Image capture and processing challenges--and solutions--in portable designs--Part I

Camera Modules
Solid-state imaging can be grouped into a few broad categories. At the low end, there are the sensors used in optical mice and photocopying machines. Higher up the spectrum are the imagers found in cell phones and basic digital still cameras and camcorders. Next come professional quality picture and image capture devices and, finally, imagers for scientific instruments. Presented graphically (See Figure 1), it can be seen that device resolution spans eight orders of magnitude and price by about four, with cell phone imagers occupying a unique position in terms of the price per pixel.

The design and manufacture of camera modules for portable electronics products is extremely challenging. The more pervasive factors include extremely high production volume, severe price erosion, restricted height and ever-increasing performance and functionality specifications. Despite this, owing to the widespread placement of camera modules on cell phones, the size of this market comprehensively dominates all others: more than 2 million camera modules are manufactured each day purely for this platform.

Solid State Camera Modules
Solid-state imaging began with the invention of the charge-coupled device (CCD) in 1969 at the Semiconductor Components Division of Bell Laboratories. The first portable digital still camera (DSC) using a CCD sensor was demonstrated by engineers at Eastman Kodak in 1985. An alternative imaging technology based on complementary metal-oxide semiconductors (CMOS) emerged in the 1970s out of work at the National Aeronautics and Space Administration Jet Propulsion Laboratory. This effort eventually led to the development of CMOS active-pixel sensors. CMOS imagers are able to furnish many on-chip functions, allowing for reduced size, lower power consumption and simplified assembly. Consequently, CMOS now dominates solid state imager technology except for niche applications where optical performance or imager resolution is paramount. Today, image sensor die are manufactured by many semiconductor companies. The smallest standardized area imager is the quarter common intermediate format (QCIF), with a resolution of 25,344 pixels, while the largest commercially available imager has 111M pixels.

A camera module has three main components: a solid state image sensor that converts photons to electrons; a lens train that comprises all the optical parts of the camera; and a housing that holds the lens train in the correct physical location with respect to the imager. A cut-away drawing of a camera module is given in Figure 2.

The lens train also has three key components: the lenses that focus the scene onto the imager, several apertures and an infrared filter. There can be as few as one lens on a QCIF camera module, or as many as four on one camera module with mega pixel resolution. Usually, the lenses are plastic for reasons of cost, but higher quality cameras sometimes use glass for the first lens because of its superior optical properties. The optical train inevitably contains at least one stop to limit the field of view and, frequently, several apertures to control optical aberrations and set the exposure speed (F-number). These are simply opaque films with a clear central region that are applied to lens surfaces. For normal applications, the lens train will always contain an infrared filter because silicon imagers are sensitive to longer wavelengths than the human eye can perceive.

The lens train is mounted in a sub-assembly called the lens turret that screws into the lens housing. The lens housing holds the lens train in approximately the right location with respect to the imager and adjustment of the screw thread of the lens turret is used to set the focus.

Market and Trends
Solid-state camera modules remained a specialist product until 2001, when a common intermediate format (CIF) camera debuted on a cell phone. Within eight years, the number of image sensors produced annually for hand-held platforms rose from thousands to approximately 1 billion. It is estimated that in 2008, more than 80 percent of all cell phones will have at least one camera, many having two, with market saturation eventually occurring at around 1.5 billion units per annum (Techno Systems Research Co. Ltd, November 2007). Other applications that use cell phone cameras include low-end DSC, automotive driver aids, web cams and toys, which together could result in an additional 1 billion camera modules per annum by 2015.

Consumers appreciate cell phones with cameras because the camera is integrated and hence they automatically carry it with them most of the time. However, cell phone cameras are unable to deliver the same quality of pictures as a DSC with the equivalent number of pixels. Consumers are increasingly recognizing this performance gap and that raw pixel count does not relate to image quality. The trend seems to be that future cell phones will have a Video Graphics Array (VGA) resolution camera for video conferencing and a higher resolution camera in the 5 to 10 Mpixel range for photography.

Given the potential ability of a mega-pixel camera to obtain good quality images, consumers are demanding DSC-like quality and more DSC-like features on camera phones. Some of the most sought-after characteristics include:

• Low light sensitivity, especially the ability to take photographs indoors without flash. Flash photography is very power hungry in a product where battery life is often a major consideration for the consumer. Flash photography can also introduce undesirable image artifacts, such as red-eye, that then need correction.
• Focus, image stabilization and optical zoom. Clearly, photographs need to be in focus. Image stabilization is, in many ways, analogous to focus because it is also image blurring, but in a lateral direction. Because the distance of the object to the camera varies, what is required is the means of either adjusting the focus to suit or extending the depth of field. Optical zoom allows the user to get closer to the subject of the photograph and details of the scene to be magnified. Digital zoom degrades image quality, so is undesirable if the captured quality is already low.
• Higher resolution. Although higher resolution does not directly translate into higher picture quality, acquisition of additional information by a higher resolution imager facilitates effective image enhancement by software.
• Size reduction. The height of camera modules is one factor limiting the thinness of cell phones where the current fashion is for extreme thinness. Camera modules are typically around 5mm high, but would ideally be less than 1.5mm tall.
• Cost reduction. Camera modules and the associated image processor are relatively expensive components and contribute to the overall handset price. This is especially true for cell phones that have two cameras, where the cost of the cameras is around 12 percent of the total handset bill of materials [Semiconductor Insights, 2008]. A long-term goal of the industry is the $1 VGA camera module.

The consumer demand for camera phones is high-quality image capture under a wide range of conditions, accomplished at a single button push. Unlike a DSC, the most desirable "feature" of a camera phone is fully automatic operation of the camera with no menus or settings to navigate. Delivering that in a highly compact package translates to system design challenges where cost is the ultimate arbitrator.