The Camera Phone Challenge: Competing in a Digital Still Camera World
By Feisal Mosleh, October 2006
The camera phone has given many people the chance to capture those special moments when all they have at hand is their cell phone. However, they are often disappointed by the quality of the image, especially in low light conditions as consumers' taste in digital pictures is based on their experience with mid-range point and shoot Digital Still Cameras (DSC). Many of these are CCD based devices of 3, 4 and 5MP resolutions. Most offer autofocus, optical zoom, a strong flash, and exposure control.
It's reasonable to assume that mobile cameras will have “arrived” once they produce pictures of comparable quality to this perceived norm.
It's not just about higher Megapixels!
Though increased resolution offers the chance for a smoother, less pixelated picture, the overall image quality is determined by several factors including low apparent levels of noise, which goes hand in hand with a good low light performance, well adjusted white balance that results in more appealing, vivid colors. It's not just about gradually increasing resolutions alone.
For example, a higher resolution camera may produce a poorer perceived picture quality because it has a worse dynamic range producing poorer contrast. A lower resolution camera may produce brighter, more pleasing and better contrast photos. Several features will play their part in improving perceived camera phone image quality over the next few years: auto focusing, optical zoom, improved lens quality, better color management, stronger flashes, more sophisticated image processing and of course higher resolutions.
Special Constraints for Phone Cameras vs. DSCs
In many ways the phone camera is advancing along a path similar to that of digital still cameras (DSCs). Yet, 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.
Cell phones are small, and getting smaller, some as thin as 8mm, and thus the camera module must fit in a very small space. Typically, a camera module may be only 8mm x 8mm for a 1.3 MP camera. This means the sensor die must be very tiny. This trend continues reducing sensor and module dimensions. For example, Avago's latest 1.3MP can fit into a 6mm x 6mm module. CMOS sensors are much smaller than CCD ones and are therefore the preferred choice. DSCs use CCD and CMOS based sensors.
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, typically only illuminate about a 2-3 meter distance. This is not generally strong enough for a bright picture in say 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 low light conditions
Many people would like to take camera phone pictures in low light 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 (opposes camera phone size constraints) and use more power.
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 the camera still works!