How to Correct a Spectral Error of the ALTA PAR Light Meter

What is a Spectral Error?

The combination of diffuser transmittance, interference filter transmittance, and photodetector sensitivity yields the spectral response of a quantum sensor.

A perfect photodetector/filter/diffuser combination would exactly match the defined plant photosynthetic response to photons (equal weighting to all photons between 400 and 700 nm, no weighting of photons outside this range), but this is challenging in practice.

A mismatch between the defined plant photosynthetic response and sensor spectral response results in a spectral error when the sensor measures radiation from sources with a different spectrum than the radiation source used to calibrate the sensor (Federer and Tanner, 1966; Ross and Sulev, 2000).

How PPFD Measurements Are Calculated By the Meter

Spectral errors for PPFD measurements made under radiation sources for growing plants were calculated for Apogee SQ-100 and SQ-500 series quantum sensors using the method of Federer and Tanner (1966).

This method requires PPFD weighting factors (defined plant photosynthetic response), measured sensor spectral response (shown in Spectral Response section on page 7), and radiation source spectral outputs (measured with a spectroradiometer).

This method calculates spectral error only and does not consider calibration, directional (cosine), temperature, and stability/drift errors. Spectral error data (listed in the table below) indicate errors less than 5 % for sunlight in different conditions (clear, cloudy, reflected from plant canopies, transmitted below plant canopies) and standard broad-spectrum electric lamps (cool white fluorescent, metal halide, high-pressure sodium), but more significant errors for different mixtures of light-emitting diodes (LEDs) for the SQ-100 series sensors.

Spectral errors for the SQ-500 series sensors are smaller than those for SQ-100 series sensors because the spectral response of SQ-500 series sensors is a closer match to the defined plant photosynthetic response.

Quantum sensors are the most common instrument for measuring PPFD because they are about an order of magnitude lower in cost than the spectroradiometers, but spectral errors must be considered. The spectral errors in the table below can be used as correction factors for respective radiation sources.

Spectral Errors for PPFD Measurements with Apogee SQ-100 and SQ-500 Series Quantum Sensors

Federer, C.A., and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654657.
Ross, J., and M. Sulev, 2000. Sources of errors in measurements of PAR. Agricultural and Forest Meteorology 100:103-125.