Measurements of transmitted irradiance under sea ice and corresponding incident solar radiation at the surface have been performed on First-Year-Ice (FYI) during the ALERT2018 campaign (Multidisciplinary Arctic Program (MAP) - Last Ice) off Alert, Nunavut, Canada in the Lincoln Sea on 10, 12, and 22 May 2018. The radiation measurements have been performed with Ramses spectral radiometers (TriOS, Rastede, Germany). All data are given in full spectral resolution interpolated to 1.0 nm, and integrated over the entire wavelength range (broadband, total: 320 to 950 nm). The irradiance sensor (cos-collector) was installed on the ROV. The transmittance is the ratio between transmitted irradiance and incident irradiance. All times are given in UTC.