The use of clear pixels allows small-pixel sensors to capture more of the available light.

Sensor maker Aptina says its Clarity+ system will allow 13MP smartphones that match the performance of 8MP models, as it gave more details of the new technology.

The company uses clear pixels and some advanced image processing to offer improved sensitivity in both low-light and daylight situations.

Sensors using clear pixels have been proposed before (including Sony’s announcement and then retraction of its plans to use clear pixels in its current generation of smartphone sensors), but Aptina says its combination of a new color filter pattern, redesigned microlenses and novel image processing allows its system to make full use of a 2X increase in light capture, compared with existing sensors.

Rather than adding clear pixels into a red, green and blue array, Clarity+ uses clear pixels in the place of green ones. It is then able to calculate green values by subtracting red and blue from the data captured at the clear pixels. The company says it can match the color accuracy of existing sensors.

The additional light capture from the clear pixels offers improved low-light performance but also allows the use of shorter exposures the rest of the time – decreasing the risk of motion-blur. It also promises improved HDR performance by decreasing the amount of time needed to capture multiple frames – decreasing the differences between frames, hence requiring less correction for motion effects.

Aptina says replacing the all green pixels with clear ones is more effective than inserting some clear pixels into a red, green, blue array. It allows full advantage to be taken of the additional light capture without an undue impact on resolution or an increase in color errors.

The company says its color filter pattern avoids the reduced resolution and color errors shown by existing, more elaborate clear pixel patterns. The similarity of its layout to conventional Bayer designs also means it can easily be used with the highly refined noise reduction, sharpening and edge detection algorithms that have been developed over the decades that the Bayer pattern has dominated the industry.

Although no products have yet been announced, the company was able to show us a fully-working (and apparently near-production standard) sample of a Clarity+ chip, which performed very well, when compared with a Bayer version of a similar sensor.

The working sample we were shown showed significant improvements in noise performance in low light.
 Bayer image  Clarity+ image

The technology is built on a combination of a number of hardware and software modifications but the breakthrough steps are the use of the clear channel to reduce noise before and during conversion to RGB, along with the re-introduction of the luminance data from the clear pixels, at the end of the processing pipeline.

“Effectively the color comes from the [calculated] red, green and blue signal and the [brightness] comes from the clear channel,” explains President and CTO, Dr Robert Gove.

The biggest single breakthrough came from re-introducing brightness (luma) information from the clear pixels, late in the processing chain. This represents the most significant change to the image processing pipeline, compared with a conventional sensor design.

This modified processing pipeline could put increased pressure on smartphone processors but the company will offer its own image processor and says it will make it easy for other processor makers to adopt its technologies. “We’ve generated a lot of intellectual property but that IP has no value if it slows the adoption of this technology,” says CEO Philip Carmack.