How To Choose The Right Handheld Light Quantum Measuring Instrument

Handheld quantum measuring instrument is a common method for field testing of plant lamps. Although use the world famous brand measuring instruments, still need pay attention to the error analysis of measurement results. The measurement error of planting parameters will affect the reliability of product design.

Let us take the LI-COR quantum measuring instrument as an example to introduce the error analysis of the handheld quantum measuring instrument.

Founded in 1971, LI-COR is widely used in agricultural planting data measurement and is a well-known brand in plant physiology and ecology research. Among them, the absolute calibration specification of LI-COR light quantum sensor is NIST traceable ±5 %, the specification value is 3%; the relative error specification value of the sensor is 5%; the cosine error is 2%, the temperature coefficient is 1%, and the absolute error and relative error generated by the instrument itself are above 11%. In practical applications, the environment is also a considered factor. Please note that this 11% does not include errors caused by user errors.

Disclaimer: The article map is mainly to provide a description of the concept of the appearance, does not mean that we recommend.

Handheld instrument sensor measurement errors include the following

1. Absolute calibration error: Calibration of standard lamp accuracy and its standard lamp.

2. Relative error: the spectral response error of the sensor.

3. Spatial error: The sensor is not responsive to radiation at various angles of incidence, including cosine and azimuth errors.

4. Temperature coefficient error: The sensor input and output are affected by the temperature coefficient.

5. Response time error: The error that occurs when the measurement source changes rapidly during the measurement.

6. Long-term stability error: The error that occurs when the calibration of the sensor changes over time can be eliminated by calibration.

7. User error: The sensor is subject to errors caused by human operation and external events.

User error is the most important to pay attention to error

User errors include: reflections of people’s clothing and buildings, obstacles, atmospheric smog, environmental dust and smoke, sparks and flashes, algae, vibrations, etc. can also cause measurement errors.

The error caused by user error will be 5%-30%, and in extreme cases it will reach 100%.

User error is uncertain, and the assessment of this error is related to test experience and expertise.

The causes for absolute error and relative error

The absolute and relative errors of a handheld measuring instrument are basically caused by the accuracy of the sensor measuring probe. The accuracy of the instrument is basically determined by the price of the instrument.

The relative error of the sensor is mainly caused by the spectral response error of the photoelectric probe. Why is the price of different precision probes so different? We need to understand what the spectral response error of the photodetector is.

For optical quantum sensors, the relative error is mainly caused by the accuracy of the photodetector and the precision of the components. As shown in the figure below, the ideal photon probe response curve is rectangular in the wavelength range of 400-700nm, but the response of the photodetector curve can’t realize this rectangle at all, but it is approximate rectangle. As shown in the figure below, this causes the spectral response error. The red, yellow and blue parts in the figure are the spectral response errors. The larger the area, the bigger the spectral response error.


Ideal response curve and actual error curve

The photodetector can be corrected by an algorithm. The relative error of the sensor cannot be eliminated anyway. The figure below shows the spectral response curve of the LI-190R sensor.

LI-COR believes that in the worst case, the total instrument error may reach 21%. The measurement error of the low and medium price handheld measuring instruments we buy on the market can be evaluated by ourselves.

Remark: To purchase a handheld measuring instrument, must let the manufacturer present the response curve of the photodetector. Otherwise, the instrument that was bought back may become the wrong start.

Below are the response curves of some optical quantum sensors that claim to be high-precision. The difference between the ideal rectangular and the rectangular can be visualized.

It need be cautious to purchase handheld photon counters. If you do not pay attention to the error of the evaluation instrument, it will fall into the measurement error trap, which seriously affects product design and parameter evaluation.

If already purchased the instrument, you can try to ask the instrument supplier or manufacturer for the probe response curve. If they can’t give it, you need to carefully evaluate the purchased instrument’s measurement error.

Handheld illuminance sensor and irradiance sensor error

The illuminance sensor’s photoelectric probe has relatively high precision, low price and relatively low measurement error. The medium priced illuminometer can meet the illuminance measurement requirements required to calculate the plant lamp parameters by calculation method.

The figure below shows the illuminance sensor response curve with a measurement error of <9%. Although the response curve of the sensor does not exactly match the visual function curve, the measurement error caused by this accuracy is acceptable in the PAR range.

The following figure is the response curve with measurement error <15%

Note: The spectral parameters of the plant lamp are not expressed in illuminance units. The purpose of measuring the illuminance value is to calculate the PPFD value by XD factor.

The irradiance sensor used in the PAR range has a relatively low measurement error. The unit for the spectral parameters of the current plant lamp is basically expressed by PPF and PPFD. The PAR irradiance measuring instrument is rarely used, so we don’t introduce here.

The spectrum corrected by the Mokley curve needs to be used with caution

We found that some gauges claim to provide spectral parameters that are corrected according to the Mokley curve (see the figure below). The purchase of such gauges requires caution. The Mokley curve does not become a national standard and is a single-blade data analysis. Plant canopy data, in the spectral weighting study of plant lights, there is still academic controversy about weighting red light or blue light. Therefore, we recommend not to use measuring instruments that claim to be weighted. Otherwise, users are impossible to communicate and study with other people in the spectral data.

RQE corrected spectral curve

Plant lights are spectra parameterized products

Plant lamp is a parameterized and technical product. The basis of parameterization and technology is the reliability of photon data. The measurement of the spectral parameters of plant lamp is a technical and complex measurement work. The spectral design of plant lamp needs to attach great importance to the measurement of spectrum. If the measurement data’s error too large, the spectral design will be invalid, and the spectral data error will leads to confusion in the planting process.

The lighting industry generally has the problem of virtual standard light source parameters. The plant light industry has also begun to appear this phenomenon, most of which are virtual standard power, chaotic parameters. Once the plant lights are made, the product itself will not have any deceptive, plant lamp functions are the representation of the parameters, the planting effect is the performance of the plant lamp spectral parameters.

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