Relationship between Radius of Curvature to its Tangent and Lateral Displacement

Consider a charged particle experiences Lorentz force and its track is bent into a circular arc. For simplicity, we let the magnetic field perpendicular to the path of the particle such that the resulting circular track is on the plane perpendicular to the viewer or camera. 

Point T represents the location where the particle enters the magnetic field and its tangent, D represents the linear path of the particle if field is absent.

When the particle exit the magnetic field at point T', it would have a lateral perpendicular displacement, x from the tangent, D. Experimentally, it is possible to determine both D and x through open source image measuring applications if calibration is done properly.

The relationship between D, x, and radius of curvature, R, is as follow;