We present the optical identification of mid-IR and radio sources detected in the European Large Area ISO Survey ( ELAIS ) areas N1 and N2 .
Using the r ’ band optical data from the Wide Field Survey we apply a likelihood ratio method to search for the counterparts of the 1056 and 691 sources detected at 15 \mu m and 1.4 GHz respectively , down to flux limits of S _ { 15 } = 0.5 mJy and S _ { 1.4 { GHz } } = 0.135 mJy .
We find that \sim 92 % of the 15 \mu m ELAIS sources have an optical counterpart down to the magnitude limit of the optical data , r ’ =24 .
All mid-IR sources with fluxes S _ { 15 } \geq 3 mJy have an optical counterpart .
The magnitude distribution of the sources shows a well defined peak at relatively bright magnitudes r ’ \sim 18 .
About 20 % of the identified sources show a point-like morphology ; its magnitude distribution has a peak at fainter magnitudes than those of galaxies .
The mid-IR-to-optical and radio-to-optical flux diagrams are presented and discussed in terms of actual galaxy models .
Objects with mid-IR-to-optical fluxes larger than 1000 are found that can only be explained as highly obscured star forming galaxies or AGNs .
Blank fields being 8 % of the 15 \mu m sample have even larger ratios suggesting that they may be associated with higher redshift and higher obscured objects .