Hi Ilya,


Yes (?); Wavelength. IIRC, wavenumber is 1/wavelength (or some such;
2pi comes to mind...).
Yes, wavenumber is 2 * pi / lambda, units m^-1

[BTW, because of non-linearity of wavelength vs wavenumber, spectral
density which constant per wavelength becomes very non-constant when
measured per wavenumber.]
Yes, but the choice is arbitrary. Since wavenumber which is proportional to
photon energy, an interesting quantity for many applications, spectroscopy
people tend towards wavenumber whereas optical people like wavelength since
resolving power scales with that.

BTW, can you substantiate your interesting assumption:
----
In any decent photographic system the most important component
of performance/price ratio is the lenses. Since the price of the
lens scales as 4th or 5th power of its linear size, decreasing
the size of the sensor (while keeping S/N ratio) may lead to
very significant improvements of performance/price.
---
with some examples?
The tradeoff of lens aperture and expense vs sensor size determines
ultimately the size and shape of the digital camera. After the 'fashion
factor' of course.

-- hans