Aptina patent application US20150146054 "Image sensors with color filter elements of different sizes" by Brian Vaartstra, Mitchell J. Mooney, and Stanley Micinski describes the small pixel challenges "Typically, each color filter element in a conventional color filter array has the same size and shape (i.e., each of color filter elements R, G, and B are square and occupy the same area). As pixel size becomes smaller with each successive generation of imaging sensor devices, one problem that may arise is the diffraction limit of red light. Due to this diffraction limitation, the quantum efficiency may be significantly degraded for pixels that are smaller than 0.7 microns on one side (as an example). It may therefore be desirable to form color filter elements of varying sizes optimized for the wavelength(s) of interest."
So, Aptina proposes to increase red color filter at the expense of the blue one:
"Color filter element 102-X may be a yellow color filter element, a green color filter element, a cyan color filter element, a magenta color filter element, an infrared-pass filter element (e.g., a filter that passes infrared light), an IR-block filter element (e.g., a filter that blocks infrared light), a clear color filter element (e.g., a filter that passes all visible light), or any combination of color filter elements (e.g., in each unit cell, one X pixel containing a green color filter and the other containing an IR pass filter)."
Another Aptina patent application US20150146055 "Imaging pixels with improved analog-to-digital circuitry" by Parthasarathy Sampath proposes a digital correction to compensate the capacitor mismatches in column-parallel SAR ADC: