We designed a lens for an ultra-compact camera system. This lens is catoptric (all reflective), and uses just two curved mirror elements. These mirrors have a shape that is called "freeform", since they are non-rotationally symmetric. Off-axis parabolic (OAP) mirrors are an example of a simple type of freeform shape, but there are much more complex mirror shapes, ones that do not have rotationally-symmetric 'parents', that are also considered freeform. An in-depth discussion of OAP mirrors and their design and alignment can be found in "Demystifying the Off-Axis Parabola," available in our Download section.

The surface shapes used for the mirrors in the present example are actually a bit more involved than simple OAPs. In ZEMAX, these surfaces are called 'Extended Polynomials' and their shapes are defined using up to 230 polynomial aspheric coefficients, which allow the surface shape to differ in the x- and y-directions, and provide a means for defining virtually any shape, including shapes that do not have rotationally-symmetric 'parents'. We show explicitly how to manipulate these 'Extended Polynomial' surfaces by giving step-by-step instructions for designing a Progressive Addition Lens (PAL) in "How to Model the Human Eye in ZEMAX," also available in our Download section.

Advances in aspheric lens machining capabilities in the past few years have made complex freeform lens designs not only physically realizable, but actually quite reasonable to produce, either in single-quantity runs (for prototyping) or in large production quantities.

 

This proprietary lens was designed to produce a sharp image on an OmniVision OV7221 VGA CMOS image sensor. This lens has an 81mm focal length, works at f/3.2, and features near-diffraction-limited performance over the entire field of view (FOV).

Because the imaging system is all-reflective, it offers identical performance over all wavelengths -- from ultraviolet (UV) to long-wave infrared (LWIR). Although most image sensors have a much more limited range of wavelengths, we can adapt this lens to provide simultaneous viewing on two different sensors -- dual-band imaging. For example, a silicon mirror may be inserted just ahead of the focal plane, to provide simultaneous visible and LWIR imaging.

This dual-band imager produces a visible image on a high-resolution CMOS sensor (such as the Silicon Imaging SI-1920HD) and an LWIR image on a GaAs infrared image sensor (such as the FLIR SC4000), simultaneously. The full FOV of this dual-band imaging system is approximately 5 degrees. Simulated images produced with this dual-band imager design are shown below.

The simulated image above, produced on the visible HD CMOS sensor, shows the Space Shuttle Atlantis at 1km target distance. Note the near-perfect image resolution on this full-HD sensor (1920x1080 pixels).

This is a simulated thermographic image (LWIR) produced on the FLIR sensor, showing the same exact space shuttle at 1km target distance. Note that this thermograph lines up precisely with the visible HD image since the sensors are co-aligned through this dual-band imaging system. Our dual-band imager provides precisely-aligned movies of distant objects at visible and LWIR wavebands.

This same freeform two-mirror lens design has also been adapted to a solid version, made of a single piece of machined (or molded) acrylic. The two curved mirror surfaces are coated with silver, for high reflectance across the visible range. For this design, imaging quality is near-diffraction-limited across the visible range (480-680nm). The focal plane array (FPA) is cemented to the plastic lens, thereby minimizing chromatic aberration, which would normally be prevalent in a single-material lens like this.

 

 

 

SUMMARY

We have shown an example of a very compact two-surface reflective imager that uses freeform optical shapes. We have designed other, proprietary systems with up to seven freeform optical surfaces. Contact us to learn more about how Contrast Optical can solve your optical design problems.