Direct light will cause a glare on the surface, which prevents inspection of the package surface. 

Indirect lighting in appropriate colour cancels out red printing on the package surface to visualize defected areas. 

 Blue light with 470nm peak wavelength cannot detect foreign materials in the paper cup.

UV light in 375nm peak wavelength enables to visualize foreign materials even in the same colour with the paper cup. 

Visible light cannot cancel printed design on the package. 

IR light transmits the design print on the package and allows to visualize the base aluminium film.