The present invention allows light leaked to outside of a light guide plate from a surface area L to pass through a diffusion sheet by covering both the bottom of an optical sheet and the surface area L of the light guide plate located between an LED FPC and the optical sheet with the diffusion sheet. This same composition is can be used to treat viral infections, particularly hepatitis C, herpes simplex, Kaposi’s sarcoma and HIV. A cover covers at least a portion of the distal end working unit.