Introducing The New Apogee Original X Series Quantum Sensors

New Original X Quantum

Starting June 1, 2021, all Apogee "Original" quantum sensors are changing and will be replaced with the new Original "X" models.

Dear Customers,

Summary: Due to a supplier issue, all Apogee "Original" black-head quantum sensors are changing to a different internal detector on June 1, 2021. This will affect the spectral response of all original sensor and meter models as well as the calibration factor for the unamplified pigtail wire models. Model numbers will be changing as well - sensors with the new detector will be denoted by an “X” after the model number (for example, SQ-100X-SS and so on). All new “X” version sensors will be calibrated to sunlight; electric calibrated models will be discontinued.

Background: Apogee was informed in 2020 by a key supplier that the optical detector used in Apogee’s low-cost “Original” black-head quantum sensor line was being discontinued. This meant we needed to find a suitable replacement component that still kept the price low. The new detector we found is high quality, but has a slightly different spectral response (see graphs below) and signal output.

The new sensors will read accurately in sunlight, but may have errors under different electric light sources, especially LEDs. Correction factors for using these sensors under electric lights will soon be updated here:

The output signal size of the new detector is also changing, which means the standard calibration multiplier for the base non-amplified analog models (SQ-110 and SQ-120) will be changing from 5 to 10 µmol/m2/s per mV for the new SQ-100X sensor. This means those who integrate these particular models into systems will need to change the multiplier going forward. The outputs of amplified and digital models are unchanged

As you can see below, model numbers will be changing as well, with new models containing an “X” in the model number. One big change is that we will no longer have separate sun and electric calibration model pigtail sensors (meters will still have SUN and ELEC modes). The pigtail wire sensors will all now be calibrated to sunlight.

We hope the following information will answer all of your questions. Please feel free to reach out to us if you need any help.

Apogee Instruments Original Quantum Sensor Change Details

If you can't access the video via Youtube, click here.

Key Points
  • The new X version "original" quantum sensors have the same prices as the older original models.
  • A single calibration factor for all light sources makes the X versions more user friendly, eliminating the need for separate electric light and sunlight sensors.
  • The new "Original X" sensors have the same cosine response.
  • Original X sensors are calibrated to sunlight and measurement errors are small under broad-spectrum lamps.
  • The new Original X sensors have higher sensitivity to violet and blue wavelengths typically used in aquarium lighting. This can provide more accurate measurements of PPFD in aquariums after applying an appropriate correction factor for the light source being used.
  • Due to the imperfect spectral response of the Original X sensors, Apogee SQ-500 or SQ-610 quantum sensors are better suited to measure PPFD under narrowband LEDs and are recommended for this application.
 Old Original Sensor Model Numbers New Original "X" Sensor Model Numbers
MQ-100  MQ-100X - Integral sensor with handheld meter
MQ-200 MQ-200x - Separate sensor with handheld meter
MQ-210 MQ-210X - Underwater separate sensor with handheld meter



SQ-100X-SS - Unamplified sun calibrated sensor



SQ-202X-SS - Amplified 0-2.5 V sun calibrated sensor



SQ-204X-SS - Amplified 4-20 mA sun calibrated sensor



SQ-205X-SS - Amplified 0-5 V sun calibrated sensor

SQ-420 SQ-420X - USB output smart sensor
SQ-421-SS SQ-421X-SS - SDI-12 digital output sensor
SQ-422-SS SQ-422X-SS - Modbus digital output sensor
PQ-100 Package PQ-100X Package - microCache and SQ-100X sensor with 30 cm cable
PQ-110 Package PQ-110X Package - microCache and SQ-100X sensor with 2 m cable
Performance Comparison

The SQ-100X calibration factor was adjusted from 5.0 to 10.0 µmol m-2 s-1 per mV. This shift makes unamplified analog sensors more user friendly. The older Original sensors had different versions that were calibrated to measure either sunlight or electric lights, with a difference of 12 % between these calibrations. The SQ-100X can measure both sunlight and many electric lights with a single calibration. This eliminates the need for multiple calibrations or adjustments to data. A comparison of the SQ-100 series and SQ-100X specifications are found below.

  Specification Comparison*
  Old SQ-100 series New SQ-100X
Power Supply Self Powered
Sensitivity 0.2 mV per µmol m-2 s-1 0.1 mV per µmol m-2 s-1 
Calibration Factor 5 µmol m-2 s-1 per mV 10 µmol m-2 s-1 per mV 
Calibration Uncertainty ± 5 % 
Calibrated Output Range 0 to 800 mV  0 to 400 mV
Measurement Repeatability Less than 0.5 % 
Long-term Drift (Non-stability) 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 410 to 655 nm 375 to 630 nm 
Temperature Response 0.06 ± 0.06 % per C
Operating Environment -40 to 70 C; 0 to 100 % relative humidity; can be submerged in water up to depths of 30 m
Mass (with 5 m of lead wire) 90 g
Immersion Effect Correction Factor 1.08 1.15

* The only change to all other amplified and digital models is the spectral response and the immersion correction factor. 

Spectral Response
Spectral Response Comparison

Mean measured spectral responses of four SQ-100 (gray line) and four SQ-100X (green line) series quantum sensors compared to PPFD weighting factors (black line), equal weighting of all photons between 400 and 700 nm (dotted black line).

Light Source Spectral Errors (%)
 Light Source  SQ-100 Series SQ-100X Series
Sun (Clear Sky) 0.0 0.0
Sun (Under Plant Canopy) 0.8 11

Cool White Fluorescent (T5)

0.0 7.2

Actinic (Violet) Fluorescent (T5)

-24.4 6.9

Blue Plus T5

-6.3 16.9

High Pressure Sodium

0.8 3.2
Metal Halide -3.2 6.9
Cool White Fluorescent LED -2.4 7.3
Red LED (667 nm) -61.9 -56.7
Warm White LED -8.8 -7.8
Blue LED (442 nm) -15 12.1

Mean measured PPFD differences [%] from four SQ-100 (middle column), and four SQ-100X (right column) series quantum sensors under various light sources relative to the mean of two Apogee SQ-500 quantum sensors. The SQ-100 differences were measured using both sunlight (not underlined) and electric light (underlined) calibrations, while the SQ-100X differences were measured using a single calibration.

Cosine Response

Cosine Response Comparison

The mean directional response of four SQ-100 (blue line) and four SQ-100X (green line) sensors determined by comparison to the mean of seven SQ-500 quantum sensors.


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Thanks for being a loyal customer and we hope your sensors serve you well for many years to come!