Light detector linearity with varying wavelength
What you are asking is how flat the spectrum of various light measuring devices is.  Good devices of either type will have datasheets that tell you this.  Usually there is something called a "spectral sensitivity" graph.

If I remember right, photodiodes have narrower spectral sensitivity than CdS cells in general, but can be made to different center wavelengths.  For example, 980nm is a common wavelength for IR detectors.

One way of getting a flat spectrum is to use multiple sensors, each peaking at a different wavelength or with a different filter in front of them.  Software then uses the various values to compute the "flat equivalent" intensity.