Photoionization calculations with an SED corresponding to a zero-age instantaneous starburst model from M03 with M_sup = 100    M_⊙. A sequence of models of increasing metallicity Z is represented by the magenta solid line, for which U is constant at 0.01. The metallicities covered a range from Z = 0.01   Z_⊙ to Z = 2.5   Z_⊙, where Z increases by 0.2 dex from model to model. Three sequences of models are superimposed in which U is increased (rather than Z), covering the range 0.001 to 0.046. Each sequence has a different metallicity, as follows: Z = 0.01   Z_⊙ (cyan dotted line), Z = 0.25   Z_⊙ (light green dotted line) and Z = 1.0   Z_⊙ (blue dotted line). In this and subsequent figures, a light gray dashed line represents the locus of equal temperatures.