• SM-500: Full-spectrum is accurate for measuring 389-692 nm under all light sources including LEDs.
• SM-600: ePAR is accurate for measuring the newly discovered extended PAR (ePAR) 383-757 nm range under all light sources.

Sensors are cosine-corrected to maintain accuracy when radiation comes from low angles (± 5 % at 75° zenith angle). Long-term non-stability determined from multiple sensors in accelerated aging test and field conditions is less than 2 % per year.

Graphical summaries of environmental measurements can be downloaded to dataloggers via Modbus or smartphones via Bluetooth with the ApogeeConnect app (iOS/Android). While the Guardian can take measurements as a stand-alone device mounted to a mast or hung by thin wires, it can also integrate into systems for greenhouses, grow rooms, and vertical farms via Modbus or Bluetooth. Bluetooth and Modbus connectivity can be simultaneous, allowing for in-person spot checks and supplementary logging even while connected to a larger controller system.

The Guardian is constructed from high-quality ASA plastic with low thermal conductivity and a gloss white finish in selective areas to lower emissivity with an IP rating of IP53. The Guardian is filled with foam to provide additional thermal insulation. Additionally, a quantum sensor is mounted on the top of the shield to measure PAR (SM-500) or ePAR (SM-600). The internal airway utilizes a small venturi constriction to maximize airflow efficiency over the thermistor, which minimizes the thermal boundary layer’s influence on temperature measurements. Simultaneously, a separate module used to measure CO₂, humidity, and pressure is aspirated by a passive, low velocity airstream using a “fluid entrainment” principle.