3.1.2 Flashcards
Explain the cobalt blue filter on the slit lamp
Maximises the excitation of fluorescein (an excitation filter)
An abraded area would absorb fluorescein and display a fluorescent green area against the blue background
Used to look for corneal abrasion or leaks
Can be used with fluorescein/ fluorescein angiography
Can enhance reflectivity of optic disc Drusen
Explain the use of the wratten 12 (yellow) filter
Select for light emitted by fluorescein ( a barrier filter)
Used in conjunction with a cobalt blue light to improve contrast by cutting out cobalt blue.
Fluorescein glows once in the eye, therefore wratten filter cuts out all the blue
Reduces UV exposure
What is the red free (green) filter used for?
Enhances contrast of blood vessels and haemorrhages
Red lesions easier to see as they appear darker
Used to determine inflammatory cells/red blood cells
(Over the illumination system)
Helps in cup to disc ratio assessment
Helped identify nerve fibre layer (NFL) dropout- a sign of glaucoma
Blocks structures below RPE
What is the neutral density filter used for?
Reduces beam brightness without changing colour
Usually over the observation system
What are the different methods of illumination?
Diffuse - examining the eye and adnexa
Diffuser with flip up system ( spreads out light from illumination system)
Direct- parallelpiped, corneal section, conical beam, van herrick
Indirect- retro illumination, specular reflection, sclerotic scatter
Describe the diffuse illumination system
Allows an overall view of the eye and only allows us to see small aspects of the eye
Or you can open up the aperture but that becomes uncomfortable for the patient
So you can put a diffuser in front of the illumination system the light will be diffused across and the whole eye will be in even illumination
Low mag gives large depth of field
Describe direct illumination
Pointing the light directly at the things we want to observe
Used in most techniques
Not great for fainter targets which might be lost in glare/scatter
Describe direct illumination
Pointing the light directly at the things we want to observe
Used in most techniques
Not great for fainter targets which might be lost in glare/scatter
Describe indirect illumination
Used to view low contrast features
3 types:
1) proximal illumination- caused by scatter from the direct beam
2) retro illumination- light reflected back from the ocular structures which act like a diffuse reflector
3) proximal illumination- from scatter of the light from retro illumination (light is bouncing back from ocular structures)
Eg
A) direct illumination
B) scatter from the direct beam illuminates features around the beam causing “proximal illumination”
C) diffuse reflections from anatomical structures (in this case the iris) illuminate corneal features behind
D) this reflected beam also scatters causing more proximal illumination
Retro illumination of the iris
Retro illumination of the iris uses the iris as the “diffuse reflector” and allows us to illuminate the cornea from behind.
It allows us to look in detail of low contrast features which may be lost in the glare of direct.
It also silhouettes features against a bright background
Direct (light pointed at the object)
Proximal (from direct)- light scattered from the direct beam
Retro- reflected light (from iris)
Proximal (from retro) light scattered from reflected light
