We are using a 4W LED as a light source. LED works as a point source because of that light disperse in all the directions. But we need a slit of colimating light.

So we have designed the optics for the slit lamp apparatus using a optics software. We have run some simulation by putting different focal length lenses in between the light source and the slit dial.

Finally we got desired simulation results by using two 45 mm focal length lenses.

Problem definition for the simulation:

Final simulation results with 45 mm focal length lenses: