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