The focal lenght

The lens (or lens system) should project a sharp image of the mask on the CCD. The focal length follows from the distance lens-mask and the distance lens-CCD, according basic optic formula. If the lens is placed in the middle between the mask and CCD, then the focal length equals 1/4 of the distance between the mask and CCD.

The lens diameter

The ratio diameter/focal length is determined by two parameters: the required image sharpness and the required signal/noise ratio of the video image. A value of 0.03 is adequate if a light source with condensor is used (field lens or Fresnel lens). The sharpness is such that the transient region of a contour has a width of about 100 microns, corresponding with about 8 pixels.
This corresponds with a lens diameter of 50 mm and a distance of 6.6 m. As 'proximity sensor' (where the distance between mask and CCD is in the order of 250 mm) an aperture of 0.3 can be obtained easely. A much simpeler light source can be used (some LEDs with an opaque diffusor). Then an aperture of 0.3 is reached with a 20 mm lens.

Lens type

If the light source is a (monochromatic) Light Emitting Diode (LED), then a single lens is adequate since chromatic abberation does not occur.
For a small aperture of 0.03 the effect of spherical abberation and 'cusion' deformation have not been observed. A simple plano-convex lens is then sufficient. For the larger aperture of the proximity sensor a double-convex lens is recommended.

Lens material

The material should be sufficiently transparant for the light. Stable and homogeneous lenses are often made from BK7 glass.

Lens manufacturers

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