Time-delay distance ($D_{\Delta t}=(1+z_l)D_l{D}_s/D_{ls}$)

The distance combination $D_{\Delta t}=(1+z_l)D_l{D}_s/D_{ls}$ converts a dimensionless thin-lens delay difference into an observed time delay. Inference from a variable lensed source also requires a mass model and control of external convergence.