A nug of spectrum knowledge....
Plants perceive light brightness in a different way- in the amount of energy the light is valued at based on wavelength. Spectrum plays a key role in this because at different wavelengths, light carries vastly different amounts of energy. The more blue, violet and ultra-violet light becomes, the shorter its wavelength is and the more energy it carries. Orange, red and far red have longer wavelengths and carry less energy. But energy isn’t everything, plants evolved to use the entire range of sunlight, both long and short wavelengths. Changing seasons bring with them different light spectrums. That’s why we use blue light in veg, it mimics spring and summer. And we use red light in flower, it mimics late summer, fall.