Apogee MQ-200 Quantum Meter

Product Description

Apogee quantum sensors measure photosynthetically active radiation. The MQ-200 features a handheld meter to display and store measurements with a separate sensor head connected by a lead. The sensor head features a fully potted, domed-shaped design making it fully waterproof, weatherproof and self-cleaning. The head can be held by hand, mounted to a leveling plate, or attached to an optional sensor wand for taking measurements in hard to reach places. Photosynthetically active radiation (PAR), or photosynthetic photon flux (PPF), is the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. Gardeners, greenhouse managers, growth chamber users and salt-water aquarists measure PAR to ensure optimal specimen health.

Measuring PAR to ensure proper light levels for underwater photosynthesis

Sufficient lighting is vital for growing healthy coral and other photosynthetic organisms in an aquarium. The two critical components of adequate lighting are intensity and spectrum. Light (or photons) in the wavelengths of 400 to 700 nanometers (nm) is the energy source for photosynthesis and is called Photosynthetically Active Radiation (PAR) or Quantum, and is usually expressed in Photosynthetic Photon Flux (PPF). Measuring the PAR output of lighting is superior to measuring LUX or footcandles, which are weighted measurements that approximate the human eye response, and thus overweight wavelengths between 550 and 600 nm and underweight wavelengths below 500 nm and above 650 nm.

The Quantum Meter manufactured by Apogee Instruments (model MQ-200) measures PAR in PPF units (μmol m^-2 s^-1) and features both sunlight and electric lighting modes. This meter has become very popular with advanced aquarists because it is ideally suited for use underwater. The separate sensor head is potted solid and is completely sealed with no hollow cavities for water to penetrate and cause measurement errors. The blue diffuser improves the spectral response to more accurately measure all wavelengths of light and has minimal error due to the immersion effect. The meter also features advanced logging capabilities such as automatically monitoring light levels on a half hour basis for up to 99 measurements and storing the daily total for over three months.

If data logging capabilites are not important to you, you can also make PAR readings using one of our stand-alone Quantum sensor heads by hooking it up to a high-quality voltmeter. The SQ-100 series sensors are considered to be self-powered and have been calibrated to 5.0 umol m-2 s-1 per mV. Use a voltmeter with a mV setting to attain the best resolution. Connect the positive lead of the voltmeter to the red wire of the SQ and the negative lead of the voltmeter to the black wire of the SQ. Once you are reading the mV output from the sensor, simply multiply this reading by 5.0. This will give you the μmol m^-2 s^-11 output from the sensor.

Apogee Quantum sensors have become indispensable tools for those wanting to ensure adequate PAR outputs while saving thousands of dollars over higher-priced spectroradiometers that provide far more accuracy than is needed for this application. Measuring PAR output can help you adjust your lighting configuration, rearrange your tank, alert you to lighting malfunctions, and let you know when it is time to replace your bulbs. Some bulbs decline in PAR output long before they burn out.

PAR Intensity Requirements for Coral

When growing a reef in a artificial environment, the PAR requirements of various types of coral will vary greatly due to the different depths and water conditions in which they existed naturally. Each tank setup requires a unique type, intensity, and duration of lighting. Many good resources exist in print and online that can help determine the PAR requirements you'll need to provide for optimum specimen health, such as this article by Sanjay Joshi. You can also get great information by asking a professional aquarist or by becoming involved in a local reef club.

Correcting for the Spectral Errors of Common Electric Lights

Because of the increased popularity of the reefkeeping hobby, the reef tank lighting market has recently grown exponentially with hundreds of lighting options now available. Unfortunately, all types of electric lights have a unique spectrum, and therefore also yield a unique set of spectral errors when measured by any commercially available PAR meter. These errors are generally minimal and shouldn't be a concern for most aquarists. However, when high precision is required, the following information, and a little bit of math, can help.

In response to emerging electric lighting technologies, Apogee has done extensive research to help customers make accurate PAR readings. Spectral errors for different commercially available lights were determined via the method proposed by Federer and Tanner (1966). These results are found in the table below.

The spectral errors of common lights such as CWF, CF, MH, and HPS are fairly straight forward. To make a high precision PAR reading for these types of lights, simply recalculate the PAR reading given by the meter with the corresponding percent error from the table below to yield a more precise PAR measurement

In recent years, LEDs have gained popularity in the marketplace due to their low power consumption and minimal heat output. This is great for cost savings, but due to the unique spectral output of the various colors (for example, very narrow wavelength ranges), LEDs present a challenge when attempting to make accurate PAR measurements. With commercially available PAR meters, certain colors of LED tend to read high, while others read low. The best device for accurate PAR measurement is a spectroradiometer, which provides intensity readings at each wavelength. However, these are often not well suited for underwater measurements and can range in price from several thousand to tens of thousands of dollars.

When used properly, the MQ-200 offers a very reliable and economical solution for precisely measuring the PAR output of LEDs. To achieve the highest level of accuracy, simply recalculate the PAR reading given by the meter with the corresponding spectral error percentage from the table below.

Federer, C.A. and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.

Light Intensity Measurements for Light Emitting Diodes (LEDs)

Customers often contact Apogee Instruments to inquire about whether quantum sensors and meters can be used to measure the radiation intensity from light emitting diodes (LEDs), as LEDs are becoming increasingly common as light sources for plant growth in controlled environments and coral growth in aquariums. Another article provided some qualitative information regarding the use of a broadband device (i.e. quantum/PAR sensors or meters) to measure a narrowband radiation source (i.e. many LEDs currently on the market), where it was stated that a spectroradiometer is the best instrument to accurately measure light intensity of LEDs (see the October 5, 2011 post titled 'Comparisons in Quantum Sensor Output for Different Light Sources'). While this is true, quantum meters can be used to measure LED intensity, and many customers use them for this application. As a result, an estimate of Apogee quantum meter accuracy for measuring LEDs is very practical.

A quantum sensor/meter is designed to measure the total number of photons between 400 nm and 700 nm, the photosynthetically active radiation (PAR) range. The error associated with a quantum meter (or sensor) measurement of light from a source that has a different spectrum than the source used to calibrate the meter is called spectral error. Spectral error arises because no quantum meters perfectly match the defined quantum response, meaning they do not respond to all wavelengths of light equally between 400 nm and 700 nm. Apogee quantum meters are sensitive to wavelengths between approximately 370 nm and 665 nm, with a relatively flat response between 450 nm and 650 nm due to the blue pigment used in the diffuser (Figure 1). However, they are not equally sensitive to the wavelengths within the photosynthetically active range (Figure 1). In order to determine spectral error, the spectral responses of the quantum meter, calibration light source, and light source to be measured are required, along with some spectra-dependent calculations (for details, see Federer and Tanner, 1966; Ross and Sulev, 2000).

Apogee quantum sensors and meters for electric lighting are calibrated in a custom chamber filled with T5 cool white fluorescent lamps. LEDs have a very different spectral output than T5 lamps (Figures 2, 3, and 4), thus some degree of spectral error is expected. For the narrowband, broadband, and mixed LEDs shown below, spectral errors are 8 % or less. Apogee quantum sensors and meters are less sensitive to blue wavelengths (near 400 nm) compared to longer wavelengths, and thus read low under blue LEDs. Conversely, Apogee quantum sensors and meters are more sensitive to green and red wavelengths (above 500 nm) compared to blue wavelengths, and thus read high under green and red LEDs. The broadband white LEDs output a small proportion of radiation beyond the upper end of the Apogee quantum sensor/meter sensitivity range (665 nm), and thus yield low measurements for the white LEDs.

IMPORTANT NOTE: LEDs that output a large proportion of radiation above approximately 660 nm will read very low and should not be measured with an Apogee quantum sensor/meter.

Apogee Instruments recently presented a comparison of spectral errors under LEDs for multiple quantum sensors at the 2012 International Meeting on Controlled Environment Agriculture. See the PDF of the poster here.

All Quantum/PAR sensors on the market experience a certain level of errors under different electric light sources. The following data can be used to adjust the PAR readings of Apogee Quantum sensors to achieve highly accurate readings. Please note that these errors apply only to quantum sensors that are pre-calibrated for electric lights, and for the Quantum Meter when it is set to "electric light" mode.

Table 1: Theoretical Spectral Errors for Apogee Quantum Meter Measurements of Multiple LED Sources