Waveguide security device

DETAILED DESCRIPTION OF THE INVENTION Referring to the figures in detail, a preferred embodiment of a waveguide security verification device is shown in FIG.
1 by the reference numeral 10.
A wide angle polychromatic illuminating light 20 is conducted into one end the waveguide device 10.
A high index fiber optic core 12 is surrounded by progressively decreasing indices claddings 14, 16, and 18.
The core and the each of the various claddings can operate as wavelength subtraction filters for the wide angle polychromatic illuminating light 20 impinging on one end and fully filling the waveguide device 10.
The numerical aperture (NA) or light-gathering ability of a lightguide or optical fiber is dependent upon the refractive indices of the core and the cladding and determines the angles at which light rays will be propagated by the fiber.
The greater the difference between the indices of the core and cladding, the larger the NA, the larger the acceptance cone of light and the larger the number of light modes will be conducted by the fiber.
A low NA only conducts highly directional light.
The relation between NA, acceptance cone angle and indices are as follows: NA=(n2-n1)1/2 where n2 is the refractive index of the core, and n1 is the refractive index of a cladding.
The half-angle (.
theta.
) of acceptance is related to the NA thus: NA=sin.
theta.
.
theta.
=arcsin(NA) The NA of a optical fiber slightly decreases with fiber length as the high-order modes, i.
e.
those that travel near the critical angle are slowly lost.
Furthermore, light entering a waveguide can be classified as an incident ray, a reflected ray, or a refracted ray.
Incident light is incoming light.
Incident light that impinges on the waveguide at less than or equal to a critical angle, known as the angle of incidence, can propagate through the waveguide, while light at greater than the critical angle is reflected away from the waveguide.
The light that enters the waveguide at less than or equal to the critical angle is transmitted through the waveguide so long as it remains at less than the critical angle (leaving aside scattering due to impurities and other imperfections)