We describe a novel approach to measure high redshift star formation by studying the evolving properties of emission-line galaxies (ELGs) in deep high-z surveys with broadband photometry. Measuring the colour offsets between dust-free ELGs and line-free models, restframe wquivalent widths (EWs) are measured for Halpha, [OIII] and [OII] from CANDELS deep multiband photometry (near-continuous U to IRAC4.5 coverage) from z=0.1 up to z=5.

Measured EWs are complementary with spectroscopic (3D-HST) and narrow-band methods (HiZELS) and median Halpha EWs evolve consistently up to z=2 and then depart to lower values than extrapolated from z below 2. The use of deep broadband filter coverage allows for a systematic measurement of EWs in galaxies up to z=5 reaching down to 1e9 solar masses.

The fraction of ELGs grows to dominate the total galaxy sample at redshifts greater than 1.5, with [OIII] displaying a larger fraction of extreme EWs than Halpha and and [OIII]/[OII] increasing with z throughout the sample.

This method was applied to all the CANDELS fields, where consistent results were found. Luminosities, star formation rates and specific star formation rates are measured for the ELG population, showing a departure of median sSFR above the "main sequence" of star formation towards higher redshifts and the line ratios are also explored to yield other physical quantities.

This method proves efficient in surveying the physical quantities of large galaxy populations and is extensible to very high redshifts, provided there is deep IRAC line-free coverage (5.8 and 8.0 bands), which allows it to be applied to any future deep multibroadband photometry, as provided by JWST.