How can you tell if lights emit harmful blue wavelengths at night?

Diffraction glasses are an inexpensive & simple way to determine whether circadian blue is emitted by a yellowish or white light.

Electric lights which emit too much sky-blue light in the evening, and provide too low a dose of circadian blue light during the day, are the key disruptors of our circadian clocks, and the cause of significant chronic ill-health.

But how can you tell whether an electric light is blue-free or blue-rich? Most lights look yellowish-white, so you cannot eas…