Selecting the sensor, lens, and lens motion system for biometric, medical, and industrial micro cameras… and putting it all together.

We are all familiar with the consumer digital cameras that are in our pockets, mobile phones, and personal computers. Thanks to incredible advances in microelectronics, CMOS image sensors, and optics, most of us have a very good camera within reach.

Now these tiny cameras are inspiring product engineers in “non-consumer” applications — such as biometric identification, medical and diagnostic devices, and machine vision — to make even greater products. In fact, markets for these new applications are projected to grow faster than consumer camera markets over the next few years.

In this article we discuss sensor and lens requirements, compare the piezoelectric M3-F focus module to voice coil motors and stepper motors for lens motion, and talk about image processing, digital-signal processing, and other system considerations.

The Need for Focus

Typically, biometric, medical, and industrial imaging applications need high-resolution images. This generally means the need for a digital sensor with more than 5 MP 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 always have 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, in 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, greater usability, and 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.

Step 1: Selecting the Image Sensor

Great advances have occurred 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 (Figure 1).  

Figure 1: Image sensors are increasingly used in non-consumer camera applications including military, security and surveillance, machine vision, and medical.

• 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.

Step 2: Selecting the Optical Lens

Advances in polymer and glass optics have kept pace with digital image sensors. Today, optical engineers 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).
• Percent transmission.
• Focal length (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, 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 diameter and total track length less than 7 mm (Figure 2). 

Figure 2: Lens assemblies from Sunex.

Inside this assembly are four or five 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 (Figure 3).  

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.

Figure 3: Example lens assembly. (Credit: Sunex)

Figure 4: The M3-F focus module is a compact piezoelectric motion system that moves a lens assembly relative to an image sensor with micrometer-scale precision and minimal tip and tilt.

Step 3: 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.

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 (Figure 4) from New Scale Technologies is best-in-class in both of these measures. It provides the most precise focus in the smallest size for your imaging system. 

Compared to voice coil motors (VCMs) the M3-F focus module:

• Is smaller.
• Moves a larger lens.
• Provides more stroke.
• Achieves greater precision.
• Has faster step and settle time (Figure 5).

Is more reliable over a wider temperature range and in high-shock environments. 

Figure 5: Step and settle time of the piezoelectric M3-F focus module compared to a voice coil motor.

The M3-F performance also exceeds stepper motor solutions in size, lens mass, stroke, and precision and robustness (Table 1).

Table 1: Comparison of a M3-F piezoelectric focus module, a voice coil motor, and a stepper motor.

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, the last step is 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.  

Figure 6: This camera system from D3 Engineering features the M3-F focus module and advanced digital signal processors (DSPs) from TI. The DSPs integrate video and image capture analytics, an interfaces that provide autofocus capability.

The auto focus algorithm measures the relative “sharpness” of the image 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 to be sent to the M3-F, demanding a tight interface and integration between your ISP and the M3-F module. New Scale partners with image processing specialists, such as D3 Engineering, to aid this process by working with advanced digital signal processors (DSPs) from Texas Instruments. The DSPs integrate video analytics and image capture analytics, as well as interfaces directly with the M3-F to provide autofocus capability in camera solutions (Figure 6).

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 advanced miniature motion systems.

For more information visit www.newscaletech.com.