What is this new back lighting on the new iPhone?

Recently, Apple announced the new iPhone and several of the hardware changes were aimed at the camera. Thanks to the infamous missing iPhone fiasco, we were expecting the 5-megapixel camera and flash, but Jobs announced that the camera was backlit. That stumped us, so we did some research on camera chips.

What is back lighting?

Stop giggling first … Okay … I admit I think they could have come up with a better name. Some of the jokes I saw on Twitter seemed to be especially crude about this feature. Backlighting is a trick for better digital photos by bringing more available light to the place where you measure.

First an overview of digital cameras

The key part of all digital cameras is a chip called a CCD (charge coupled device) that detects light falling on its surface. Light affects the charge on a grid of millions of tiny capacitors created in the silicon when the chip is manufactured (making chips is called fabrication and involves a lot of etching and depositing of thin layers and different materials in precisely controlled ways). For simplicity, think of black and white CCDs, where the matrix of capacitors corresponds to the matrix of pixels that make up a digital image (color sometimes uses a lot more tricks). The chip has an additional circuit that measures the charge of all the capacitors, which is basically how light levels are read from the image. In order to read all of these capacitors, there are tiny wires running across the top. They are not actually wires, but rather a thin layer of aluminum or copper that is effectively sprayed onto the chip and then carefully eaten using acids to leave connection traces, which are practically metal wires attached to the chip.

Why back lighting?

All those wires and other parts of the capacitors are laid on top of a square silicon tile (called a matrix) with the capacitors on the bottom. The circuitry and cables don’t obstruct it too much as it works, but it does block out some light and scatter the surrounding capacitors a bit, reducing the quality of the captured image, especially in limited lighting. Let’s go back to that square silicone tile. Silicon is the main ingredient in normal glass (also known as silicon dioxide), but the material used to make chips is super-pure and transparent silicon crystal. With back lighting the problems with traditional bottom capacitor CCDs are literally backwards and now it illuminates the image on what was the bottom of the silicon tile and lets the light shine through the silicon to the capacitors. This way you avoid all the cables and bring more light to the capacitors.

With this new camera chip, Apple has shown some beautiful images, but the key performance area will be low lighting. Capturing scenes like the last light of a fading sunset. Previous iPhones (Original and 3G) had very basic fixed focus 2 megapixel cameras that struggled in low light. The 3GS brought autofocus and auto white balance which improved performance, but still struggled with those scenes, often distorting colors. Now with the new camera chip, the iPhone should take much improved photos, along with the built-in flash to handle low lighting indoors.