# A sensor question

Larry Colen lrc at red4est.com
Mon Apr 11 01:19:54 EDT 2011

```On Apr 10, 2011, at 9:36 PM, Sandy Harris wrote:

> The usual sensor uses basically three types of element -- R, G and B
> -- in a particular layout.
> Why not X Y Z where X = R+G, Y = R+G+B, Z = G+B ?

Interesting,  my enlarger has CMY where
C=G+B  M=R+B and Y=R+G

remapping yours

Y=R+G, W=R+G+B, C=G+B

But, the normal pattern is four pixels R,G,G,B so you could do a four pixel:
C,M,Y,W  which is almost the same as what printers use: CMYK

poking at various wikipedia entries on color (googling color wavelength)

violet	668–789 THz	380–450 nm
blue	631–668 THz	450–475 nm
cyan	606–630 THz	476–495 nm
green	526–606 THz	495–570 nm
yellow	508–526 THz	570–590 nm
orange	484–508 THz	590–620 nm
red	400–484 THz	620–750 nm

I'll use capital letters for positive results and lower case for negative
Let's say green light hits your array, we get
YWC
if white light hits it we get
YWC

with a CMYW array green gives us
CmYW
white gives us
CMYW
so we can at least tell the difference between green and white.

>
> You can get RGB from XYZ easily enough:
>
>  Y-X = R+G+B - R+G = B
>  Y-Z = R+G+B - B+G = R
>
>  X+Z-Y = R+G + B+G - R+G+B = R
>
> But the total light you are accepting is 2+2+3 = 7 rather than
> 1+1+1=3, so you are getting more photons overall. Isn't that
> beneficial?

At first glance it looks great.

>
> Y also gives you a straightforward monochrome.

But why additive rather than subtractive color.

I suspect that part of it has to do with the fact that light isn't composed of R,G,B photons, it's just that our eyes are composed of RGB cones:
Cone type	Name	Range	Peak wavelength
S	β	400–500 nm	420–440 nm  violet-green peaking in low violet
M	γ	450–630 nm	534–555 nm  blue-red peaking in green
L	ρ	500–700 nm	564–580 nm  green-red peaking in yellow-orange

If a 600 nm (orange) photon hits our eyes, the M&L cones are activated, or in the RGB parlance the red and green sensor sites, I'm not sure which of your sensors it would trigger and in what percentage.

Could you explain your sensor idea to me in terms of photon wavelengths?  I got this far and am not clever enough to work it out.

I also suspect that additive color math is a lot easier than subtractive color math.

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