apogee INSTRUMENT MQ-620 Quantum Meter

apogee INSTRUMENT MQ-620 Quantum Meter-fig1


EU Declaration of Conformity 

  • This declaration of conformity is issued under the sole responsibility of the manufacturer:
    • Apogee Instruments, Inc. 721 W 1800 N
    • Logan, Utah 84321
    • USA
  • for the following product(s):
    • Models: MQ-620
    • Type: Extended Range PFD Meter
  • The object of the declaration described above is in conformity with the relevant Union harmonization legislation:
    • 2014/30/EU: Electromagnetic Compatibility (EMC) Directive
    • 2011/65/EU : Restriction of Hazardous Substances (RoHS 2) Directive
    • 2015/863/EU : Amending Annex II to Directive 2011/65/EU (RoHS 3)
  • Standards referenced during compliance assessment:
    • EN 61326-1:2013: Electrical equipment for measurement, control, and laboratory use – EMC requirements
    • EN 50581:2012:Technical documentation for the assessment of electrical and electronic products with respect to the restriction of hazardous substances
  • Please be advised that based on the information available to us from our raw material suppliers, the products manufactured by us do not contain, as intentional additives, any of the restricted materials including lead (see note below), mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), polybrominated diphenyls (PBDE), bis (2-ethylhexyl) phthalate (DEHP), butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), and diisobutyl phthalate (DIBP). However, please note that articles containing greater than 0.1 % lead concentration are RoHS 3 compliant using exemption 6c.
  • Further note that Apogee Instruments does not specifically run any analysis on our raw materials or end products for the presence of these substances, but we rely on the information provided to us by our material suppliers.
  • Signed for and on behalf of:
    • Apogee Instruments, February 2022apogee INSTRUMENT MQ-620 Quantum Meter-fig23
    • Bruce Bugbee President
    • Apogee Instruments, Inc.


  • Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total radiation across a range of 400 to 700 nm. PAR is almost universally quantified as photosynthetic photon flux density (PPFD) in units of micromoles per square meter per second (µmol m-2 s-1, equal to microEinsteins per square meter per second) summed from 400 to 700 nm (total number of photons from 400 to 700 nm). However, ultraviolet and far red photons outside the defined PAR range of 400-700 nm can also contribute to photosynthesis and influence plant responses (e.g., flowering).
  • Sensors that measure PPFD are often called quantum sensors due to the quantized nature of radiation. A quantum refers to the minimum quantity of radiation, one photon, involved in physical interactions (e.g., absorption by photosynthetic pigments). In other words, one photon is a single quantum of radiation. Sensors that function like traditional quantum sensors, but measure a wider range of wavelengths can be thought of as an ‘extended range’ quantum sensor.
  • Typical applications of traditional quantum sensors include incoming PPFD measurement over plant canopies in outdoor environments or in greenhouses and growth chambers, and reflected or under-canopy (transmitted) PPFD measurement in the same environments. The Extended Range PFD Sensor detailed in this manual uses a detector that is sensitive to radiation up to about 1100 nm, well beyond the range of wavelengths that influence photosynthesis and plant responses. This means this particular sensor should only be used for photon flux density measurements under LEDs.
  • Apogee Instruments MQ-620 meters consist of a handheld meter and a dedicated sensor that is connected by cable to an anodized aluminum housing. SQ-600 series Extended Range PFD Sensors consist of a cast acrylic diffuser (filter), photodiode, signal processing circuitry mounted in an anodized aluminum housing, and are potted solid with no internal air space. MQ series extended range PFD meters provide a real-time PFD reading on the LCD display, that determine the radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all angles of a hemisphere. MQ series quantum meters include manual and automatic data logging features for making spot-check measurements.


Apogee MQ series quantum meters covered in this manual are self-contained and come complete with handheld meter and sensor.

apogee INSTRUMENT MQ-620 Quantum Meter-fig2

A sensor’s model number and serial number are located on a label on the backside of the handheld meter.


Calibration Uncertainty ± 5 % (see Calibration Traceability below)
Measurement Range 0 to 4000 µmol m-2 s-1


Less than 0.5 %
Long-term Drift


Less than 2 % per year
Non-linearity Less than 1 % (up to 4000 µmol m-2 s-1)
Response Time Less than 1 ms
Field of View 180°
Spectral Range 340 to 1040 nm ± 5 nm (wavelengths where response is greater than 50 %; see Spectral

Response below)

Directional (Cosine)


± 2 % at 45° zenith angle, ± 5 % at 75° zenith angle (see Directional Response below)
Azimuth Error Less than 0.5 %
Tilt Error Less than 0.5 %
Temperature Response -0.11 ± 0.04 % per C
Uncertainty in Daily Total Less than 5 %
Housing Anodized aluminum body with acrylic diffuser
IP Rating IP68
Operating Environment -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m
Meter Dimensions 126 mm length, 70 mm width, 24 mm height
Sensor Dimensions 30.5 mm diameter, 37 mm height
Mass 140 g (with 5 m of lead wire)
Cable 2 m of two conductor, shielded, twisted-pair wire; additional cable available; TPR jacket
Warranty 4 years against defects in materials and workmanship

Calibration Traceability
Apogee MQ series quantum meters are calibrated through side-by-side comparison to the mean of four transfer standard quantum sensors under a reference lamp. The reference quantum sensors are recalibrated with a 200 W quartz halogen lamp traceable to the National Institute of Standards and Technology (NIST).

Spectral Response

apogee INSTRUMENT MQ-620 Quantum Meter-fig3

Mean spectral response measurements of six replicate Apogee MQ-600 series Extended Range PFD Sensors. Spectral response measurements were made at 10 nm increments across a wavelength range of 300 to 1100 nm in a monochromator with an attached electric light source. Measured spectral data from each PFD sensor were normalized by the measured spectral response of the monochromator/electric light combination, which was measured with a spectroradiometer.

Cosine Response

apogee INSTRUMENT MQ-620 Quantum Meter-fig4

Directional, or cosine, response is defined as the measurement error at a specific angle of radiation incidence. Error for Apogee MQ-600 series Extended Range PFD Sensor is approximately ± 2 % and ± 5 % at solar zenith angles of 45° and 75°, respectively.


  • Apogee MQ series quantum meters are designed for spot-check measurements, and calculation of daily light integral (DLI; total number of photons incident on a planar surface over the course of a day) through the built-in logging feature. To accurately measure PFD incident on a horizontal surface, the sensor must be level. For this purpose, each MQ model comes with a different option for mounting the sensor to a horizontal plane.apogee INSTRUMENT MQ-620 Quantum Meter-fig5
  • The AL-100 leveling plate is recommended for use with the MQ-620 (AL-100 leveling plate pictured). To facilitate mounting to a cross arm, the AL-120 mounting bracket is recommended.
  • The AM-310 Sensor Wand accessory incorporates a mounting fixture at the end of an extendable telescopic wand (up to 33 inches/84 cm). The wand is not suited for wet environments; however, it is excellent for greenhouses and growth chambers. Its ability to retract to a smaller size also makes it ideal for travel use.apogee INSTRUMENT MQ-620 Quantum Meter-fig6
  • The AM-320 Saltwater Submersible Sensor Wand accessory incorporates a mounting fixture at the end of a 40-inch segmented fiberglass wand and is well-suited for saltwater use. The wand allows the user to place the sensor in hard-to-reach areas such as aquariums.apogee INSTRUMENT MQ-620 Quantum Meter-fig7
    NOTE: The handheld meter portion of the instrument is not waterproof. Do not get the meter wet or leave the meter in high humidity environments for prolonged periods of time. Doing so can lead to corrosion that could void the warranty.



  • Use a Phillips head screwdriver to remove the screw from the battery cover. Remove the battery cover by slightly lifting and sliding the outer edge of the cover away from the meter.
  • To power the meter, slide the included battery (CR2320) into the battery holder, after removing the battery door from the meter’s back panel.apogee INSTRUMENT MQ-620 Quantum Meter-fig8
  • The positive side (designated by a “+” sign) should be facing out from the meter circuit board.
    NOTE: The battery cradle can be damaged by using an incorrectly sized battery. If the battery cradle is damaged, the circuit board will need to be replaced and the warranty will be void. To avoid this costly problem, use only a CR2320 battery.


  • Press down on the battery with a screwdriver or similar object. Slide battery out.apogee INSTRUMENT MQ-620 Quantum Meter-fig9
  • If the battery is difficult to move, turn the meter on its side so that the opening for the battery is facing downward and tap the meter downward against an open palm to dislodge the battery enough so that it can be removed with your thumb to slide the battery out of the battery holder.


MQ series quantum meters are designed with a user-friendly interface allowing quick and easy measurements.

  • apogee INSTRUMENT MQ-620 Quantum Meter-fig10 Press the power button to activate the LCD display. After two minutes of non-activity the meter will revert to sleep mode and the display will shut off to conserve battery life.
  • apogee INSTRUMENT MQ-620 Quantum Meter-fig11 Press the mode button to access the main menu, where manual or automatic logging are selected, and where the meter can be reset.
  • apogee INSTRUMENT MQ-620 Quantum Meter-fig12 Press the sample button to log a reading while taking manual measurements.
  • apogee INSTRUMENT MQ-620 Quantum Meter-fig13 Press the up button to make selections in the main menu. This button is also used to view and scroll through the logged measurements on the LCD display.
  • apogee INSTRUMENT MQ-620 Quantum Meter-fig14 Press the down button to make selections in the main menu. This button is also used to view and scroll through the logged measurements on the LCD display.
  • The LCD display consists of the total number of logged measurements in the upper right-hand corner, the real-time PPFD value in the center, and the selected menu options along the bottom.apogee INSTRUMENT MQ-620 Quantum Meter-fig15
  • Logging: To choose between manual or automatic logging, push the mode button once and use the up/down buttons to make the appropriate selection (SMPL or LOG). Once the desired mode is blinking, press the mode button two more times to exit the menu. When in SMPL mode press the sample button to record up to 99 manual measurements (a counter in the upper right hand corner of the LCD display indicates the total number of saved measurements). When in LOG mode the meter will power on/off to make a measurement every 30 seconds. Every 30 minutes the meter will average the sixty 30 second measurements and record the averaged value to memory. The meter can store up to 99 averages and will start to overwrite the oldest measurement once there are 99 measurements. Every 48 averaged measurements (making a 24-hour period), the meter will also store an integrated daily total in moles per meter squared per day (mol m-2 d-1).
  • Reset: To reset the meter, in either SMPL or LOG mode, push the mode button three times (RUN should be blinking), then while pressing the down button, press the mode button once. This will erase all of the saved measurements in memory, but only for the selected mode. That is, performing a reset when in SMPL mode will only erase the manual measurements and performing a reset when in LOG mode will only erase the automatic measurements.
  • Review/Download Data: Each of the logged measurements in either SMPL or LOG mode can be reviewed on the LCD display by pressing the up/down buttons. To exit and return to the real-time readings, press the sample button. Note that the integrated daily total values are not accessible through the LCD and can only be viewed by downloading to a computer.
  • Downloading the stored measurements will require the AC-100 communication cable and software (sold separately). The meter outputs data using the UART protocol and requires the AC-100 to convert from UART to USB, so standard USB cables will not work. Set up instructions and software can be downloaded from the Apogee website (http://www.apogeeinstruments.com/ac-100-communcation-cable/).apogee INSTRUMENT MQ-620 Quantum Meter-fig16

Immersion Effect Correction Factor

  • When a radiation sensor is submerged in water, more of the incident radiation is backscattered out of the diffuser than when the sensor is in air (Smith, 1969; Tyler and Smith, 1970). This phenomenon is caused by the difference in the refractive index for air (1.00) and water (1.33) and is called the immersion effect. Without correction for the immersion effect, radiation sensors calibrated in air can only provide relative values underwater (Smith, 1969; Tyler and Smith, 1970). Immersion effect correction factors can be derived by making measurements in air and at multiple water depths at a constant distance from a lamp in a controlled laboratory setting.
  • Apogee MQ-620 series ePFD sensors have an immersion effect correction factor of 1.25. This correction factor should be multiplied by PPFD measurements made underwater to yield accurate PPFD.
    NOTE: The handheld meter portion of the instrument is not waterproof. Do not get the meter wet or leave the meter in high humidity environments for prolonged periods of time. Doing so can lead to corrosion that could void the warranty.
  • Further information on underwater measurements and the immersion effect can be found on the Apogee webpage (http://www.apogeeinstruments.com/underwater-par-measurements/).
  • Smith, R.C., 1969. An underwater spectral irradiance collector. Journal of Marine Research 27:341-351.
  • Tyler, J.E., and R.C. Smith, 1970. Measurements of Spectral Irradiance Underwater. Gordon and Breach, New York, New York. 103 pages


  • Downloading data to a computer requires the AC-100 communication cable and the free ApogeeAMS software. The meter outputs data using the UART protocol and requires the AC-100 to convert from UART to USB, so standard USB cables will not work.
  • The most recent version of ApogeeAMS software can be downloaded at http://www.apogeeinstruments.com/downloads/.
  • When the ApogeeAMS software is first opened, it will show a blank screen until communication with the meter is established. If you click “Open Port” it will say “connection failed.”apogee INSTRUMENT MQ-620 Quantum Meter-fig17
  • To establish communication, make sure the meter is plugged into your computer using the AC-100 communication cable. To connect click the dropdown menu button and “COM#” options will appear. For more details on how to figure out which COM is the right one, watch our video.apogee INSTRUMENT MQ-620 Quantum Meter-fig18
  • When you have connected to the correct COM#, the software will say “Connected”.
    Click “Sample Data” to view saved sample readings.apogee INSTRUMENT MQ-620 Quantum Meter-fig19
  • “Daily Totals” shows all the saved Daily Light Integral (DLI) totals per day.apogee INSTRUMENT MQ-620 Quantum Meter-fig20
  • Click “30 Min Avg” to see the meter’s 99, 30-minute averages.apogee INSTRUMENT MQ-620 Quantum Meter-fig21
  • To analyze the data, click on “File” and “Save As” to save the data as a .csv file.
    Or you can highlight the numbers, copy, and paste them into a blank Excel spreadsheet. Data will need to be comma delimited.apogee INSTRUMENT MQ-620 Quantum Meter-fig22


  • Moisture or debris on the diffuser is a common cause of low readings. The sensor has a domed diffuser and housing for improved self-cleaning from rainfall, but materials can accumulate on the diffuser (e.g., dust during periods of low rainfall, salt deposits from evaporation of sea spray or sprinkler irrigation water) and partially block the optical path. Dust or organic deposits are best removed using water or window cleaner and a soft cloth or cotton swab. Salt deposits should be dissolved with vinegar and removed with a soft cloth or cotton swab. Never use an abrasive material or cleaner on the diffuser.
  • Although Apogee sensors are very stable, nominal accuracy drift is normal for all research-grade sensors. To ensure maximum accuracy, we generally recommend sensors are sent in for re calibration every two years, although you can often wait longer according to your particular tolerances.


  • Verify Functionality
    Pressing the power button should activate the LCD and provide a real-time PPFD reading. Direct the sensor head toward a light source and verify the PPFD reading responds. Increase and decrease the distance from the sensor to the light source to verify that the reading changes proportionally (decreasing PPFD with increasing distance and increasing PPFD with decreasing distance). Blocking all radiation from the sensor should force the PPFD reading to zero.
  • Battery Life
    • When the meter is maintained properly the coin cell battery (CR2320) should last for many months, even after continuous use. The low battery indicator will appear in the upper left hand corner of the LCD display when the battery voltage drops below 2.8 V DC. The meter will still function correctly for some time, but once the battery is drained the pushbuttons will no longer respond and any logged measurements will be lost.
    • Pressing the power button to turn off the meter will actually put it in sleep mode, where there is still a slight amount of current draw. This is necessary to maintain the logged measurements in memory. Therefore, it is recommended to remove the battery when storing the meter for many months at a time, in order to preserve battery life.
  • Low-Battery Error after Battery Replacement
    A master reset will usually correct this error, please see the master reset section for details and cautions. If a master reset does not remove the low battery indicator, please double check that the voltage of your new battery is above 2.8 V, this is the threshold for the indicator to turn on.
  • Master Reset
    • If a meter ever becomes non-responsive or experiences anomalies, such as a low battery indicator even after replacing the old battery, a master reset can be performed that may correct the problem. Note that a master reset will erase all logged measurements from memory.
    • Step 1: press the power button so that the LCD display is activated.
    • Step 2: Slide the battery out of the holder, which will cause the LCD display to fade out.
    • Step 3: After a few seconds, slide the battery back into the holder.
    • The LCD display will flash all segments and then show a revision number (e.g. “R1.0”). This indicates the master reset was performed and the display should return to normal.
  • Error Codes and Fixes
    • Error codes will appear in place of the real-time reading on the LCD display and will continue to flash until the problem is corrected. Contact Apogee if the following fixes do not rectify the problem.
    • Err 1: battery voltage out of range. Fix: replace CR2320 battery and perform master reset.
    • Err 2: sensor voltage out of range. Fix: perform master reset.
    • Err 3: not calibrated. Fix: perform master reset.
    • Err 4: CPU voltage below minimum. Fix: replace CR2320 battery and perform master reset.
  • Modifying Cable Length
    Although it is possible to splice additional cable to the separate sensor of the appropriate MQ model, note that the cable wires are soldered directly into the circuit board of the meter. Care should be taken to remove the back panel of the meter in order to access the board and splice on the additional cable, otherwise two splices would need to be made between the meter and sensor head. See Apogee webpage for further details on how to extend sensor cable length: (http://www.apogeeinstruments.com/how-to-make-a-weatherproof-cable-splice/).


Apogee Instruments will accept returns within 30 days of purchase as long as the product is in new condition (to be determined by Apogee). Returns are subject to a 10 % restocking fee.

  • What is Covered
    • All products manufactured by Apogee Instruments are warranted to be free from defects in materials and craftsmanship for a period of four (4) years from the date of shipment from our factory. To be considered for warranty coverage an item must be evaluated by Apogee.
    • Products not manufactured by Apogee (spectroradiometers, chlorophyll content meters, EE08-SS probes) are covered for a period of one (1) year.
  • What is Not Covered
    • The customer is responsible for all costs associated with the removal, reinstallation, and shipping of suspected warranty items to our factory.
    • The warranty does not cover equipment that has been damaged due to the following conditions:
      1. Improper installation, use, or abuse.
      2. Operation of the instrument outside of its specified operating range.
      3. Natural occurrences such as lightning, fire, etc.
      4. Unauthorized modification.
      5. Improper or unauthorized repair.
        Please note that nominal accuracy drift is normal over time. Routine re calibration of sensors/meters is considered part of proper maintenance and is not covered under warranty.
  • Who is Covered
    This warranty covers the original purchaser of the product or other party who may own it during the warranty period.
  • What Apogee Will Do
    At no charge Apogee will:
    1. Either repair or replace (at our discretion) the item under warranty.
    2. Ship the item back to the customer by the carrier of our choice.
      Different or expedited shipping methods will be at the customer’s expense.
  • How To Return An Item
    1. Please do not send any products back to Apogee Instruments until you have received a Return Merchandise Authorization (RMA) number from our technical support department by submitting an online RMA form at
      www.apogeeinstruments.com/tech-support-recalibration-repairs/. We will use your RMA number for tracking of the service item. Call (435) 245-8012 or email techsupport@apogeeinstruments.com with questions.
    2. For warranty evaluations, send all RMA sensors and meters back in the following condition: Clean the sensor’s exterior and cord. Do not modify the sensors or wires, including splicing, cutting wire leads, etc. If a connector has been attached to the cable end, please include the mating connector – otherwise the sensor connector will be removed in order to complete the repair/re calibration.
      Note: When sending back sensors for routine calibration that have Apogee’s standard stainless-steel connectors, you only need to send the sensor with the 30 cm section of cable and one-half of the connector. We have mating connectors at our factory that can be used for calibrating the sensor.
    3.  Please write the RMA number on the outside of the shipping container.
    4. Return the item with freight per-paid and fully insured to our factory address shown below. We are not responsible for any costs associated with the transportation of products across international borders.
      • Apogee Instruments, Inc.
      • 721 West 1800 North Logan, UT
      • 84321, USA
    5. Upon receipt, Apogee Instruments will determine the cause of failure. If the product is found to be defective in terms of operation to the published specifications due to a failure of product materials or craftsmanship, Apogee Instruments will repair or replace the items free of charge. If it is determined that your product is not covered under warranty, you will be informed and given an estimated repair/replacement cost.

For issues with sensors beyond the warranty period, please contact Apogee at techsupport@apogeeinstruments.com to discuss repair or replacement options.


  • The available remedy of defects under this warranty is for the repair or replacement of the original product, and Apogee Instruments is not responsible for any direct, indirect, incidental, or consequential damages, including but not limited to loss of income, loss of revenue, loss of profit, loss of data, loss of wages, loss of time, loss of sales, accruement of debts or expenses, injury to personal property, or injury to any person or any other type of damage or loss.
  • This limited warranty and any disputes arising out of or in connection with this limited warranty (“Disputes”) shall be governed by the laws of the State of Utah, USA, excluding conflicts of law principles and excluding the Convention for the International Sale of Goods. The courts located in the State of Utah, USA, shall have exclusive jurisdiction over any Disputes.
  • This limited warranty gives you specific legal rights, and you may also have other rights, which vary from state to state and jurisdiction to jurisdiction, and which shall not be affected by this limited warranty. This warranty extends only to you and cannot by transferred or assigned. If any provision of this limited warranty is unlawful, void, or unenforceable, that provision shall be deemed severable and shall not affect any remaining provisions. In case of any inconsistency between the English and other versions of this limited warranty, the English version shall prevail.
  • This warranty cannot be changed, assumed, or amended by any other person or agreement


  • 721 WEST 1800 NORTH, LOGAN, UTAH 84321,
  • USA TEL: (435) 792-4700
  • FAX: (435) 787-8268
  • Copyright © 2022 Apogee Instruments, Inc.

Documents / Resources

apogee INSTRUMENT MQ-620 Quantum Meter [pdf] Owner's Manual
MQ-620 Quantum Meter, MQ-620, Quantum Meter


Leave a comment

Your email address will not be published. Required fields are marked *