You can then remeasure this configuration periodically. Drastic changes are a cause for concern. For spectral radiometers, measuring a fluorescent lamp is useful because you can make sure the lamp's spectral lines show up where they are supposed to. A radiometer takes the incident photons and measures those in each waveband separately, so that the noise for each measurement is greater. My radiometer has difficulty at the low end of my monitor's gamma curves.
Spectral radiometers vary considerably in how finely they sample the spectrum.
It is possible to pay a lot of money and get sampling every nanometer or pay a lot less money and get sampling every 10 nm. In addition to how finely they sample the spectrum, these instruments differ in the bandwidth measured at each sample point. One thing that you want to be true is that light at every wavelength is accounted for somewhere. That is, it would be bad to have a device that sampled every 10nm with a 1nm bandwidth, because then your instrument would be completely blind to light between the sample points. I assume that most devices you can buy are set-up correctly in this regard, but I have never actually checked.