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Spectacle Microscopes
Spectacle Microscopes
Definition
A spectacle mounted +ve lens used to produce magnification forreading or near point tasks.
Usually of greater power than the traditional maximum for a reading add (+3.00DS)
Magnification = Relative Distance Magnification
RDM = reference distance = Fequivalent
old distance 4
A +ve lens as a magnifying aid allows a Px to obtain an increased retinal image size by holding the object closerto the eye without placing extra demands on accommodation.
The object is placed at the anterior focal point of the lens so that parallel light emerges from the device.
f
Relative distance Magnification
Angular Magnification
Magnification (F/4)g 2x 3x 4x 5x 6x 8x
Working Distance
(cm)12.5 8.33 6.25 5 4 3
Most commonly used between 3x – 6x
Can be considered to cover the range between 1.2x – 20x
Spectacle Microscopes can be sub-divided into 3 types:
1. High reading additions
2. Simple Spectacle magnifiers
3. Compound spectacle magnifiers
High reading additions
Defined as adds which result in a working distance less thanthat which the Px would normally have preferred.
Generally binocular.
Usually > +3.00DS but < +10.00DS
Simple Spectacle magnifiers
Monocular Lens
Magnification 2x to 10x
Compound spectacle magnifiers
When > 10x magnification is required
Monocular
Working distance is very critical
Binocular vs Monocular Correction
Binocular vision provides many advantages.
Close working distances required when using spectacle microscopes may simply demand too much convergence.
Base-in prism is often incorporated
1 for each eye per dioptre of add
+10.00DS give 10 in RE & LEi.e. total = 20
Likelihood of achieving good binocular vision:
+6.00 (1.5x) - easy
+8.00 (2x) - difficult
+10.00 (2.5x) - risky
+12.00 (3x) - highly unlikely
Advantages
1. Px has hands free.
2. The field of view is maximal as the lens is close to the eye.
3. Similar cosmetic appearance to conventional spectacles.
Disadvantages
1. Very reduced working distance
• limits writing, sewing etc.• postural discomfort• illumination difficult• convergence demands
2. Have to be removed when walking around.
3. Need to use head movements
Stand Magnifiers
f
Relative distance Magnification
Angular Magnification
Hand Magnifier
M = Fequivalent 4
fh
Magnifier to object distance is < focal length of the lens
Light is divergent when leaving the magnifier
The Px must therefore accommodate to neutralise this divergence
FA
fh
FM
z
Feq = FM + FA - z FM FA
A stand magnifier with a +20.00DS lens has a stand height = 4cms
FM = +20.00DS object distance l = -0.04m
L’ = L + FM = -25.00DS + (+20.00DS) = -5.00DS
L’ is the emerging vergence
L = 1/-0.04 = -25.00DS
L is the vergence of light entering the lens
The image distance l’ = 1/-5.00DS = -0.20m = 20 cm.
If the Px’s eye is directly behind the magnifier (z = 0) then a reading addition of +5.00DS will be required to neutralise the emerging vergence.
However, this is rarely the case.
z typically is = 20cm but may be a little as 5 – 10 cm for high powered magnifiers.
Assuming that z = 20 cm then the new image distance is
z + l’ = -20 + -20 = -40 cm
1/-0.40 = -2.50 DS
Therefore an add of + 2.50DS will be required at this distance.
Magnification
Feq = FM + FA - z FM FAM = Fequivalent
4
When z = 0 FM = +20.00DS & FA = +5.00DS
Feq = +20.00 + 5.00 - 0 = +25.00DS
M = 25 /4 = 6.25x
When z = 0.2m FM = +20.00DS & FA = +2.50DS
Feq = +20.00 + 2.50 - (0.2 x 20.00x 2.50) = +12.50DS
M = 12.5 /4 = 3.1x
- the position of the image created by the magnifier- the way in which the magnifier is used
The magnification is clearly dependent upon:
These factors are rarely taken into consideration by manufacturerswhen marking the device.
There is no standard reading addition or accommodative effort hence each individual situation has to be assessed.
Low-Powered Stand Magnifiers
Characteristics Low powered Stand Mag
Magnification Range
Typical emergent vergence
Lens form & Diameter
Typical linear field of view
Other features
< +16.00DS (4x)
Up to –10.00DS
Spherical up to 75mm
Aspheric up to 100mm
40 mm with z = 25cm
Design usually allows illumination of and access to object plane
Medium/high powered Stand Magnifiers
Characteristics Med/high powered Stand Mag
Magnification Range
Typical emergent vergence
Lens form & Diameter
Typical linear field of view
Other features
+16.00DS to +80.00DS
(4 – 20x)
plano to –4.00DS
Aspheric up to 50mm
Doublet lenses up to 20 mm
100 mm with z close to 0
Usually internally illuminated
Advantages of Stand magnifiers
- accurate working distance- hands free use- can have internal illumination systems
Disadvantages of Stand magnifiers
- Px may require a special pair of reading spectacles- Stand can prevent access to object
Bar & Flat Field Magnifiers
Bar Magnifier
Solid hemi-cylindrical lens which magnifies vertically
Flat Field Magnifier
Solid hemisphere magnifies in all meridians