Magnifiers and Other Optical Devices
It is also common for users to report difficulty in
following lines of text due to the reduced area of the unit and
it is advisable to use some form of line guide, such as a typograph,
to assist. To assist with reading lines of text a number of manufacturers
have produced magnifiers with built in line guides.
These take the form of simple bar magnifiers with
engraved red line guides, dome magnifiers with part frosted bases
(so called Visual Tracking Magnifiers), magnifiers with additional
bar magnifiers built into the handles and carefully designed lighting
systems that produce a bright line of light within the magnifier's
field of view.
Magnifiers will also magnify any slight hand tremor
or movement and it is important that the units are supported firmly
when being used. The use of a rigid table or clipboard is essential
as the magnification increases. One final family of magnifiers use
a theory that results in a very thin sheet magnifier.
By moulding concentric prismatic ridges on the surface
of a sheet of plastic, Fresnel magnifiers can be made to enable
areas as large as A4 sheets. However there is a penalty to pay fresnel
magnifiers are of a relatively low power and there is some image
degradation caused by the design. A similar theory is used for the
TV screen magnifier.
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