Q4: How can RGBW have double the sensitivity of Bayer when only half of its pixels are W, and W has only double the sensitivity of G?
observed the following on a gray image patch: the W
signal from the image sensor is roughly twice as
strong as the G signal which is roughly twice as
strong as the R, B signal. Both the Quantum
Efficiency of the photodiode and the transmissivity
of the color filters account for these sensitivity
1/2 the pixels are W, 1/4 are G and 1/8 each are R,
B. If we assume W to generate 1 unit of signal, G
generates 1/2 and R, B each generate 1/4. Total
signal for a gray image patch then is 1/2 + 1/4 *
1/2 + (1/8 + 1/8) * 1/4 = 11/16
the other hand, has 1/2 G pixels and 1/4 R, B pixels
each and generates 1/2 * 1/2 + (1/4 + 1/4)*1/4 =
6/16 on the same image patch.
analysis indicates that RGBW should be roughly twice
as sensitive as Bayer. In practice the relative
sensitivities of R, G, B, W vary from sensor to
sensor with W often being a little less than twice
as sensitive as G but G often being more than twice
as sensitive as one or both of R, B.
prototype sensors manufactured by 3 vendors and
their sensitivity improvement over Bayer range from
6dB to 6.9 dB in low light where the sensor is read
noise limited, and 3dB to 3.5dB in bright light
where the sensor is photon shot noise limited.