It is disclosed a method and an apparatus of determining the distance (D)
between a collimator lens (13) and an object. Low-coherence light is
emitted from a light source (1) and directed the low-coherence light
through a collimator lens (13) to the object. The reflected light from
the collimator lens (13) and the object is directed to a beam splitter
(4) and split into two beams. Within the reference arm the frequency of
the beam is shifted by an acousto-optical modulator (5) to a certain
frequency and within the delay arm the time delay of the beam is scanned
by a variable delay line (7). After combining both beams in a beam
combiner (8), the presence of a frequency component equal to the
frequency shift of the acousto-optical modulator (5) is detected and the
distance (D) between the collimator lens (13) and the object is
calculated.