  Objects sensed by laser interferometers are usually not stable in position or
orientation. This angular instability can lead to a coupling of angular tilt to
apparent longitudinal displacement -- tilt-to-length coupling (TTL). In LISA
this is a potential noise source for both the test mass interferometer and the
long-arm interferometer. We have experimentally investigated TTL coupling in a
setup representative for the LISA test mass interferometer and used this system
to characterise two different imaging systems (a two-lens design and a
four-lens design) both designed to minimise TTL coupling. We show that both
imaging systems meet the LISA requirement of +-25 um/rad for interfering beams
with relative angles of up to +-300 urad. Furthermore, we found a dependency of
the TTL coupling on beam properties such as the waist size and location, which
we characterised both theoretically and experimentally.
