The need for automated focus
Typically, biometric, medical, and industrial imaging
applications need high-resolution images. This generally means the
need a digital sensor with more than 5MP and a sharply focused image
of the subject. Usually the subject is less than one meter from the
camera.
A camera with a fixed lens produces images that are always a
compromise in focus for a wide range of subject distances. For these
markets, this compromise is simply not good enough.
If a camera has a fixed lens the subject must be moved relative to
the camera to achieve a sharp focus. For example for iris scanning
the person must move forward and backward in front of the camera
until a valid and in-focus image is captured. Another example is
barcode scanning where the operator must move the camera relative to
the barcode until a valid data capture is achieved.
For industrial and medical imaging the optical requirements are
typically a smaller field of view and a shorter depth of focus,
which makes it even harder to position the subject.
So whether it’s imaging a face or iris for biometric ID or sample
scanning for medical diagnostics, manual positioning to achieve
“best focus” is time-consuming, not user-friendly and produces lower
quality images.
When you add precise lens movement to your camera, you can
automatically focus the image with less need to carefully position
the subject. The camera will have higher throughput with the ability
to capture images faster, it will have greater usability, and it
will deliver higher image quality at the same time.
So what does a system design engineer need to know in order to
create a focus camera for these applications? It’s basically a three
step process.
Selecting the image sensor
 The
first step is selecting your image sensor. There have been great
advances in CMOS digital image sensors driven by consumer
applications. This industry produces more than 1 billion sensors per
year which has created opportunities for even more growth in the
“long tail” of non-consumer applications.
Selecting the best image sensor for your application requires
close discussions with leading CMOS sensor suppliers including
Aptina, OmniVision, and SONY Digital. Some of the most important
parameters to consider include:
- Your targets for image resolution and camera size. These
will determine the number of pixels and size of pixels you will
need in the sensor. Today very high quality sensors are
available with more than 5 MP in a 1/3” format.
- Your need for color vs. monochrome imaging.
- Your need for visible or infrared sensing.
- Your lighting conditions. These will determine the pixel
requirements for light sensitivity and dynamic range.
Selecting the optical lens
The second step is selecting the optical lens. Advances in
polymer and glass optics have kept pace with digital image sensors.
Optical engineers today can select from many high quality polymer
materials in addition to traditional glass materials. In general,
molded polymer lenses are used for the highest volume and lowest
cost cameras while not sacrificing quality.
Some of the more important optical parameters you need to
consider include:
- wavelength of light (visible or infrared)
- % transmission
- F#
- back focal length (BFL)
- total track length (TTL)
- point spread function (PSF)
A good optical company can help you design the best possible lens
for your system. For example New Scale often works with Sunex
optical engineers in California (www.optics-online.com),
who help our camera customers source the lens assembly that best
fits their requirements.
Typically using a standard M12 threaded lens and barrel assembly we
can quickly specify a lens that will be compatible with a 1/3”
sensor in less than an 8 mm dia. and total track length less than 7
mm.
Inside this assembly are 4 or 5 optical elements of molded glass or
polymer. The front aperture lens captures the light and a sequence
of carefully aligned lens elements bends this light into a flat
field of focus on the image plane of the digital sensor.
Sunex lens assemblies and example lens diagram (courtesy of Sunex).
The distance of the lens assembly from the focused image plane
changes as the distance of the subject from the camera changes.
Thus, by precisely moving the lens assembly, you can focus the
subject precisely on the image plane.
Selecting the focus solution
 Now that you’ve identified the image sensor and the lens
assembly, you’re ready to add focus to your camera solution. This
can be as simple as adding the M3-F focus module into your camera.
Focus requires precise movement of the lens assembly relative to
the image sensor. Precision movement includes two components:
Micrometer-scale motion of the lens assembly relative to the image
sensor, and precise control of the tip and tilt of the lens as it is
moving. The M3-F focus module is best-in-class in both of these
measures.
The M3-F also offers a number of additional benefits. It provides
the most precise focus in the smallest size for your imaging system.
Compared to voice coil motors (VCMs) the M3-F:
- is smaller
- moves a larger lens
- provides more stroke
- achieves greater precision
- has faster step and settle time
- is more reliable over a wider temperature range and in
high-shock environments
The M3-F performance also exceeds stepper motor solutions in all
categories including size, lens mass, stroke, and precision and
robustness.
| parameter |
M3-F |
VCM |
Stepper Motor |
| Lens mass |
5 g |
0.2 g |
1 g |
| Relative size |
1 |
1.5 |
2 |
| Stroke |
>3 mm |
0.3 mm |
3 mm |
| Resolution |
0.5 µm |
5 µm |
20 µm |
| Repeatability |
5 µm |
20 µm |
40 µm |
| Accuracy |
30 µm |
40 µm |
0 µm |
| Dynamic tilt |
< 0.1 degree |
0.3 degree |
0.2 degree |
| |
|
|
|
The M3-F focus module out-performs voice coil
motors and stepper motors for non-consumer camera applications
Additional system considerations
Your auto focus camera system design is nearly complete. You have
selected the image sensor, the optical lens assembly and the motion
module for your camera system. Integrating these together, you have
one final step: to consider the software and hardware needed for
your image pipeline.
Some image sensors are already integrated with an ISP or image
signal processor. An ISP will include such things as your video or
image analytics, interface with your hardware, and the interface
with the M3-F module. It may also include an autofocus algorithm.
The auto focus algorithm measures the relative “sharpness” of the
image just captured. The typical output is a unit-less number that
indicates the relative focus of some subset of the total image.
Using this value, many options for finding the best focus position
of the lens may be used.
For example, one popular method is a “hill climbing” algorithm.
After each move, a new image is captured and the new sharpness value
compared to the previous sharpness value. After several iterations
the best focus position of the lens is found and maintained by the
M3-F.
All focus algorithms require position commands be sent to the M3-F,
requiring a tight interface and integration between your ISP and the
M3-F module. New Scale partners with image processing specialists
such as D3 Engineering (www.d3engineering.com) who work with
advanced digital signal processors from Texas Instruments. These
DSPs integrate video analytics and image capture analytics, as well
as interfaces directly with the M3-F to provide autofocus capability
in camera solutions.
Camera system from D3 Engineering integrates New
Scale’s M3-F focus module with advanced digital signal processors
from Texas Instruments. These DSPs integrate video analytics and
image capture analytics, as well as interfaces directly with the
M3-F to provide autofocus capability.
Conclusion
Exciting new applications for biometric, medical, and industrial
imaging are emerging every day. These new opportunities are possible
because of the availability of higher-resolution image sensors,
polymer optics, faster microelectronics, advanced software and New
Scale’s M3-F focus module. Contact us for help in creating the
optimum autofocus imaging solution for your product.
Contacts
New Scale Technologies
Victor NY, 14564
Phone +1 (585) 924-4450
email
sales@newscaletech.com
www.newscaletech.com
Sunex
Carlsbad, CA 92010
Phone +1 (760) 597-2966
www.optics-online.com
D3 Engineering
Rochester, NY 14623
+1 (585) 429-1550
www.d3engineering.com
Learn more
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