PRINCIPLES OF GLASS-BEAD REFLECTORIZATION

THE CHEMICAL COMPOSITION, PHYSICAL PROPERTIES AND PRINCIPLES OF APPLICATION OF GLASS BEADS ARE REVIEWED. ALL THE PROPERTIES OF GLASS WHICH SIGNIFICANTLY AFFECT BEAD REFLECTORIZATION ARE DIRECTLY RELATED TO COMPOSITION. CHEMICAL DURABILITY AND SERVICEABILITY ARE NOT SYNONYMOUS TERMS IN DEFINING THE QUALITIES OF GLASS BEADS. MANY GLASSES OF RELATIVELY POOR DURABILITY GIVE EXCELLENT SERVICE. CHEMICAL DURABILITY IS NOT AS SIGNIFICANT AS SERVICEABILITY AND OPTICAL TESTS ARE THE MOST RELIABLE METHOD OF EVALUATING SERVICEABILITY. COLOR OF THE BEADS IS IMPORTANT FROM POSSIBLE OBJECTIONABLE MODIFICATION OF THE COLOR OF THE PAINTED STRIPE. THE AMOUNT AND DISTRIBUTION OF THE REFLECTED LIGHT IS LARGELY DETERMINED BY THE REFRACTIVE INDEX OF THE GLASS COMPOSING THE BEADS AND ITS RELATION TO THE INDEX TO THE VEHICLE AND PIGMENTS IN THE PAINT. THE DEPTH AND FIRMNESS OF EMBEDMENT OF BEADS IN PAINT DEPENDS ON COMPATABILITY WITH RESPECT TO THE ABILITY OF THE PAINT TO WET THE BEAD SURFACES. OPTIMUM GRADATIONS OF BEADS FOR MAXIMUM USEFULNESS HAVE NOT BEEN COMPLETELY DETERMINED. TWO KINDS OF IMPERFECTIONS RECEIVING MOST ATTENTION ARE NONROUNDNESS AND GAS INCLUSIONS. DEPTH OF EMBEDMENT IS AN IMPORTANT FACTOR IN REFLEX REFLECTION OF PAVEMENT MARKING BEADS, BECAUSE OF THE VERY SMALL ANGLES OF INCIDENCE INVOLVED, THE EFFECTIVE REFLEX-REFLECTING ZONE OF THE BEAD IS EXTREMELY NARROW. FROM AN ANALYSIS OF BEAD INTERFERENCE IN RELATION TO CRITICAL SEPARATION DISTANCES, IT APPEARS THAT SOME BEAD ECONOMY MAY BE ACHIEVED BY THIS FURTHER STUDY OF APPLICATION RATES. EXPERIENCE INDICATES THAT THE USE OF BEADS LENGTHENS THE LIFE OF THE PAINTED STRIPE, AND THAT BEADED STRIPES LAST LONGER ON BITUMINOUS PAVEMENTS THAN ON CONCRETE.