Single wavelength light is emitted by:
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
QTH
LED
Argon laser
PAC