Photometric Units and Nomenclature

The subject of photometric units and nomenclature has attracted some attention recently in the technical press, and a wish had been expressed by more than one writer that there might come into use a more systematic and uniformly accepted nomenclature. Hering, who had given the subject a good deal of attention and has published some valuable articles on it, remarks that many writers are vague in their expressions, using such terms as intensity, quantity, brightness illumination, flux, etc., in quite different senses. He says:

Moreover, the application of these useful laws (of light distribution) would be much better understood if we had a clearer physical conception of what these various quantities really mean, instead of merely calling them by indefinite names.
The following discussion is an attempt to bring out the physical meaning of the various quantities referred to, and to show that some of the names of units that have been been objected to are really useful and contribute to clear thinking. May of the theorems derived are not new, but they are nevertheless useful is developing the desired relations between the various photometric quantities. Acknowledgment is made to Blondel, Palaz, Liebenthal, Hering, Kennelly, Sharp, Hyde, Jones and others, whose writing and discussions have done much to develop the subject.

In what follows some of the names are used in a different sense from that ordinarily obtaining, and slight changes have been made in some of the symbols. These changes are in the interest of a more systematic arrangement, and it is hoped they may not be found confusing.