Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total radiation across a range of 400 to 700 nm. PAR is almost universally quantified as photosynthetic photon flux density (PPFD) in units of micromoles per square meter per second (µmol m-2 s-1, equal to microEinsteins per square meter per second) summed from 400 to 700 nm (total number of photons from 400 to 700 nm). However, ultraviolet and far-red photons outside the defined PAR range of 400-700 nm can also contribute to photosynthesis and influence plant responses (e.g., flowering).
Sensors that measure PPFD are often called quantum sensors due to the quantized nature of radiation. A quantum refers to the minimum quantity of radiation, one photon, involved in physical interactions (e.g., absorption by photosynthetic pigments). In other words, one photon is a single quantum of radiation. Sensors that function like traditional quantum sensors, but measure a wider range of wavelengths can be thought of as an ‘extended range’ quantum sensor.
Typical applications of traditional quantum sensors include incoming PPFD measurement over plant canopies in outdoor environments or in greenhouses and growth chambers, and reflected or under-canopy (transmitted) PPFD measurement in the same environments. The Quantum Light Pollution Sensor uses a detector that is sensitive to radiation up to about 1100 nm, well beyond the range of wavelengths that influence photosynthesis and plant responses. This means this particular sensor should only be used for photon flux density measurements under LEDs.
| SQ-640 | SQ-644 | SQ-646 | SQ-647 | |
|---|---|---|---|---|
| Power Supply | Self-powered | 12 to 24 V DC | 5 V USB power source | 5.5 to 24 V DC |
| Current Draw | - | Maximum of 20 mA | 61 mA when logging | 1.4 mA (quiescent), 1.8 mA (active) |
| Sensitivity | 1 mV per µmol m-2 s-1 | 0.08 mA per µmol m-2 s-1 | - | - |
| Output Type | 0 to 200 mV | 4 to 20 mA | USB | SDI-12 |
| Resolution | - | - | 0.1 µmol m-2 s-1 | - |
| Calibration Factor | 1 µmol m-2 s-1 per mV | 12.5 µmol m-2 s-1 per mA | Custom for each sensor and stored in the firmware | Custom for each sensor and stored in the firmware |
| Calibration Uncertainty | ± 5 % | ± 5 % | ± 5 % | ± 5 % |
| Measurement Range | 0 to 200 µmol m-2 s-1 | 0 to 200 µmol m-2 s-1 | 0 to 200 µmol m-2 s-1 | 0 to 200 µmol m-2 s-1 |
| Measurement Repeatability | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % |
| Long-term Drift per Year | Less than 2 % per year | Less than 2 % per year | Less than 2 % per year | Less than 2 % per year |
| Non-linearity | Less than 1 % (up to 4000 µmol m-2 s-1) | Less than 1 % (up to 4000 µmol m-2 s-1) | Less than 1 % (up to 4000 µmol m-2 s-1) | Less than 1 % (up to 4000 µmol m-2 s-1) |
| Response Time | Less than 1 ms | Less than 1 ms | Software updates every second | Less than 0.6 s |
| Field of View | 180° | 180° | 180° | 180° |
| Spectral Range | 340 to 1040 nm ± 5 nm (wavelengths where response is greater than 50 % of maximum) | 340 to 1040 nm ± 5 nm (wavelengths where response is greater than 50 % of maximum) | 340 to 1040 nm ± 5 nm (wavelengths where response is greater than 50 % of maximum) | 340 to 1040 nm ± 5 nm (wavelengths where response is greater than 50 % of maximum) |
| Directional (Cosine) Response | ± 2 % at 45°; ± 5 % at 75° zenith angle | ± 2 % at 45°; ± 5 % at 75° zenith angle | ± 2 % at 45°; ± 5 % at 75° zenith angle | ± 2 % at 45°; ± 5 % at 75° zenith angle |
| Azimuth Error | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % |
| Tilt Error | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % | Less than 0.5 % |
| Temperature Response | -0.11 ± 0.04 % per C | -0.11 ± 0.04 % per C | -0.11 ± 0.04 % per C | -0.11 ± 0.04 % per C |
| Uncertainty in Daily Total | Less than 5 % | Less than 5 % | Less than 5 % | Less than 5 % |
| Housing | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic diffuser |
| IP Rating | IP68 | IP68 | IP68 | IP68 |
| Operating Environment | -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m | -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m | -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m | -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m |
| Dimensions | 30.5 mm diameter, 37 mm height | 30.5 mm diameter, 37 mm height | 30.5 mm diameter, 37 mm height | 30.5 mm diameter, 37 mm height |
| Mass (with 5 m of cable) | 140 g | 140 g | 140 g | 140 g |
| Warranty | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship |
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