[natmaka]

Source?

AFAIK traditional artificial light sources do indeed dissipate much more energy for the same amount of 'useful light', and also produce so much heat that they cannot be placed near the plant.

Is artificial light mandatory in urban farming? Aren't some (non fiber-optics-based) "light tubes" able to transmit light along with the necessary UV?

[mandelbrotwurst]

Nope! You can grow plenty of potatoes with sunlight in a plastic barrel on the balcony of your high-rise apartment.

[hanniabu]

Artificial light isn't necessary, but it's more space efficient and allows you to grow more plants per area which helps with the typically high land cost per square foot in urban areas.