Gravitational lensing phenomena occur when a massive object, such as a galaxy (lens), is located close to the line of sight between an observer and source. Observations of the gravitational lensing effect can produce small shifts in the positions of the background object. Thus the gravitational lensing phenomena can result in producing multiple images of a source. The multiple paths that light can travel in curved spacetime result in multiple images of the source and hence the difference in path lengths and traversed gravitational potentials results in a time delay between the lensed images which can be predicted assuming a model of the lens. However, conversion of the predicted time delay into physical units of the time requires knowledge of cosmological distances. Therefore, time delays can be used to infer cosmological parameters.

The general methodology, in the context of monitoring of such lenses by the upcoming 4m-ILMT, will be discussed.