# Cylindrical Fresnel Lens Focus Problem

How to focus beam from the fresnel lens on a flat surface.

In my case, instead of producing the light beam, I am receiving the light beam. So, the beam comes from the right and then focuses at a focal point.

My problem is that I am using cylindrical fresnel lens

And light beam can come from any direction (360 deg). Therefore the focal point is in the middle of the cylinder.

I am sensing the beam, using photodiode. Since photodiode has a flat surface, if i will simply put it in the middle of the cylinder it will not absorb most of the energy of a beam. Therefore somehow I need to reflect the beam from the focal point in the center of a cylinder to the flat surface, say the bottom of a cylinder, where I could put my photodiode.

How can this be done?

There is no one focal point. It is more like a focal axis. The whole longitudinal centre line of the cylinder are focal points. this means you can line up many diodes from top to bottom as long as they are in the middle of the cylinder.

Because the detector is a flat plane, you can only utilise 180 degrees of the cylinder per diode (light coming from the other 180 degrees will focus light to the rear of the detector), and if you are only using one diode, the height of the cylinder need only be as tall as one diode. Any excess height is useless.

• Could you please clarify why there is no focal POINT. As far as I understand, if the parallel beam strikes the lens (pic. 1) from the right, the beam deviates from each point of the lens such a way that rays meet in particular point (point L on the picture) only. – Herfox Mar 4 '14 at 17:11

The focal point of your lens does not necessarily coincide with the center of the cylinder!

Let's call z the axis of the cylinder and r a radial direction. Since the Fresnel pattern is only along the z direction, the lens will only focus a beam of light coming along r within a plane that contains z-r (and not perpendicularly to z).

The focus point is behind the lens at a distance f from the surface. f is the focal length and is determined by the curvature of the glass and the "riples" in the r-z plane. It is independent from the curvature of the cylinder in the plane perpendicular to z.

Top view:

Cross section:

Now if f is equal to the radius of the cylinder, all radial beams focus to a point on the z-axis. It is great when you want to send beams from different radial directions. If not they will focus a little off the axis in the direction of the beam (r).

If your light beam is large, the focus will be more like a line in the plane perpendicular to z (figure 1)

I hope it helps!