Understanding how
quantum meters work

measure photosynthetic engery in light


Spectral response. An ideal quantum sensor would give equal emphasis to all photons between 400 and 700 nm and would exclude photons above and below these wavelengths. The response of such a sensor is shown in the adjacent graph below. The most accurate way to measure this radiation is with a spectroradiometer, which costs over $15,000. However, quantum meters that approximate the ideal response with filters are commercially available for under $1000. These meters are accurate to within about  ±3 % for common light sources.
The spectral response of the Sensor used in Quantum Meters and the Quantum Sensor is shown at right. As the figure indicates, the sensor underestimates the 400 to 500 nm wavelengths (blue light), overestimates the 550-650 wavelengths (yellow and orange light), and has little sensitivity above 650 nm (red light). Fortunately, common light sources are mixtures of colors and the spectral errors offset each other. The sensor measures green light (500-550 nm) accurately, so it can be used to measure the radiation inside and at the bottom of plant canopies.
Quantum Sensor Spectral Response
 Because the spectral response of the sensor and the spectral output of electric lamps is constant, the errors under different lamp types can be calculated. This meter is calibrated for either sunlight or electric lamps. The errors under other light sources are shown below:



Lamp Type


Lamp Type


Cool White Fluorescent
Metal Halide
2% low
Cool White Fluorescent
10% high
High Pressure Sodium
2% high
Metal Halide
8% high
10% low
High Pressure Sodium
12% high



Some of the radiation coming into a sensor at low angles is reflected, which causes the reading to be less than it should be. To partly correct for this problem, sensors are often enclosed in a black cylinder with a small raised translucent disk in the top. This cosine corrected head helps to capture radiation at low angles. The Apogee sensor is cosine corrected by surrounding it with a dense foam gasket that blocks the radiation at very low angles. A small gap between the edge of the sensor and the gasket allows the correct amount of low angle radiation to be captured by the sensor. The cosine error for typical applications is less than 1%.



Increasing temperature increases the output of most radiation sensors. This meter was calibrated at 20°C. It reads 0.6% low at 10°C and 0.8% high at 30°C. This temperature error is insignificant for most applications.



The output of all radiation sensors tends to decrease over time as the detector ages. Our experience indicates that the average decrease of the sensor in this meter is about 1% to 2 % per year. We recommend returning the meter for recalibration every 3 years.
CALIBRATION  The meter includes a potentiometer that can be used to change the calibration. Please contact AgriHouse if you would like the meter recalibrated. Do not attempt recalibration without a radiation standard.




Long-Term Test Results



What are some properties of solar radiation?
Answer discusses basic terminology, radiation wavelength ranges and provides general facts about solar radiation.
What is photosynthetic radiation and why should it be measured to predict plant growth?
Answer details concepts of photosynthetic radiation including the difference between measuring PAR (photosynthetically active radiation) -also know as PPF (photosynthetic photon flux) and footcandles.
What are the conversions from PPF (umol m-2 s-1) to other units of measurment?
Answer gives different conversions from PPF to lux, footcandles, umol m-2 d-1, Einsteins and PPF to Watts per meter squared conversion for UV sensors. 
What are the best PPF (umol m-2 s-1) levels for optimum plant growth?
Answer discusses general recommendations for light levels for the ideal growing conditions.

Also see:

Understanding Plant Photosynthetic Light Energy

Quantum Meter Product List

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