We present two miniature all plastic megapixel panomorph lenses for consumer electronics (total track length (TTL) of 6.56 mm) and mobile devices (TTL of 3.80 mm) showing the unique challenges from specification, design, manufacturing and testing phases of these new generation of miniature 180° FoV wide-angle lenses.
In 2014, miniature camera modules are applied to a variety of applications such as webcam, mobile phone, automotive, endoscope, tablets, portable computers and many other products. Mobile phone cameras are probably one of the most challenging parts due to the need for smaller and smaller total track length (TTL) and optimized embedded image processing algorithms. As the technology is developing, higher resolution and higher image quality, new capabilities are required to fulfil the market needs. Consequently, the lens system becomes more complex and requires more optical elements and/or new optical elements. What is the limit? How small an injection molded lens can be? We will discuss those questions by comparing two wide angle lenses for consumer electronic market. The first lens is a 6.56 mm (TTL) panoramic (180° FOV) lens built in 2012. The second is a more recent (2014) panoramic lens (180° FOV) with a TTL of 3.80 mm for mobile phone camera. Both optics are panomorph lenses used with megapixel sensors. Between 2012 and 2014, the development in design and plastic injection molding allowed a reduction of the TTL by more than 40%. This TTL reduction has been achieved by pushing the lens design to the extreme (edge/central air and material thicknesses as well as lens shape). This was also possible due to a better control of the injection molding process and material (low birefringence, haze and thermal stability). These aspects will be presented and discussed. During the next few years, we don’t know if new material will come or new process but we will still need innovative people and industries to push again the limits.
Almost every aspect concerning the design of modern panoramic lenses brings new challenges to optical designers.
Examples of these include ray tracing programs having problems finding the entrance pupil which is moving through the
field of view, production particularities due to the shape of the front lenses, ways of tolerancing these systems having
strong distortion, particular setups required for their characterization and calibration, and algorithms to properly analyze
and make use of the obtained images. To better understand these modern panoramic lenses, the Optical Engineering
Research Laboratory at Laval University has been doing research on them during the past few years. The most
significant results are being presented in this paper.
Controlled distortion, as in commercial panomorph lenses (Immervision), is used to image a specific part of the object
with more pixels than in a normal fisheye lens. This idea is even more useful when a zone of interest vary in time with
dynamically adjustable distortion as in a panoramic locally magnifying imager. Another axis of research is the use of
modern computational techniques such as wavefront coding in wide-angle imaging systems. The particularities of such
techniques when the field of view is large or with anamorphic imagers are considered. Presentation of a novel circular
test bench in our laboratories, required to calibrate and check the image quality of wide-angle imaging system, follows.
Another presented setup uses a laser and diffractive optical elements to compactly calibrate wide-angle lenses. Then, a
discussion of the uniqueness in tolerancing these lenses, especially the front elements due to the large ratio between lens
diameter and entrance pupil diameter, is included. Lastly, particularities with polarization imaging and experiments of
triangle orientation detection tests before and after unwrapping the distorted images are briefly discussed.
We present a lens with a constant total field of view and real-time variable resolution in certain zones
of interest. This smart imaging lens uses an active optical element to modify as desired the local
distortion. This way, while keeping the total field of view constant, the resolution can be increased in a
zone of interest, at the expense of decreasing it somewhere in the remaining part of the field of view.
We first present the concept of this lens, using a deformable mirror as the active surface. Computer
simulations are done with Zemax in which a magnifying power of 2 in a zone of interest representing
10% of the full field of view is achieved, using a f=12.5 mm lens and a F/# of 18. Different
combinations of theses parameters would allow different performances and results. We then present
experimental results of this lens with a prototype built using a ferrofluidic deformable mirror as the
active element. Experimental results of a zone of increased resolution with a magnification of 1.32 and
a zone of decreased resolution with a magnification of 0.80 are obtained.
Panoramic imaging is of growing importance in many applications around the world spurred by the development of
digital imaging. Panoramic lens characteristics are unique and their careful characterization can be a challenge. For
example, the price to pay for a large field of view in this type of lens is high distortion in the image. For vision
applications like security or inspection, a precise knowledge of the distortion introduced by panoramic lenses is essential
to produce natural unwrapped views to the operator. Of special concern is the image quality which must be uniformed
over the entire field of view because all directions are equally important. In addition, two hemispheric images can also
be stitched together to create a complete spherical image. For these reasons, we have developed a dedicated setup to
study the distortion and the image quality produced by panoramic lenses. The test setup is made of a 75-cm radius
cylindrical structure with targets placed on it. Using referenced equally-spaced targets, we obtained the radial image
mapping curves for various azymuthal angles, allowing us to calculate the full-field resolution map. Also, transition
targets were used to find field-dependent spatial frequency where the MTF is 50%. We tested four different panoramic
lenses, two panomorph lenses and two fisheyes. For each lens, we discussed the experimental resolution and MTF
curves and compared some of those results to theoretical design data.
Tolerancing a lens is a basic procedure in lens design. It consists in first defining an appropriate set of tolerances for the
lens, then in adding compensators with their allowable ranges and finally in selecting an appropriate quality criterion
(MTF, RMS spot size, wavefront error, boresight error...) for the given application. The procedure is straightforward
for standard optical systems. However, it becomes more complex when tolerancing very wide angle lenses (larger than
150 degrees). With a large field of view, issues such as severe off-axis pupil shift, considerable distortion and low
relative illumination must be addressed. The pupil shift affects the raytrace as some rays can no longer be traced
properly. For high resolution imagers, particularly for robotic and security applications, the image footprint is most
critical in order to limit or avoid complex calibration procedures. We studied various wide angle lenses and concluded
that most of the distortion comes from the front surface of the lens. Consequently, any variation of the front surface will
greatly affect the image footprint. In this paper, we study the effects on the image footprint of slightly modifying the
front surface of four different lenses: a simple double-gauss for comparison, a fisheye lens, a catadioptric system
(omnidirectional lens) and a Panomorph lens. We also present a method to analyze variations of the image footprint. Our
analysis shows that for wide angle lenses, on which the entrance pupil is much smaller than the front surface,
irregularities (amplitude, slope and location) are critical on both aspherical and spherical front surfaces to predict the
image footprint variation for high resolution cameras. Finally, we present how the entrance pupil varies (location, size)
with the field of view for these optical systems.
We present the research status of a deformable mirror made of a magnetic liquid whose surface is actuated by a
triangular array of small current carrying coils. We demonstrate that the mirror can correct a 11 μm low order aberrated
wavefront to a residual RMS wavefront error 0.05 μm. Recent developments show that these deformable mirrors can
reach a frequency response of several hundred hertz. A new method for linearizing the response of these mirrors is also