In response to your request for a specification that will apply to glass beads for reflectorizing surfaces of signs, paints, and other night driving aids, we recommend the attached material which is drawn up in the form of a proposed specification. In general, the procedures. and ranges of performance characteristics conform with things that have been discussed by the A.S.T.M. Subcommittee on Traffic Paints. Actually, A.S.T.M. has not adopted a specification, but probably they will have one within the next year or two.
You will note the refractive index and size distribution or gradation are referred to in the specification, but the actual requirements are left open and subject to information contained in each invitation for bids. This provides a leeway within which you can obtain beads best suited for the use which you have planned.
In general, we would expect prices to increase as the refractive index increases, but also the reflective qualities of the beads become greater. The difference between a bead with a refractive index of 1.5 (which is equivalent to ordinary glass) and a refractive index of 1.75 is so great that the effectiveness of the sign would be increased tremendously through the use of beads with the higher refractive index rather than beads of ordinary glass. You can be certain that prospective vendors are in a position to know the refractive indices of their beads, and they will not hesitate to bid with a requirement of this sort in the invitation. Incidentally, the test that we think should be used is included in the specification and it is not difficult to perform.
In his memorandum which is attached, Mr. Havens has outlined some of the considerations that you will want to give to refractive index and to size distribution. He will be glad to discuss this further if you wish, and after you have some beads of different characteristics on hand he would welcome the opportunity of making observations in different places where they are applied in order to give you some specific data on the effects of changes in refractive indices and bead sizes.
Digital Object Identifier
Havens, James H., "Proposed Special Specification for Reflectorizing Glass Spheres" (1952). Kentucky Transportation Center Research Report. 1297.