often show a much greater than expected improvement in visual acuity.