Unless you follow the latest trends and features of digital cameras, there's a good chance Apple's use of a backside-illuminated sensor in the iPhone 4 is the first you've heard of the technology. These sensors have been popping up in digital still cameras and camcorders since Sony rolled out its HDR-XR500V and the HDR-XR520V camcorders featuring the company's own Exmor R sensor in February 2009.
The key benefits are improved speed and dynamic range, as well as better low-light performance with lower noise.
Camera phones generally use a front-illuminated CMOS sensor because they are low power and fast. However, its structure isn't ideal for capturing the best images.
Here's how Sony explains it:
Conventional CMOS sensors have a multi-layered structure that includes a layer of transistors and metal wiring between on-chip lenses and photodiodes. These are used for voltage input and output. As a result of this structure, incident light may bounce back if it strikes metal wiring in the light path, or it may be bent by refraction at the boundary of the insulating film layer. Therefore, the transistor and metal wiring can prevent light (gathered by the on-chip lenses) from reaching the photodiodes efficiently.
When the number of pixels is increased to enhance definition performance, the area per pixel is reduced. If the metal wiring is increased to create a high-speed drive system, the amount of light collected by the photodiodes is reduced, causing further deterioration in the light collection ratio.
In contrast with the conventional CMOS pixel structure, Sony's back-illuminated CMOS sensor is exposed to light from the back of the silicon substrate. The result is a dramatic improvement in photographic performance, including approximately double the sensitivity and a reduction in noise. Sony has succeeded in the development of a CMOS sensor with 1.75µm square pixels, a resolution of 5 megapixels (effective), and a speed of 60 frames per second. Sony has now commenced mass-production of this new chip, the "Exmor R" back-illuminated CMOS sensor.
Judging by the announced specs, it seems Apple may be using a newer version of this Sony sensor (it was originally announced in June 2008) for the iPhone 4's sensor. The good news being Sony's Exmor R-based point-and-shoots do produce some of the best low-light photos and fastest shooting performance.
Now, does this combined with the built-in LED flash mean you can finally drop your point-and-shoot? Not really. While I expect the photo quality, particularly in low-light conditions, to be better than your average camera phone, there's much more that goes into getting a good snapshot than the sensor. The lens is still tiny and likely not high-quality plastic. The 5x digital zoom is nice, but will never replace an optical zoom. And just because the sensor is fast doesn't mean the actual processing of the images will be improved; it could still take forever to launch the camera, save an image, and shoot again, to say nothing of shutter lag.
But for getting that picture of your friend face down on a table in a dimly lit bar, it'll probably be great.
Update: According to an iPhone 4 teardown done by Chipworks, the image sensor was designed by OmniVision.