Digital BW, The Print

Austin Franklin wrote:

> I do have questions on the diagrams they show in that 
> referenced link.  I am
> NOT a physicist...but I am an engineer with varying degrees.  
> They show on
> the top, that blue filter blocks green and red, and green 
> filter blocks red
> and blue etc.
> 
> Well, on the bottom, they show blue passes green and red, and 
> green passes
> red only.  How does that work?
> 

Hmmm, maybe here's how . . .the gory details are in the 1998 patent
#5,965,875

"It is well known that the greater the wavelength of light incident upon a
silicon substrate, the deeper the light will penetrate into the silicon
before it is absorbed. FIG. 5 shows light absorption length in silicon for
light in the visible spectrum. As indicated in FIG. 5, blue light, having a
wavelength of 400-490 nm, will be absorbed in a silicon substrate primarily
at a depth of about 0.2-0.5 microns. Green light, having a wavelength of
490-575 nm, will be absorbed in the silicon substrate at a depth of about
0.5-1.5 microns. Red light, having a wavelength of 575-700 nm, will be
absorbed in the silicon at a depth of about 1.5-3.0 microns. 

Taking advantage of these differences in absorption depth in silicon of
light of different wavelength, as shown in FIG. 6, a preferred embodiment of
the present invention provides a triple-well color photosensor structure
formed in a silicon substrate 100 of P-type conductivity (approx. 10.sup.15
atoms/cm.sup.2). The color photosensor structure includes a deep N-type
doped well region 102 (approx. 10.sup.16 atoms/cm.sup.2) formed in the
P-type silicon substrate 100. The junction depth of the N-doped region 102
is between about 1.5-3.0 microns, and preferably about 2 microns, i.e. the
approximate absorption depth of red light. Thus, the pn junction between the
deep N-doped region 102 and the P-type substrate 100 forms a red-sensitive
photodiode between the two regions."