  NGC 4815 is a distant and populous open cluster, which lies in the galactic
plane in a region of strong absorption. As a consequence, its membership,
spatial distribution and luminosity function are not well determined. In this
paper, we present an algorithm which uses both positional and photometric data
simultaneously to search for open cluster members. The contribution of the
field stars is estimated by our Galaxy model software. The method has been
developed for deep CCD photometric observations, but it can also be used to
investigate the membership in an n-dimensional space, including proper motions,
radial velocity and metallicity. A clean and well defined colour-magnitude
diagram (CMD) for NGC 4815 has been obtained after decontaminating from field
stars. We have investigated the spatial distribution of cluster members and the
luminosity function. We have shown that the projected surface distribution of
stars is well-represented by an exponentially decreasing function with a scale
length of 1.67 $\pm$ 0.06 arcmin. We have found evidence for mass segregation.
The luminosity function for NGC 4815 is similar to that of the Hyades cluster,
and shows a gap at $V$ = 15 mag.

Astrophysics