"Colorimeter"

Not quite, but, close enough...
How can I determine the spectrum of incident light on a sensor,
in general?  Then, how many corners can I cut to sacrifice resolution
and accuracy?
I've worked with true colorimeters (dual wavelength) in the past.
But, they were optimized to look for specific wavelengths.
I calibrate the light emitted by my monitors with a device,
but it controls the light source to do so.
With no knowledge of the actual (visible) spectrum impinging on
a sensor (and a bit of time to integrate results), how can I
do this short of swapping individual filters in front of the
sensor(s)?