Лако-красочные материалы — производство

5.5.1

Introduction

Luminescent materials (or phosphors) are generally characterized by the emission of light with energy beyond thermal equilibrium, therefore the nature of luminescence is different from black body radiation. As a consequence, external energy has to be applied to luminescent materials to enable them to generate light. Luminescence can occur as a result of very different kinds of excitation, like photo — or electro­luminescence. In practice, the luminescence generally is due to excitation with X — rays, cathode rays or excitation with UV or even visible light.

Luminescence originates from electronically excited states in atoms and molecules and the emission process is governed by quantum mechanical selection rules. For­bidden transitions generally are slower than allowed optical transitions. Emission originating from allowed optical transitions, with decay times of the order of ps or faster is called fluorescence; the term for emission with longer decay times is phos­phorescence. The time in which the emission intensity decreases to 1/e or 1/10 (for exponential decay and hyperbolic decay, respectively) is called the decay time.

Luminescent materials have changed the world. Energy saving lamps, many kinds of displays and modern medical equipment all rely on luminescent materials and it is hardly imaginable that large scale application of luminescent materials started only slightly more than 100 years ago (for an overview see e. g. Ref. [5.198]).

In this section, historic aspects of luminescent materials will be discussed first, followed by a short treatment of luminescence mechanisms and luminescence exci­tation schemes. Thereafter, devices based on luminescent materials and the way in which luminescent materials determine their operational performance will be dis­cussed. Preparational aspects of luminescent materials will be described and then this section will end with an outlook.

5.5.2