Fujifilm has a history of designing (and using) digital camera sensors that diverge from the traditional design used by almost every other digital camera manufacturer.
The sensor used to in digital cameras are actually only capable of detect how much light hits the sensor at each light receptor (pixel). The light hitting the sensor has to be filtered so that different pixels are hit by different colors of light. Some are hit by red, others by green or blue. These values are then used to render a color image.
Bayer Pattern
Most digital cameras use a Bayer pattern in the filter used to produce color information.
This regular pattern works well, most of the time. Problems can occur when photographing something that also has a regular pattern, like a window screen. The two patterns interact with each other to form an artificially pattern, called a moire pattern, in the end image. (Diagonal lines of alternating colors for example.)
Sensors that use this Bayer pattern will use a "low-pass" or "anti-aliasing" filter in front of the sensor to avoid this pattern in the end image. This filter works by slightly blurring the light as it paces though the filter.
This results in slight loss of detail.
X-Trans Filter
This is a color filter developed by Fujifilm to address moire. It replaces the regular pattern found in the Bayer filter with a less regular pattern.
This enables Fujifilm to remove the low pass filter without increasing the risk of moire patterns occurring when using their cameras. This increases the level of detail the camera can capture slightly.
The Latest Patent
The latest sensor patent filed by Fujifilm combines a sensor array that utilizes different pixel sizes with a filter that utilizes clear filters as well as colored.
The addition of clear filter areas and larger pixels should make the sensor more sensitive in low light conditions, lowering noise taken in low light.
The downside is a sensor that is more complex to manufacture. This should increase its cost when compared to the Bayer based sensors used in other cameras.
It also looks like the sensor may have more open space when compared to the Bayer sensor. (This could just be due to the drawing used in the patent.)
Alternatively, Fujifilm might be able to produce similar result simply by alternating the filter used with the current sensor. Adding clear areas and using a a pattern like this:
might allow Fujifilm to emulate the results of the patented sensor without have to increase manufacturing costs.
BTW, if you're wondering why the green areas are larger than the blue and red, that's because the human eye is more sensitive to that wavelength (color) of light than the other two
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