Three ice-tethered / floating systems measuring sub - sea-ice Photosynthetic Active Radiation (PAR) were deployed within the MOSAiC ice camp from February 2020, and recorded hourly data through to later summer 2020. These Optical Chain And Logger (OptiCAL) systems capture PAR-like data at 12 logarithmically-spaced levels down to 50 m depth, and, along with positional data, transmit these data hourly via Iridium. The PAR sensors have a wide dynamic range, able to detect PAR from 5x10⁻⁵ µMol s⁻¹ m⁻² to 100 µMol s⁻¹ m⁻² and thereby graph a wide range of light behaviour from winter to summer and from surface to depth. Sampling at the 12 levels is accomplished by resin-containment 'nodes' located along a power and communication cable that hangs vertically from the underside of the surface spar-buoy. The nodes are at nominal depths of 1, 2, 3, 4, 5, 6, 7, 9, 14, 21, 32, and 50m. Each node has six 'PAR' sensors, three detecting downwelling light and three upwelling light. Two of each set of three sensor sets have simple optical filters: one transmitting 'orange' the other 'blue'. The third sensor, being unfiltered, is 'white'. PAR data are archived in CSV files as 'counts' from the sensors, and are in units of millihertz (mHz). The detail of the CSV format is provided in this directory (see OptiCAL 'hh' buoy: Detail of the CSV format under Documentation), but to convert from mHz to the expected SI unit of µMol s⁻¹ m⁻² requires a conversion function. Details of this conversion are provided in this directory (see OptiCAL 'hh' buoy: Data description under Documentation). The 'hh' unit was deployed in the Dark site Second Year Ice, next to ITBOB and Salt/Light Harps. Its drift started at N 88.0651900, E 79.20462533 on 16th Feb 2020 and last data transmission was on 9th August at 13:00 UTC due to battery power being exhausted. Intended recovery was not possible before positional data were lost.This instrument was deployed as part of the project Ridges - Safe HAVens for ice-associated Flora and Fauna in a Seasonally ice-covered Arctic OCean (HAVOC), funded by the Research Council of Norway, project number: 280292 and Arctic Ocean ecosystems - Applied technology, Biological interactions and Consequences in an era of abrupt climate change (Arctic ABC Development, Research Council of Norway, project number: 245923). Ship-time associated to the participation of the DEARice team (D. Divine on leg 2) on the MOSAiC Expedition has been supported by the European Union's Horizon 2020 research and innovation programme under grant agreement No 730965 (ARICE).