The Cage System
The cage system is the essential optomechnical component at the back port of the microscope. It is responsible for adjusting the focus and the TIRF angle.
First, the two lenses with focal lengths of 25 mm and 100 mm are positioned with a distance of d = f1 + 2*f2. The second lens focusses on the back focal plane of the objective. By this the laser beam is not focussed on the sample but leaves the objective orthogonal with a parallel beam. The width of the beam is given by the widening by the first lens.
The whole system can be moved in parallel to the optical axis by using linear positioning stages, which are mounted at the side of cage and keep the cage in position. By doing this the beam is shifted from the central position (where it also hits the objective at the central position) to the side. Entering the side of the objective leads to refraction inside the objective so that the beam does not leave the objective orthogonal but is tilted with a certain angle. The more the beam is shifted to the side the higher the angle is. Reaching a critical angle total internal reflection occurs. High NA objectives (for instance 100x with NA > 1.4) are necessary to reach these angles.