> Is there a name for a light source which has a constant emission intensity across a given range of wavelengths?

The issue with this is that you can’t necessarily define ‘constant’ in a meaningful way. Being a density function rather than a point function, the energy irradiance from a source changes changes depending on whether you measure in terms of wavelength or frequency. I recommend [0] as an introduction to the problem.

[0] https://oceanopticsbook.info/view/light-and-radiometry/level...