Slit lamp are used in the examination of the anterior segment of the eye . Where they allow the practitioner to perform a careful examination of cornea ,conjunctiva and lids. In modern instrument the microscope and light source are coupled so that the light shines on the same part of the eye that the microscope is focused .
This coupling facilitates examination of the eye . The instrument is called a slit lamp because in the normal mode of operation . The light source produces a thin vertical slit of light at the eye . When this slit of light shines along a different axis to that of the microscope the clinician see a section of living eye.
Main parts of slit lamp Silt lamp microscopes Illumination systems . Mechanical coupling .
Attachments of slit lamp Fundus examination Gonioscopy Tonometry Laser photo coagulation Measuring the visual acuity in patients with hazy media .
Slit lamp microscopes The magnification of slit lamp microscope is usually with in the range of *6 to * 40 . At higher magnifications the small and often uncontrollable movement of the patient’s eye make the image jump around so much that an examination cannot be performed satisfactorily . This range of magnifications and the large working distance mean that compound rather than simple microscope have to be used .
In most basic form composed of microscopes are composed of two optical elements . An objective and an eyepiece .[in the fig shown as two simple positive lenses. In reality the eye piece and occasionally the objective are composed of a series of lenses to reduce aberrations .
Range of magnifications The use of different objectives. The use of different eyepieces. The Littmann- Galilean telescope principle . With a zoom system .
The use of different objectives This is one of the oldest and possibly still the most frequently used technique for obtaining different magnifications . The different objectives are usually placed on a turret type of arrangement that allows them to be fairy rapidly changed during an examination .
The system is usually limited to two sets of objective lenses due to the confinements of space in and around the objective area of slit-lamp.
The use of different eye pieces This technique is usually used in the cheaper slit lamps it is the only means of changing the magnification . It is not a very convenient technique as it requires the practitioner to pull out the current eye pieces and replace them Normally they are kept in a drawer attached to the instrument table .
More than two pairs of eye pieces to be provide with the slit lamp . The technique alone provides a very limited range of magnifications.
The littmann Galilean telescope principle The Galilean magnification changer developed by littmann  It is completely separate optics that sit nearly bw the objective and eye piece lenses and does not require either of them to change . It provides a larger range of magnifications than the other technique .
Typically five via a turrent arrangement which is completely enclosed within the microscope’s body . It is called Galilean system because it utilizes Galilean telescopes to alter the magnification . The Galilean telescopes have two optical components a positive and a negative lens .
The parallel light rays both enters and leaves the system and under goes some degree of magnification which is dependent upon the power and separation of two lens . Galilean telescope fits with in the standard slit lamp microscope along with a rely lens in the manner . By reversing the order of the lenses in the telescope a different magnification can be achieve with out altering any optical elements.
Zoom systems Recently expensive slit-lamps have been produced with a zoom systems that allow a continuously variable degree of magnification . The Nikon instrument contains the zoom system with in the objective of the microscope and offers a range of magnifications from *7 to *35.
Illumination systems The objective of the slit-lamp illumination system is to produce a bright light evenly illuminated ,finely focused ,adjustable slit of light at the eye . Almost all slit –lamp manufactures have adopted the koller illumination system .
Koller illumination system The filament of the bulb is imaged by the condenser lenses at or close to projector lens . The projector lens forms an image of the slit at the eye . The diameter of the projection lens is usually fairly small in size. This has two advantage :- first is keep the aberrations of the lens down ,which results in a better quality image .
Second :- it increases the depth of focus of the slit and thereby produces a better optical section of the eye . While the optics of the condenser are not as critical as that of the other elements of the slit – lamp care must be taken in their design so as not to introduce too much chromatic aberration which will tend to cause fingers at the slit image .
To reduce the aberration is normally achieved by using two or more lenses in the condenser system . The light source used in slit –lamp illumination systems is a tungsten filament bulb . For safety reasons ,they are run on relatively low voltages Slit-lamp can now be obtained with halogen filled bulbs.
The diag slit-lamp It use a optic fiber to convey the light from a high –power halogen bulb . It is housed in a special fan cooled box to the slit lamp .
Advantage Decrease the weight of slit- lamp Allow it to be hand held by the practitioner .
Mechanical coupling of microscope and illumination system Microscope and illumination system slit-lamp are linked together around a common axis or rotation that co insides with their focal plans. Microscope focused where the illumination system is focused . Without changing the focus the microscope and the illumination system can rotated around their axis .
The movement towards or away from the eye and from side to side is usually achieved via a joy stick . Up and down movement is obtained via some screw device .