what are low vision optical devices|telescope|low vision aids|low vision definition according to who|Medical world

what are low vision optical devices

Low vision definition according to WHO

According to World Health Organization is defined as A person with low vision, who has impairment of visual functioning even after treatment, and/or standard refractive correction, and has a visual acuity less than 6/18 to light perception or a visual field is less than 10 degrees from the point of foxation.

what are low vision optical devices

low vision is a condition when vision cannot be improved further with the help of traditional spectacle lenses,medication and or surgery. person with low vision often rely on the use of different number of different instruments, called Low vision devices.

low vision devices make thing appear larger, make thing brighter or clearer and even improve contrast

• Enlargement of object
• Contrast Enhancement
• Selective fixation
• Optical magnification
• Electronic Magnification

Principal of magnification at near point

Relative distance magnification

Moving the object closer to eye causing a proportional enlargement of the retinal image. e.g. sitting in front of the television, holding a book close while reading etc.

Relative size magnification

Enlarging the physical size of an object, such as large print rather than standard print. e.g. Large print books, writing large latter etc

Angular magnification

using an optical system such as telescope, magnifiers

Types of optical devices

they are divided into two terms-

• optical devices for distance
• optical devices for near

Optical Aid for Near

hey are divided into 3 terms-

• Hand-held magnifiers
• Stand magnifiers
• Dom and Bar magnifiers
• Spectacle magnifiers

Hand-held magnifiers

• These are handy and easy to use.
• They have to be used at the focal length of the lens
• higher the power havier is the magnifier, more distortion lesser is the field of view
• it can be used along with bifocals or reading glasses

Uses

• Used for near viewing e.g. reading or or any near activity
• The individual can move it around the print or object and will be able to see an elarged image of the target
• One hand is in use.
• Available in different magnification and sizes

Advantages

• Variable eye to lens distance
• Normal reading distance
• Convenient for short term task
• t is readily available and fairly inexpensive
• Portable

Disadvantages

• Decreased field of view
• Tricky and steady handling is required
• One hand is occupied

Instruction to patient with Hand-held magnifier

• Place the magnifier at the top of the paper
• Bring it slowly towards the eyes till the latters are clearest
• Move head to the best position above the magnifier. Try out the comfortable
• Moving the magnifier from left to right while keeping the same distance between page and magnifier
• When the line ends, move back across the line to the beginning.
• Move down one line and start reading again

Stand magnifier

Introduction and uses

• It is an optical low vision devices used for near
• Helpful for school aged children
• Can be used in high magnification
• Image is always sharp focus because of the rigid lens mounting
• Person moves it around the print and can see enlarged image of the print or any object

Advantages

• Device of choice for Tremors, arthritis, constricted fields
• May have its own light source
• Portable

Disadvantages

• Image distortion
• Reduced field of view
• Needs constant head down for readings.
• Requires a flat surface to place reading material
• Requires the use of accommodation or a near prescription

Instruction to patient with Hand-held magnifier

• Place the magnifier at the top of the paper
• Bring it slowly towards the eyes till the latters are clearest
• Move head to the best position above the magnifier. Try out the comfortable
• Moving the magnifier from left to right while keeping the same distance between page and magnifier
• When the line ends, move back across the line to the beginning
• Move down one line and start reading again

Dom and Bar Magnifiers

• Single Solid lens of hemispherical type or flat field type
• It is made of glass or acrylic plastic
• They are plano-convex lenses
• Used to enlarge word on page

Advantages

• They are easy to use
• No habitual reading posture
• Binocular viewing of the image is possible
• The patient does not need to hold the lens
• They are useful to Arthritis

Disadvantages

• These lenses are often large up to 90mm diameter and heavy, since they are often made to glass
• they are only suitable for reading on a firm and flat surface
• Limited level of Magnification available

Spactacle Magnifier

• These are high plus reading lenses
• Both angular magnification and relative distance magnification are used
• These lenses have short focal length
• They are commonly prescribed low vision spectacle aid

• they are saveral design options are available depending upon-
• Mobility requirment
• Power needed
• Field of view

• Designs of spectacle microscopes are-
• Full field microscpe
• Spherical lens design (up to +8.00D
• Aspheric lens design (+10.00 to +48.00D)
• Doublet lens design (2x-20x)
• Half eye microscopes (+5.00-+12.00D)
• Bifocal microscopes (up to +40.00D)

Uses

• Spectacle microscope are used for readings. looking at object from close distance.
• May sometimes be used for writing tasks

Advantages

• They give a more conventional appearance
• They provide a large field of vision
• Allows both hand free
• Good for patient with hand tremors
• Useful for prolonged reading
• Patient can acheive a high degree of success and satisfaction with their use

Disadvantages

• spactacle microscope are more fragile then magnifiers
• Must hold object close to face, limited working distance.
• Limited task performed.
• Discomfort for head positioning needed

Instruction to petient with spactacle microscope

• Sit upright in normal position
• Wear the magnifying glasses while sitting
• hold the book in both hands
• Bring the book nearer slowly till the latters are clear
• Start reading by moving the book slowly from right to left in steady movement keeping the same reading distance
• At the end of the line MOve the book to the right
• Move the book slightly up so that the eyes are at the level of next line-start reading again

Optical Aid Distance

They are divided into two terms-

• Contact lens-Telescope system
• Telescope

Contact lens-Telescope system

A contact lens may be used in a design of a Galilian telescopic system. The ocular or eyepiece is a high powered concave contact lens and the object is a lowered powered convex spectacle lens.

Advantages

• Incresed field of view
• Light in weight
• Conventional appearance

Disadvantages

• Difficult to fit because of a high power features
• limited range of magnification
• Contact lens movement causes apparent field movement

Telescope

• Telescope is an focal system where both incident ray reflected ray are parallel
• An optical instrument used to magnify the apparent size of a distant object

In Low vision rehablitation
• used whenever approach magnification and conventional optical correction is not possible. Refracting type(Positive objective lens)
• Uses angular magnification

Optics of Telescopes

They are divided into two terms-

• Galilian
• Keplarian

Galilian Telescope

• The eyepeice is negative lens and the objective lens is a positive lens
• Resultant image is virtual and errect
• Loss of light reduces the brightness of the image to some extend
• field quality is relatively poor

Keplerian Telescope

• Both the eyepiece and objective lens are positive
• Resultant image is real and inverted, prism are incarporated to erect the image.
• Loss of light is more in this system so there is a greater loss of brightness in this system
• Field quality is relatively good

Types of Telescopes

They are divided into following terms-

Hand-held- Monocular and Binocular

elescope is held by hand in front of the dominant eye to focus and visualize the target. Smaller the power of the telescope larger is the field of view. because they are hand held, they are used for short term distance task.

Spectacle mounted- monocular, binocular

Telescope can be clipped on the spectacle frame. smaller power telescopes are only available

Uses

• Reading black board from a distance in school
• finding and recognizing people or animals
• Watching television
• Watching games

Advantages

• The telescope make objects appear closer
• Elargment can be achieved at a longer working distance then most other forms of equivelent low vision aid
• Some telescope can be focused to give both distance and near enlargement
• It can incarporate the patient prescription or the telescope focus can be adjusted.

Disadvantages

• They require good contrast
• One takes time to locate the next on the blackboard
• The telescope are expensive
• The patient needs extensive training to be able to use it routinely