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We present ELDAR, a new method that exploits the potential of medium- andnarrow-band filter surveys to securely identify active galactic nuclei (AGN)and determine their redshifts. Our methodology improves on traditionalapproaches by looking for AGN emission lines expected to be identified againstthe continuum, thanks to the width of the filters. To assess its performance,we apply ELDAR to the data of the ALHAMBRA survey, which covered an effectivearea of $2.38\,{\rm deg}^2$ with 20 contiguous medium-band optical filters downto F814W$\simeq 24.5$. Using two different configurations of ELDAR in which werequire the detection of at least 2 and 3 emission lines, respectively, weextract two catalogues of type-I AGN. The first is composed of 585 sources($79\,\%$ of them spectroscopically-unknown) down to F814W$=22.5$ at $z_{\rmphot}>1$, which corresponds to a surface density of $209\,{\rm deg}^{-2}$. Inthe second, the 494 selected sources ($83\,\%$ of themspectroscopically-unknown) reach F814W$=23$ at $z_{\rm phot}>1.5$, for acorresponding number density of $176\,{\rm deg}^{-2}$. Then, using samples ofspectroscopically-known AGN in the ALHAMBRA fields, for the two catalogues weestimate a completeness of $73\,\%$ and $67\,\%$, and a redshift precision of$1.01\,\%$ and $0.86\,\%$ (with outliers fractions of $8.1\,\%$ and $5.8\,\%$).At $z>2$, where our selection performs best, we reach $85\,\%$ and $77\,\%$completeness and we find no contamination from galaxies.

ELDAR, a new method to identify AGN in multi-filter surveys: the ALHAMBRA test-case