Simple analytic expressions are derived for describing the radiation from an arc confined in a cylindrical constrictor and operating in the general 'gas-heating' or 'inlet' regime. For initial comparisons, radiation from 'fully developed' arcs in argon and nitrogen has been measured as a function of the arc current and the gas pressure in the confining cylinder. These measurements are compared to the appropriate limit in the analytical predictions and the correlation is found to be good, except for the electric field dependence upon current. It was found possible to improve the correlation by modifying the results of the analysis to include, in an approximate manner, the nonlinear relationships that actually exist among the properties of the plasma. Similarity parameters, which are inferred from the analysis and approximately derived from the basic equations, are applied successfully to the measurements with argon and nitrogen and to extensive calculations with nitrogen.