Biomicroscope 1

Slit lamp consists of a:

- Observation unit
- Illumination unit

Consists of :
- Bulb – light source
- Condenser lens – to reduce divergence and keep light in the system
- Slit --- Allows divergent rays to pass through ---- object
- Projection lens – forms image of the slit onto the eye

This system is shown horizontally but in reality, it is vertical

So a mirror is added after the projection lens for the light to change direction onto the
eye

The slit formed onto the eye needs to have certain features:
- Focused, uniformly illuminated beam with sharp edges
- Adjustable brightness, height, width, orientation and tilt
- No chromatic aberrations
Features of a slit lamp:
- Provides optic section of cornea and lens
- Range of magnification for the eye
- Rotation of illumination system about vertical axis

The illumination system and observation system share an isocentre --- they swing about
on the same axis i.e. if illumination system is pointing at point A , then observation
system is also pointing at point A.
Removing isocentricity is done by offsetting the system ---- indirect illumination

Parfocal – when you change the mag you don’t need to change the focus (move the SL) -
--light beam and observation system are focused onto the same plane

In a simple microscope to get a higher mag you would need to move the observation
system closer → NOT THE CASE IN A SLIT LAMP

When observing an eye in a SL converging light rays hit the eye, and diverging light rays
return to the system , hitting the objective lens forming an inverted image ---- the
eyepiece focuses on this inverted image
We don’t want to see an inverted image, we also don’t want the SL to be very long
INTRODUCE PRISMS
- Cuts down the length light needs to travel (shortens microscope)
- Reinverts the image --- image is now upright

4 ways to change magnification in a slit lamp:

1. Change the objective lens –
2. Change the eyepiece ---- higher power eyepiece
3. Zoom system ---- increase/decrease the distance between the objective and
eyepiece lens
4. Littman-Galilean telescope principle
A Littmann- Galilean telescope
Consists of a positively powered objective lens and a negatively powered eyepiece

There is a Litman-Galilean telescope / turret inside a slit lamp

This is separate from the objective lens of the slit lamp

Objective lens MUST be towards the object (cornea etc. ) so that the image remains big,
if it was the other way around the image would be too small
This is the magnification knob you see at the side of a slit lamp
- each lens is opposite to a corresponding lens of the opposite power
- HOWEVER, negative lens will never be infront of the patient, only positive to
ensure the image is big

Birds eye view:

As you can see the Galilean telescope has the positive powered lens towards the patient’s
eye and the negative towards the eyepiece
Fundus examination:
- The Slit lamp itself cannot place a slit onto the retina
- Thus another lens must be put infront of the eye to do this and bring the image
outside
This is done by either 2 different lenses:
1. Highly powered negative lens – Hruby lens
- - 55 D
- Produces erect image of the fundus
- Small FOV around 4mm
Because the objective lens of the SL is positive --- meaning that light is not focussing far
enough back in the eye – the negative lens increased divergence to move the light rays
back
- In this you move the objective lens as close as possible to the patient before using
the Hruby lens

2. Highly powered positive lens

- Either +90D or +78D
- Produces aerial image between the SL and LENS
- Larger FOV around 12mm

- In this you move the objective lens as far as possible from the patient’s eye

- Because the converging light rays are meeting before the cornea --- they are now
diverging rays – allowing the positive lens to focus onto the retina
Seeing the retina from the objective lens is indicated by a red coloured patch of light

Stereoscopes:
- Left eye and right eye receive similar, but slightly different images (shot at
different angles)
- The brain finds the overlapping space between the two images and fuses them
- Depth perception is formed by seeing something from two different angles and
the brain fusing the overlapping images
- Stereoscope prevents both eyes from seeing the image of the other eye

Stereoscopes require a lot of accommodative demand, and thus are equipped with plus
lenses on both eyes. E.g. if image is at 20cm, +5D lens is used

More retinal disparity --- better depth perception

By moving the image back and forth from 20cm , you are training the patient to
accommodate in 3 dimensions --- train the person to converge and diverge
Biomicroscope 2 :
4 main illumination techniques:
1. Diffuse
2. Direct
3. Indirect
4. Retro

Diffuse:
A diffuser is used to illuminate a wide portion of the eye (put infront of the mirror)
Usually used to examine the anterior segment of the eye and the adnexa

Direct illumination:
- Utilises isocentric aspect of illumination and observation system

What you are illuminating is what you are observing

Most common and has several types:
- Conical beam
- Specular reflection – endothelium
- Optic section – lens and cornea
- Parallelepiped
- Tangential
- Broad beam

Optic section:
- Angle between illumination system and observation system is 45 degrees
- Very thin slit width
- Thin slit passes through transparent layers of the eye , reflecting light
- Optic section of the cornea and lens
Parallelepiped:
- Very similar to optic section except the slit is thicker 0.5mm to 2mm
- Gives surface and depth --- 3-dimensional block view of structure being observed
- More information about layers of the structure
Conical beam:
- Very short and narrow ---- 1mm x 1mm
- Angle of 45 degrees
- Focused onto the anterior chamber
- No reflections --- not normal --- indicates something is in the aqueous/anterior
chamber as it should not be reflecting light (something alongside aqueous
humour)

Tangential illumination:
- Illumination system is tangent to what you want to observe
- Angle between illumination system and observation system is 90 degrees
- Elevations and depressions are highlighted --- elevations cast a shadow to provide
depth perception

Specular reflection:
- Uses concept of angle of incidence = angle of reflection
- Endothelial cells
- Angle is 60 degrees
- Viewing down the path of the reflected ray
Indirect illumination:
- You are illuminating a structure next to the one you want to observe
- Point of illumination and point of observation are not the same
- Sclerotic scatter

For sclerotic scatter you don’t need to offset (no longer isocentric)
OFFSET is used when the point you want to observe is not within your field of view of
your microscope

This is known as proximal viewing

Retro-illumination:

- Reflected back light from the iris is hitting the cornea --- cornea is backlit
- Illuminate structure behind area of interest – illuminating area of interest