Single wavelength light is emitted by:
QTH
LED
Argon laser
PAC
Argon laser light has wavelength of 470 nm which is monochromatic (single wavelength or frequency) in nature. 
It produces an intensity of 200 to 300 mW.

​Ref: Textbook of Operative Dentistry, Nisha and Amit Garg, 3rd edition, Pg no:277