Metameric effects on sensors
To start with, please look at the spectral sensitivity characteristics of a sensor. For numeric example we will use here curves of SONY ICX285AQ, see p.7 of SONY publication
http://products.sel.sony.com/semi/PDF/ICX285AQ.pdf
Digitizing the curves on page 7, we will have a table similar to:
400nm 450nm 500nm 550nm 600nm 650nm 700nm
R 0.03 0.02 0.04 0.07 0.96 0.94 0.82
G 0.03 0.13 0.56 0.9 0.31 0.04 0.15
B 0.22 0.66 0.55 0.04 0 0.01 0.01
Now let's take 4 wavelengths, 450, 500, 550, and 600nm; and play with them a little. Can we find a source that emits 450 and 550 nm that will provide a response equivalent to another source of light, emitting 500 and 600nm? Solving a simultaneous linear equation, we see that sensor will respond to first power source, emitting 65.4mW at 450nm and 41.6mW at 550 nm exactly the same way as to the second power source, emitting 81.5mW at 500nm and 1mW at 600nm.
Further, it is easy to see that we can find an infinite number of mixtures of those 4 wave lengths that will produce exactly the same sensor response. That means that for ICX285AQ a lot of colours between cyanish blue and cyanish green will trigger the same sensor response.
Reproduction of many shades of reds is even more challenging then that. If the series of wavelengths is 400, 500, 600, and 700nm with this sensor orange, brown, and even some shades of green trigger same response.
The above problem is emphasized when shooting conditions are far from native sensor colour temperature, and with "wrong" exposure. Yes, resulting colour depends not only on the white balance, but on exposure too. Speak of ETTR
It is important that this kind of metamerism is very far from the way our human perception tends to interpret colours. The example above shows the colours where humans see distinctly different colour. To prove this one can compute tristimulus values computed as per CIE.
Here are two examples of rendition of ambiguous colours mentioned above - the particular sensor will render them into the same numbers, and demosaicing will further interprete them as a same colour:
cyanish blue to cyanish green:
orange-brown-green:
You can have a better view in a colour-savvy application. Images are in sRGB colour space.
http://products.sel.sony.com/semi/PDF/ICX285AQ.pdf
Digitizing the curves on page 7, we will have a table similar to:
400nm 450nm 500nm 550nm 600nm 650nm 700nm
R 0.03 0.02 0.04 0.07 0.96 0.94 0.82
G 0.03 0.13 0.56 0.9 0.31 0.04 0.15
B 0.22 0.66 0.55 0.04 0 0.01 0.01
Now let's take 4 wavelengths, 450, 500, 550, and 600nm; and play with them a little. Can we find a source that emits 450 and 550 nm that will provide a response equivalent to another source of light, emitting 500 and 600nm? Solving a simultaneous linear equation, we see that sensor will respond to first power source, emitting 65.4mW at 450nm and 41.6mW at 550 nm exactly the same way as to the second power source, emitting 81.5mW at 500nm and 1mW at 600nm.
Further, it is easy to see that we can find an infinite number of mixtures of those 4 wave lengths that will produce exactly the same sensor response. That means that for ICX285AQ a lot of colours between cyanish blue and cyanish green will trigger the same sensor response.
Reproduction of many shades of reds is even more challenging then that. If the series of wavelengths is 400, 500, 600, and 700nm with this sensor orange, brown, and even some shades of green trigger same response.
The above problem is emphasized when shooting conditions are far from native sensor colour temperature, and with "wrong" exposure. Yes, resulting colour depends not only on the white balance, but on exposure too. Speak of ETTR
It is important that this kind of metamerism is very far from the way our human perception tends to interpret colours. The example above shows the colours where humans see distinctly different colour. To prove this one can compute tristimulus values computed as per CIE.
Here are two examples of rendition of ambiguous colours mentioned above - the particular sensor will render them into the same numbers, and demosaicing will further interprete them as a same colour:
cyanish blue to cyanish green:
orange-brown-green:
You can have a better view in a colour-savvy application. Images are in sRGB colour space.