Visual Impairment 3- Near LVAs Flashcards
Where should an object be placed for parallel lights to emerge?
Anterior focal length of lens
What is the definition of a magnifier?
A plus lns allowing px to obtain increased retinal image size by holding objects closer to eye without extra demands on accommodation
How would a hand held mag be calculated?
Mag= Feq/4
What happens if the is not at anterior focal point?
Distance closer= not parallel, need reading add
Dist further= rays converging, image blurred
Why does dist between object and mag matter?
If there’s no dist= magnification is minimum
Dist increases= magnification increases
Why does dist between mag and pxs eye matter?
Effects the FOV
Relative distance mag needs to decrease as mag and eye dist increases , otherwise no effect on mag
How can we maximise the FOV and factors would reduce the FOV?
Increase FOV= Large diameter
Decrease FOV= aberrations and high powers
What type of lens may give a increase d field of view with minimal peripheral aberrations?
Aspheric lenses
Whar is the linear field calculation?
FOV= D/ d (dist b/w mag and cornea) x Feq
How is FOV increased in a hand held mag?
Move closer to lens
What image is created at the focal point?
Virtual imagev
What is required to help magnify an image if the object is closer than focal length?
Near add
When is illumination added to a mag?
If the power increases= the lens diameter decreases= the focal length decreases—– illumination added
How is a clear image produced in a stand magnifier news why?
The focal length is less than the object dist to len
Better quality image and less aberrations
What are the 2 components to a stand magnifier?
Magnifier
Reading add lens
What is the calculation to the 2 component system?
Feq= FM + FA - zFM . FA
What are the advantages of using a stand magnifier?
Accurate WD
Hands free
Internal illumination
Prolog reading
What are the disadvantages to a stand magnifier?
Special reading specs required
Prevents access
Flat surface
More bulky
When will the FOV be effected?
FOV will reduce as px moves AWAY from the lens
What mag calculation is used in a stand magnifier?
Mag= Fequiv/4
What ways can the mag be increased in a stand mag?
Increase reading add
Decrease separation between Reading add and magn
What is the starting point reading add for a spectacle microscope to become considered an LVA and why?
+4.00
Why?… standard add as 1/4= 25cm
What are the different types of spectacle magnifier?
SV readers Hyperoculars (12x) Prismatic half eyes (+12.00D) Bifocal units Clip to lenses (4x) Head band magnifier (3x)
What is the mag calculation used for a spectacle mag?
Mag= Fnew/4
What are the advantages of a spectacle microscope?
Hands free
FOV max as lens close to eye
Similar to conventional specs
What are the disadvantages of spectacle microscope?
Reducing WD- writing/sewing Postural discomfort Illumination difficult Convergence demands More head movement
What 3 categories are spectacle mags subdivided into?
High read additions
Simple spec mag
Compound spec mag
What powers are typically found in high reading adds specs?
> +3.00-
What mags are typically seen in a simple spec mag?
2x- 10x
When would a compound spectacle magnifier be useful?
When >10x mag required
What does a compound spec look like?
2 lenses cemented together
Can be mistaken for telescope
What is the problem with binocular spec magnifiers?
Close WD demand too much convergence
If the convergence demand is too much, what rule of thumb is used?
Every 1D in reading add= 1 prismD added to each eye
At what power and mag does it become difficult to achieve good BV?
> +8.00 (2x)
What happens to one of the lenses in spec mag if only one lens out of the 2 is being used?
A balance, frosted, chavasse or semi opaque tape is used over the lens
Which way does a bar magnifier magnify an image?
Vz not hz
Which way does a flat field magnifier magnify an image?
All meridians
What are the advantages of a bar/flat field magnifier
Self support
Inconspicuous
Light gathering properties
Binocular viewing and normal WD
What are the disadvantages of a bar/flat field magnifier
No access underneath
Flat surface
What do manufacturers give magnification based on?
F/4+1
What are the advantages of a hand magnifier?
Discrete Light Easy to use Inexpensive Convenience Portable and compact Used c dist rx Internal illumination
What are the disadvantages of a hand magnifier?
Handling- occupy a hand and steady hand required Restrictive field of view Can case shadows Become tired Shorter/spotting tasks
