At the heart of the camera is the beam splitter. The typical design uses dichroic filters to split light into short (blue), mid (green) and long wavelengths (red). Dichroic filters use thin-film coatings on glass (the prism), and through optical interference, reflect or pass light depending on the wavelength. The coatings use many layers of different refractive indices to tune the filter band pass characteristics. (See Figure 1.)
The individual sensors are bonded to the prism after careful positioning to ensure the sensors are centered at the optical axes and that the photo-sites align.
The beam splitter has low absorption of light, so the majority of the incident light reaches the sensors. For the CFA sensor, half the green light and three-quarters of the blue and red light is lost in the absorption filters in the Bayer pattern. The result is that for a given sensor size, the beam-splitter design is inherently more sensitive.
Early single-tube consumer cameras used methods such as colored stripes to derive color information but suffered from a large difference between horizontal and vertical spatial resolution, and lacked the necessary performance for professional applications.
The signature patent in the development of single-sensor imaging was from Bryce E. Bayer, and assigned to Kodak. This detailed an RGBG CFA using transmissible filters superimposed in registration on the sensor matrix. The patent details luminance sensor elements (green) alternating with chrominance sensor elements, such that every other sensor is for luminance, thus matching the sensitivity of the retina, and much like the NTSC and PAL color systems, where the color information is subsampled. Bayer’s patent details two color channels: red and blue.
The output from the sensor is demosaiced into RGB components, and then an algorithm is applied to reconstruct an R, G and B value for each pixel. This process does lead to artifacts, especially in the red and green channels, that are not found with the three-sensor cameras. Typical artifacts include aliasing in fine red and blue detail.
The regular pattern of the sensor inevitably leads to spatial aliasing, but this can be reduced with the use of an optical low-pass filter (OLPF) in front of the sensor. With a three-sensor camera, the design of the OLPF is matched to the resolution of the sensor. However, with a Bayer sensor, the photosite density for red and blue is half that of the green photosites. The effective fill factor is also different, so the design of the OLPF is more of a compromise with the single-sensor camera.
The Bayer RGBG layout is just one of several alternative CFA schemes that are used in cameras. One RED Digital Cinema camera even offers a single sensor without a CFA for the cinematographer who wants to shoot in black and white.