Advanced Imaging


Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

Megapixel Madness: Myth or Reality?

Image still suffers without quality optics and display
Figure 1
© Tessera, Inc.
Figure 1: Cut-away drawing though a camera module assembled using COB processes.
Figures 2a, b & c
© Tessera, Inc.
Figure 2a, b, c: Formation of a wafer-level cavity package. Left: the device wafer containing five die. Center: application of the seal material to form a picture frame around the perimeter of each die. Right: attachment of a lid material to seal the cavity over each die. Singulation frees the packaged die from the wafer, as shown in Figure 6.
figure 3
© Tessera, Inc.
Figure 3: Image sensor die with exposed bond pads on all four sides and provided with a glass cover, fabricated using a wafer-level process (Shellcase® CF).
Figure 4
© Tessera, Inc.
Figure 4: Image sensor packaged at the wafer level with the light-sensitive face protected by a glass wafer and a BGA interface for attaching the component to a PCB by soldering.
Table 1

Table 1: Standard format and resolution of conventional solid state imagers. Source: Video Electronics Standards Association. Click here to view larger image.

Figure 5
© Tessera, Inc.
Figure 5: Cell phone cameras with conventional optics can only focus over a limited distance (left). By using a special lens, proprietary algorithms and some surplus pixels, most objects can simultaneously be in focus (right).
Figure 6
© Tessera, Inc.
Figure 6: Conventional camera module and a next-generation product where both the imager packaging and lens stack assembly are done at the wafer level.

By Giles Humpston, Tessera, Inc.

For most consumers, when it comes to choosing a new digital still camera (DSC) or a camera phone, a simple relationship applies: more megapixels equal better picture quality equals higher price.

The demand for more pixels is clearly evident, as shown by the growth in imager resolution for DSCs and cell phones over the last few years. Both curves exhibit a rising trend. The question is, when will they level off? There are two likely end points. One is at 25 megapixels. At this resolution, the imager should be able to record the same detail as 35mm photographic film. This has provided sufficient quality for 99 percent of picture-taking applications, and probably is still adequate for most consumers. Professional still cameras with 39 megapixels already exist, while many high-end consumer-grade cameras boast 10-plus megapixels. A second possible asymptote is around 120 megapixels. This is the resolution of the human eye in the center of its focus. Clearly, there is not much point in having a camera that can record more detail of a scene than can be seen while actually there. Imagers with 111 megapixel sensors are manufactured commercially for scientific instruments, although they are too large and too expensive for use in mass-produced portable electronics products.

The relationship between megapixels, perceived image quality and price can be traced back to when the first solid-state imagers were introduced to the market. Moving from a common intermediate format (CIF) resolution camera to a video graphics array (VGA) format provides three times the number of pixels and a very noticeable improvement in picture quality. As seen in Table 1, a CIF format camera has very few pixels, which are sufficiently small in number as to be individually discernable in the image. A VGA camera module has just enough pixels for the image to appear to be analog under most circumstances. The same generally cannot be said when exchanging a 1.3 megapixel camera for one branded at 5 megapixels.

The premise that high numbers of megapixels are required to obtain a high-quality image is a myth, as it ignores two very important aspects, namely the camera optics and the screen on which the electronic picture is viewed.

Camera Optics

To transform a solid state imager into a camera, the semiconductor device must be married to an optical train. For reasons of cost, many of the lenses used with DSCs and cell phones are made of optical polymers so they may be formed by injection moulding. These materials tend to be inferior to glass, particularly in terms of their transmittance and color purity, which further degrades the image quality. It is worth noting that the stunning pictures sent back by the Mars Rovers were taken with a 1.0 megapixel imager coupled to an f/20 lens train, the measured distortion of which is less than 0.01 percent across the entire field area. As this demonstrates, it is not really a question of how much boosting the megapixel count can help to increase the image quality of a miniature camera if, at the same time, the materials and form factor of the lens train compromises the optical performance. It is no accident that professional-quality cameras, both film and digital, have large-diameter glass lenses mounted in deep lens bodies.

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