  An experimental technique is developed to simultaneously measure both
temporal and spatial coherences of a light source by altering a standard
Michelson interferometer, which has been primarily used for measuring temporal
coherence only. Instead of using simple plane mirrors, two retroreflectors and
their longitudinal and lateral movements are utilized to incorporate spatial
coherence measurement using this modified Michelson interferometer. In general,
one uses Young double slit interferometer to measure spatial coherence.
However, this modified interferometer can be used as an optical setup kept at
room temperature outside a cryostat to measure spatio-temporal coherence of a
light source placed at cryogenic temperatures. This avoids the added
complexities of modulation of interference fringe patterns due to single slit
diffraction as well. The process of mixing of spatial and temporal parts of
coherences is intrinsic to existing methods for dual measurements. We addressed
these issues of spatiotemporal mixing and we introduced a method of temporal
filtering in spatial coherence measurements. We also developed a curve overlap
method which is used to extend the range of the experimental setup during
temporal coherence measurements without compromising the precision. Together,
these methods provide major advantages over plane mirror based standard
interferometric systems for dual measurements in avoiding systematic errors
which lead to inaccuracies especially for light sources with low coherences.
