 Helium – cadmium lasers are a common source of blue @-@ ultraviolet laser light . They operate at either 325 or 422 nm in fluorescence microscopes and various laboratory experiments . Cadmium selenide quantum dots emit bright luminescence under UV excitation ( He @-@ Cd laser , for example ) . The color of this luminescence can be green , yellow or red depending on the particle size . Colloidal solutions of those particles are used for imaging of biological tissues and solutions with a fluorescence microscope . 
