The possibility of interpolating the internal quantum efficiency of silicon photodiodes using a model with three adjustable parameters is investigated. The three parameters are determined from self-calibration measurements at 351, 476, and 800 nm. The internal quantum efficiency is then interpolated to 407 and 677 nm using the model. The calculated results are compared with direct measurements referenced to an electrical substitution radiometer. A difference of 0.6% was observed at 407 nm. This is probably significant, arising from inadequacies in the internal quantum efficiency model and possibly from volume recombination that is not accounted for by the self-calibration procedure. An insignificant (>0.1%) difference was observed at 677 nm.