Silicon is naturally less sensitive to blue and violet wavelengths of light than it is to green, and it is most sensitive to red. I am not really sure that adding additional pixel types with, say, a violet filter, will really improve the quality of violet in digital photos taken with such a sensor. Current bayer CFA's use fairly broad filter ranges for blue, green, and red pixels. Blue filters allow in light down to around 380nm (very violet), while red filters allow in light up to and beyond 700nm. Green overlaps both just a tad.
Assuming you did actually involve say violet, blue, cyan, green, yellow, orange, red, deep red CFA pixels in some "advanced" sensor design. One way or another, you still need to "coalesce" that greater variety of colors into RGB image pixels in the end anyway. You would have to do some weighted mathematical averaging of blue with violet and cyan, green with cyan and yellow, and red with deep red, orange and yellow. There are a few drawbacks to such an approach, and potentially no real benefits:
- You need to be extremely careful about how you lay each color out in the CFA to ensure that color distribution is effective to capture enough luminance per pixel and still be capable of interpolating an output pixel (today most interpolation is based on 2x2 quads of bayer pixels with slightly weighted algorithms to eliminate color artifacts, zippering, etc.) Fuji's mew X Pro-1 uses 6x6 blocks of pixels to produce an output pixel, and it still only uses RGB albeit with less pattern repetition.
- You need a more complex RAW file format to acomodate the increase in various CFA pixel color types.
- You need considerably more complex interpolation algorithms to convert larger blocks of input CFA pixels into output RGB pixels.
- You need more advanced logic to convert multiple input colors into fewer output channels per output pixel with appropriate color accuracy.
- When working RAW, you need to reprocess the bayer pixels and send them through your entire pipeline of edits to render the image on screen, so more complex interpolation and color conversion will impact the performance of post-process workflows
I think more can be done by moving to a Foveon X-3 style sensor, where each color channel is stacked at a single photosite. Silicon is naturally sensitive to most of the visible wavelengths of light, and considerably more sensitive to near infrared than it is to UV. As such, it also naturally filters light the deeper it penetrates into a silicon well, making it pretty easy to capture blue at the surface, green in the middle, and red deep into the well (which is exactly what Foveon sensors do.) By eliminating an alternating pattern of RGB pixels, you improve a LOT of the characteristics of the sensor directly, and since you have full color data at every single pixel, you also have full color fidelity. Foveon sensors are fairly well known for having richer blues than bayer-type sensors, without the need for a lot of complexity. You also eliminate color moire, minimize monochrome moire, reduce noise in the blue channel, etc.
