LOW VISION Telescopes
Freedom Eye Technology Ltd http://www.freedom-eye.com | date:2016-11-01  | Source: www.freedom-eye.com 

LOW VISION Telescopes

Definition

Low-vision devices are designed to improve visual performance in children with low vision, thus enabling academic and social adaptation and providing enrichment of daily experiences. They can be optical or optical and electronic.

Types of Devices Optical

When conventional lenses do not provide required visual range, aids that have optical properties capable of promoting better visual performance through lenses are indicated.

Types of optical aids for

  • Distance
  • Intermediate distance
  • Near

Optical Aids for Distance and Intermediate Distance

The telescopic system (TS) or telescope is an optical instrument that improves the resolution of an object by increasing the size of the image projected on the retina, making it closer. . It is available for far, near, and middle distances


Figure 1. Telescope for distant view.


A telescope enables greater participation in daily and social activities such as watching television and reading white boards, street signs, house and building numbers, billboards, and menu boards. On the other hand, restriction of visual field and illumination, difficulty in locating and focusing on objects quickly, and limited focus depth are disadvantages of the telescopes. Telescopes are not universally accepted due to expense, difficulty in using the devices, and asthetic considerations. Good coordination and training are essential.


Types of Telescopes

  • Galilean or Keplerian
  • Hand-held, spectacle-mounted, or clip-on
  • Monocular or binocular
  • Fixed focus, focusable telescope, or autofocus

Figure 2. Telescopes for distance, near, and middle distance.


Galilean Telescope

The Galilean telescope is a simple system consisting of 2 lenses:

  • Objective lens: convex (plus) lens, closest to the object
  • Ocular lens: minus lens, closest to the eye

The distance between the 2 lenses is determined by the difference in their focal lengths. The image produced is real and erect. It's lighter, shorter, and cheaper than the Keplerian type, thus the first-choice prescription for children. It is also the first choice in cases of peripheral field loss; in these cases, the lens order is designed in reverse (plus lens closer to the eye), providing a wider visual field.


Keplerian Telescope

Also known as astronomical or prismatic, the Keplerian telescope is an optical system that uses 2 convex (plus) lenses, the objective lens being of smaller diopter power than the ocular. The distance between the lenses is the sum of their focal lengths. The image is real and inverted, requiring a prism to reverse the image, making it longer and heavier. It produces greater visual field and better optical quality than Galilean type and costs more.

Hand-Held, Spectacle-Mounted, or Clip-On Telescope

A hand-held telescope is simpler, lighter, and cheaper than the Galilean and Keplerian types. It is particularly indicated for short activities and could be a first prescription choice for children (Figure 2).


A spectacle-mounted telescope leaves the hands free. It is useful for prolonged activities and activities that require visualization of details. However, they both weigh and cost more

The clip-on model has the advantages of both: It is lighter than the spectacle-mounted model, however, it can scratch the lenses and reduce the visual field to further distances.


Monocular or Binocular

A monocular telescope is indicated when there is a significant difference in visual acuity (VA) between the 2 eyes. It is more discreet, lighter, and cheaper. It can be used in the dominant or better-seeing eye. Binocular telescope is indicated when there is similar VA in both eyes, with the purpose of increasing the visual field, and for nystagmus. The binocular style both weighs and costs more than monocular (Figure 2)

.

Fixed-Focus, Focusable, or Autofocus Telescope

A fixed-focus telescope is indicated for children with poor motor coordination. Nowadays it is rarely prescribed; a focusable telescope reaches far, near, and intermediate distance and is preferred for and by children. The autofocus telescope both weighs and costs more and it does not constitute the first choice prescription for children.

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