Many current rapid transmittance algorithms, specifically the Optical Path Transmittance (OPTRAN), are based on use of effective transmittances to account for the effects of polychromatic radiation on the transmittance calculations. We document how OPTRAN was modified by replacing the effective transmittance concept with a correction term. Use of the correction term solves some numerical problems that were associated with use of effective transmittances, greatly reduces the line-by-line computational burden, and allows for the efficient inclusion of more gases. This correction method can easily be applied to any other fast models that use the effective transmittance approach.