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Method Of Using The Slit Lamp
Apr 20, 2017

<Note>:

1. The slit lamp offers a variety of illuminating and observing methods which do not all have to be used in routine examination, but the ability to perform them should be practiced. Not all slit lamps will allow all methods to be performed.

2. A good routine is one that you feel comfortable with and that allows you to detect and investigate any irregularities and note the normal condition (base-line data) of the eye.

3. The setting-up and use of each kind of illumination must be practiced until the proceedings become automatic.


<Diffuse illumination>:

This is a good method of observing the eye and adnexa in general. Diffusers are sometimes supplied with slit lamp and can be 'flipped-over' the illuminating system. Diffusers are generally ground glass plates that cover the light source, if these are not available a piece of tissue might suffice to diffuse the light although this can sometimes be precarious as the tissue often falls off! The slit should be opened wide and the magnification should be set as low as possible to enable a large field of view. 


<Retro-illumination>:

This is another form of indirect viewing. The light is reflected off the deeper structures, such as the iris or retina, while the microscope is focused to study the more anterior structures in the reflected light. The most typical use is to study the cornea in light reflected from the iris, or the lens in light reflected from the retina. Features that are opaque to light appear dark against a light background (e.g. scars, pigment, and vessels containing blood). Features that scatter light appear lighter than the background (e.g. oedema of the epithelium, corneal precipitates). Note that this method is useful for examining the size and density of opacities, but not their location. 


<Indirect illumination>:

This simply means looking at tissue outside the area which is directly illuminated and can be used in conjunction with most of the above techniques. When using the slit lamp direct and indirect illumination are viewed simultaneously, structures viewed in the illuminated field are seen under direct illumination, but as this does not fill the whole of the field of view, anything which reflects or scatters light from outside the illuminated area is being viewed by indirect.


<Specular Reflection:>

This type of viewing is achieved by positioning the beam of light and microscope such that the angle of incidence is equal to the angle of refection. The light can be reflected from either the anterior (i.e. tears & epithelium) or posterior (i.e. endothelium) corneal surface. Note that the reflected light should pass through only one eyepiece, and therefore this method is monocular.


<Oscillatory Illumination>:

A beam of light is rocked back and forth by moving the illuminating arm or rotating the prism or mirror. Occasional aqueous floaters are easier to observe. Can also be used to determine the extent of opacities in the crystalline lens.


<Tangential Illumination>:

The iris is examined under very oblique illumination while the microscope is aligned directly in front of the eye. Useful for examining tumours and naevi of the iris.


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