This light sensor developed by SDEC France is designed to measure the photosynthetically active radiation (also called P.A.R. sensor). It effectively measures radiation between 400 and 700 nm.
The sensor has an output proportional to the number of photon recieved regardless of their energy levels (photon flux density : µmol.m-2.s-1).
Since the energy level of each photon depends on its wavelength, the spectral response of a P.A.R. sensor using the photovoltaic effect must be higher in the far red part of the spectrum (around 700 nm) than in the blue wavelengths (around 400 nm).
The spectral response of the gallium arsenide photodiode built in the unit is close to the theoritical curve (see dart curve below). Its sensitivity is a bit low for blue radiation. With an additionnal blue filter (glass filter, tinted in the block, “SCHOTT” type), the spectral response is well balanced.
An acrylic diffuser tightly fitted into the sensor cuts off at 400 nm. It is opaque below 400 nm and has a constant transmission (around 50%) after 420 nm.
Although the red cut-off occurs at 685 nm instead of 700 nm (which is a property of the gallium arsenic photodiode), the sensor doesn't need extra expensive filters. This isn't the case when using silicon photocells.
Moreover, with this total unsensitivity above 685 nm, the JYP 1000 sensor (GaAsp photodiode) is well adapted for canopy transmittance measurements. For these measurements, 700 nm is a threshold value so an early cut-off is preferable to a late one.
In order to have a voltage output (millivolts), the unit has a built in precision resistor (metallic film /15 ppm/0,1%). It can be connected to any millivoltmeter with a high impedance :
- Numerical multimeter.
- Graphical recorder- data logger.
The sensor has a linear response until 5000µmol.m-2s-1.
Its cosine response is nicely corrected up to an 80° zenithal beam angle.
NO EXTRA EXPENSIVE FILTERS ARE NECESSARY
Height : 26 mm.
Diameter : 22 mm.
Material : Polycarbonate
Diffuser : Acrylic Type.
Blue filter : SCHOTT glass tinted in the block.
Sensor fixation system : by stainless steel screw situated under the sensor (screw : 1/4) as photographic device.
Output cable : Two cables with screen.
Lenght cable : 2 meters in standard version. Longer cables are available (contact us).
Charge resistor : The value depends on factory calibration. The average value is about 220ohm.
Output level : 10 mV for 536 µmol.m-2.s-1.
Each sensor is delivered with an individual
Temperature sensibility : The effect of temperature on the sensor is very low (better than 0,1%/°C at +20°Celsius).
Stability : JYP 1000 sensor is remarquably stable.
Each sensor has its own manufacturing number.