Cameras in handsets evolving from novelty to DSC performance, despite constraints

Special constraints for phone cameras vs. DSCs
In many ways the camera phone is advancing along a path similar to that of digital still cameras (DSCs). In some ways it isn't. Here are some constraints that mobile camera system makers must heed carefully to ensure their success and which makes the mobile camera's market path a bit different from DSCs, both in pace and direction.

Size
Cell phones are small, some as thin as 8mm, and the camera module must be small to fit in a very small space. Typically, a camera module may be only 8mm x 8mm for a 3 MP camera. This means the sensor die must be tiny. This trend continues reducing sensor and module dimensions. For example, Kodak's latest 5MP can fit into a 8mm x 8mm module whereas previous 5MP generations needed at least 10 x 10mm. CMOS sensors are much smaller than CCD ones and are therefore the preferred choice. DSCs use CCD and CMOS based sensors.

Power consumption
As with most other features on a cell phone, the camera system must consume minimal power to conserve battery life. This power consumption has to be low in both standby and active camera modes. A typical consumption in active mode may be as low as 120 mW. If the unit has a flash, it must not drain too much power. This is why most cell phones have LED based flashes which consume less power than Xenon flashes but also usually only illuminate about a 2-3 meter distance. This is not really strong enough for a bright picture in, for example, a shot of friends in a dimly lit restaurant.

The need for low power consumption is also why cameras in camera phones are CMOS based and not CCD based. How many times have you replaced your DSC alkaline battery cells or even replaced them with higher performance rechargeable Lithium ion cells?

Produce good results in dark conditions
Many people take camera phone pictures at home, in the office and in the evening. This places a requirement of being able to produce good, low noise images in low light conditions -- which CCD sensors are better at doing than CMOS sensors, but CMOS sensors use far less power and are much smaller in size. Most DSCs have the same or more stringent low light performance requirements but because they have less constraining power consumption specifications they can use CCD sensors and xenon flashes which need a large capacitor (which goes against camera phone size constraints) and use more power.

Mechanical robustness
Cell phones must survive a stringent mechanical drop test so the camera module and its sensor must be capable of withstanding strong mechanical shocks. They must pass the famed cell phone drop test that tests in positive and negative directions in the x, y and z dimensions from a height of no less than six feet. When was the last time you dropped your DSC from your shoulder? Did it survive the crash? My cell phone survives several falls a week and its camera still works!

Consumer market supply chain and volume production
Cell phones are amongst the highest volume consumer electronics devices in the industry. Mobile phone cameras must possess a highly resilient supply chain that withstands sudden consumer demand surges for cell phones. A high performance, proven supply chain is critical. An example is that all camera components have a dual supply source for exceptional reliability and can ramp from say a million units a month to say 3 million units per month in as little as three months while maintaining excellent manufacturing consistency and high yields.

The manufacturing consistency and quality also has to be very high in volumes of millions if units per months, much higher than even the highest volume DSCs.


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Figure 1: Camera phone challenges

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