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. The most accurate way to measure this radiation is with our spectroradiometer, which costs less than $4000. Our Quantum Meters are accurate to within about ±3 percent for common light sources.

The spectral response of the Apogee 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.

Cosine Response

A "cosine corrected" sensor is designed to maintain its accuracy when radiation comes from different angles. For quantum sensors this is important for measuring scattered light, typical in under-canopy measurements. Apogee Quantum Sensors achieve this through the curvature and optical qualities of the convex lens.

Our long term tests indicate that the cosine errors between completely sunny and heavily overcast days are less than 0.5 percent. Cosine errors between the summer and the winter solstice are also less than 0.5 percent.

Use Under Different Light Sources

This table shows the quantum sensor's output under varying light sources. The sensor's spectral response, the spectral output of electric lamps, and these errors are all constant -- allowing the user to calculate the correct output for each light source.

For example, if the MQ-100 Quantum Sensor set to electric lamps is used in sunlight, and a value of: 1,500 μmol m^-2 s^-1 is given, the actual light is 1,665 μmol m^-2 s^-1.

In a controlled environment with multiple lamp types (such as a mixture of MH and HPS), a more precise percent error can be determined when the amount (%) of each type of light present is known.

View graph comparing spectral output.

Temperature Response

Increasing temperature decreases the output of most silicon photodiodes. This quantum sensor was calibrated at 20 C. It reads 0.6 percent high at 10 C and 0.8 percent low at 30 C. This temperature error is insignificant for most applications.

Recalibration

The output of all radiation sensors tends to decrease over time as the detector ages. Our measurements indicate that quantum meters should be recalibrated every 3 years.

Do not attempt recalibration without a radiation standard.

Pricing

Visit our recalibration section for info on how to request an RMA to send in your quantum sensor for recalibrating.