Motorola Moto X camera: Will RGBC “Clear Pixel” technology provide good low-light performance?

After years of racing to be the megapixel king, smartphone vendors have wised up and started paying attention to overall image quality. In particular, low-light performance has (belatedly) received lots of love.

Most famously, Nokia dropped its smartphone camera resolution from 41-megapixels to 8.7-megapixels to allow for much larger 1.4-micron pixels and improved low-light performance – and then later realised that 7:1 supersampling would provide much of the same benefit in the Lumia 1020, while retaining astonishing resolving power.

HTC followed suit with the even more radical Ultrapixel sensor in the HTC One. Featuring massive 4-micron pixels and industry leading low-light performance, the camera only captures images at 4-megapixel resolution.

To provide improved low-light performance for the Moto X, Google decided to take a more adventurous approach. It is attempting to maintain a relatively high resolution of 10-megapixels, while improving low-light performance through an unusual sensor design.

Instead of having all coloured pixels (traditionally two green, one red, and one blue in repeating blocks of four called a Bayer array), one pixel from each block is clear. This “RGBC” design, that Google calls “Clear Pixel,” allows much more light to be captured in low-light situations.

The unspoken drawback of clear pixels

What the press releases and news stories about the technology don’t tell you is what a camera gives up by having 25 per cent of its pixels ultra-sensitive to light and not colour. In bright light, those clear (or as Sony calls them, white) pixels blow out way before the coloured pixels.

That means that the camera has to be set to a lower exposure or a quarter of the image information is lost. If the exposure is kept low, then the colour pixels don’t have very much information, increasing noise. If the exposure time is increased to get more information in the colour channels, then not only are the clear pixels blown out, but blooming can cause image quality issues with surrounding sensor sites. In addition, some of the colour information in the image is lost by removing half of the green pixels.

Aptina believes it has overcome the inherent colour issues with the RGBC technology through clever design and on-board signal processing, while retaining the 3-4 decibel signal-to-noise ratio improvement provided by the clear pixels. It has been rumoured that a version of the Aptina technology is in the Moto X, but Motorola has not confirmed it.

Combining the relatively large 1.4-micron pixels – the Galaxy S4 features 1.1 micron pixels – of the Moto X’s 10-megapixel sensor with Google’s Clear Pixel technology, the Moto X will definitely be a great low-light camera. We’ll have to wait until the product ships to know if Motorola has finally cracked the code for successfully using a clear pixel without sacrificing overall image quality.

Of course, we’ve just learned this morning that the Moto X will be US-only – but Motorola is planning a range of devices coming to Europe “from the same family,” so the cameras on those handsets will hopefully benefit from this tech, too.

For more on the Moto X, see our hands-on preview.

[Image Credit: Stanford SCIEN]