D70 Native Color Space
Prepared 2005-11-25 (150/93864) by Bill Claff
Revised 2007-04-21 (150/41554) by Bill Claff

The D70 has a native color space.
The exact values of the tristimulus values and white point are proprietary and are only of interest to people writing raw decoders.
But it is interesting to get an idea of what the D70 "sees".

By carefully measuring red, green, blue, and gray test points in CIE space I reverse-engineered the D70 tristimulus and equal energy points.

Here is a CIE 1931 representation of that result:

The red, green, blue, and magenta points are my test data. (Magenta are the gray values.)

CIE 1931 diagrams are useful but can also be very misleading.
For one thing no value can go beyond the reverse diagonal between 0,1 and 1,0 since x+y must be less than or equal to 1.
The D70 red and green values are essentially on this line; the D70 is very sensitive to red and green.
Note however, that the dominant wavelength of green sensitivity is shifted somewhat towards red implying that the D70 cannot "see" some greens in the Adobe RGB gamut.

Although this is for the D70 I believe the spectral sensitivity of most digital SLRs is quite similar so this information probably applies to other Nikon DSLRs such as the D200, D80, etc.

From the gamuts it's clear to see why Adobe Camera Raw (ACR) uses ProPhoto RGB internally.
There appears to be a strong argument for using ProPhoto RGB as one's working space provided you are using 16-bit data.