We present the microlensing optical depth towards the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis ( DIA ) survey .
This analysis encompasses three years of data , covering \sim 17 million stars in \sim 4 deg ^ { 2 } , to a source star baseline magnitude limit of V = 23 .
The DIA technique improves the quality of photometry in crowded fields , and allows us to detect more microlensing events with faint source stars .
We find this method increases the number of detection events by 85 \% compared with the standard analysis technique .
DIA light curves of the events are presented and the microlensing fit parameters are given .
The total microlensing optical depth is estimated to be \tau _ { total } = 2.43 ^ { +0.39 } _ { -0.38 } \times 10 ^ { -6 } averaged over 8 fields centered at l = 2 \fdg 68 and b = -3 \fdg 35 .
For the bulge component we find \tau _ { bulge } = 3.23 ^ { +0.52 } _ { -0.50 } \times 10 ^ { -6 } assuming a 25 \% stellar contribution from disk sources .
These optical depths are in good agreement with the past determinations of the MACHO [ Alcock et al .
( 1997a ] and OGLE [ Udalski et al .
( 1994 ] groups , and are higher than predicted by contemporary Galactic models .
We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars , if we adopt the stellar mass function of Scalo ( 1986 ) as our lens mass function .
However , we note that as there is still disagreement about the exact form of the stellar mass function , there is uncertainty in this conclusion .
Based on our event timescale distribution we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge .