Measuring PAR to ensure proper light levels for underwater photosynthesis
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
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
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
Electric Calibration Error [%]
Blue (448 nm peak, 10 nm FWHM)
Green (524 nm peak, 15 nm FWHM)
Red (635 nm peak, 10 nm FWHM)
Red, Blue Mixture
Red, Green, Blue Mixture