The author’s two PAR meters. On the left, a Li-Cor Model LI-189; on the right, an Apogee Instruments’ Model QMSW-SS (with optional BNC connector). Both show wear and tear from years of use, but have held up remarkably well. The black threaded fitting below the Apogee meter is part of a homemade underwater housing.
I began the research for this article in 1999 and only recently have had the time and interest to complete a comparison of my two quantum meters – One made by Li-Cor, Inc.; the other by Apogee Instruments. This comparison includes measurements made across a wide range of light intensities and spectral qualities (4,000 – 20,000 K metal halide lamps). This particular Apogee meter has a switch for alternation between measurements of ‘Sunlight’ and ‘Electric Lamps’, and I can make some casual recommendations for which to use with particular metal halide lamps.
This article is also in response to various challenges made to the validity of PAR measurements made with Apogee Instruments’ relatively inexpensive meter. (For those with an inclination to read these debates, see Advanced Aquarist’s ‘Forums’ section for comments about Mike Kirda’s article “Light in the Reef Tank: Some Real Data” and an article by Miguel Olaizola and myself entitled “Lighting the Reef Aquarium: Spectrum or Intensity” – Advanced Aquarist Online – Volume 2, Number 8, and Volume 1, Number 2, respectively).
PAR meters (also called quantum meters) are the preferred instruments for measuring light intensity when photosynthesis is involved. Ideally, quantum meters measure (and report as equal) the amount of energy in light wavelengths between 400nm and 700 nm, and report it in units of micromol per square meter per second (µmol·m²·sec). Full intensity sunlight is generally ~2,000 µmol·m²·sec. ‘Micromol’ is the preferred unit for reporting purposes, as opposed to ‘microEinstein’, a unit that is commonly used in older literature. However, any measure of PAR or PPFD (either µmol·m²·sec or µE·m²·sec) are still somewhat of ‘outlaw’ units, not recognized as SI units, and something of buzzwords to those obsessed with botany or phycology.
However, the Li-Cor series of quantum meters has been the standard for use in measuring photosynthetically active radiation for a number of years. If price is indeed equal to quality and accuracy of light measurements, then the Li-Cor meter will beat most any other competitor. But does ‘price’ equal ‘right’?
Perhaps we should examine each meter’s features before we get into the comparative data. Let’s begin with:
On the face of it, sensors are simply light-measuring photodiodes within protective housings. Their jobs are relatively simple – collect light energy (photons) and convert it to measurable current (electrons), and as long as the sensor’s housing is waterproof, it would seem that all sensors are pretty much equal. Not so.
Collector Design: 2 pi v. 4 pi
One of the first considerations is the ‘type’ of light you’ll want to measure. For our purposes, there are two sorts of light – downwelling (such as that emitted from the sun or an artificial source) and upwelling (such as that light reflected from a white sand bottom, live rock, etc.). Unless you are growing phytoplankton (which can collect light from all sides), you’ll probably want to obtain a sensor that measures only downwelling light. This is called a 2 pi sensor, and is the type used for the measurements reported in this article. Li-Cor offers a 4 pi sensor (Model Li-193SA) which resembles a standard frosted light bulb in appearance – it measures up-welling and down-welling light. Upwelling light can contribute a significant amount to the total amount of light. This should not be a concern with corals. If one wishes to measure upwelling light, simply point the sensor toward the bottom of the tank to get an estimate of reflected light. According to the measurements made by my mentor in lighting (Pete Mohan, then of Sea World Ohio), there is not a deal of difference in measurements made in aquaria by 2 pi and 4 pi sensors.
Spectral response has been the loudest argument used in debates of comparing measurements between brands of quantum meters. A quantum meter sensor’s ideal spectrum response would report as equal the energy in each wavelength between exactly 400nm and 700nm. However, neither the Li-Cor nor the Apogee response is ideal. The Apogee underreports blue wavelengths more so than the Li-Cor. The cutoff in the red portion of the spectrum is at ~650nm for the Apogee; the Li-Cor sensor extends its range (albeit slightly) into the near-infrared (cutoff at 700nm, but with a slight response to ~720nm). Near-IR radiation can cause substantial errors in certain cases.
Apogee maintains in their advertisements that most lamps’ spectral qualities will negate any ‘deficiencies’ in their sensors’ responses. We’ll later see if this claim is valid!
Cosine Collector and Correction
Both Apogee and Li-Cor sensors are cosine corrected to 80 degree angle of incidence. In plain English this means that the sensors can detect radiation coming into the sensor at angles of up to 80 degree from the center of the sensor ‘window’ or diffuser. Sensors without cosine correction will not accurately report intensities of diffused light (such as that of cloudy days, low sun angles, from fluorescent lamps). The Li-Cor Instruction Manual states that cosine correction overcompensates in air and under-compensates in water.
Sensor Construction and Photodiode Type
The Li-Cor sensor is robust, with the housing being made of copper alloy. There is a diffuser of white acrylic protecting internal filters and a silicon-based and blue-enhanced photodiode. These filters and diffuser are long lived. I had my LI-192SA sensor rebuilt after 9 years of heavy use (and 50,000 measurements). The rebuild was necessary due to ‘fogging’ of the acrylic diffuser.
The Apogee sensor is much smaller than the Li-Cor, and is made of plastic (both a plus in closed system aquaria with strong coral growth). It is 24mm in diameter and 25mm tall. The photodiode is made of gallium arsenide.
The Immersion Effect refers to light loss caused by different refractive indices of air and water – there is less internal reflection of light within an immersed sensor. Li-Cor quantum meters have a water calibration factor in order to overcome this loss. Apogee’s meters do not offer such an option.
And while on the subject of immersion, one should be concerned about the water resistance of the submersible sensors. Li-Cor advertises a pressure rating of the 2 pi LI-192SA Underwater Sensor as 800 psi (54 bars); The 4 pi Spherical LI-193SA is rated for 500 psi (34 bars). Apogee does not advertise a pressure rating, but I can speak from personal experience that their sensor is waterproof to depths of 37 meters (~120 feet) during SCUBA dives (the meter itself is in a waterproof housing). Suffice it to say that I doubt if either of these sensors will ever fail in even the deepest aquarium!
Effects of Temperature
Temperature affects meter performance and sensor response. Apogee advertises their instruments to perform in temperatures of up to 50 degree C (depending upon non-condensing relative humidity); Li-Cor advertises their meters to withstand temperatures of up to 55 degree C and 90% RH (non-condensing) are appropriate for proper operation.
Instrument Range and Resolution
The Apogee quantum meter reports up to 1,999 µmol·m²·sec (in increments of 1 µmol·m²·sec), while the Li-Cor reports on the order of a magnitude higher – 19,999 µmol·m²·sec, with resolution ranging from 0.01 µmol·m²·sec to 1 µmol·m²·sec, depending upon light intensity. It is possible for high wattage metal halide lamps to produce more radiation than the Apogee meter is capable of reporting. There are some options – one being use of a neutral density filter to attenuate the amount of light entering the sensor. A cheaper (and perhaps more convenient way) is to use a small piece of non-metallic window screen. Compare readings in a lower light environment to arrive at the appropriate correction factor for use in extreme light conditions. It is also possible to max out the Li-Cor meter – we managed to do that while beta-testing a 1,000 watt Fusion sulfur lamp a few years ago.
Portability and Battery Life
Both brands of quantum meters are light weight and easily portable. They use one standard 9v battery, and battery life is excellent (100-200 hours). A ‘low battery’ warning is standard on both meters.
Periodic Factory Calibration
Sensor response drifts as a result of use more than time (according to Apogee). However, both meters should be periodically recalibrated by the factory. Li-Cor recommends recalibration every 2 years (and charges $75 for the service). Apogee charges $40, but instead of recommending a recalibration period etched in stone, they offer this neat trick for meters with ‘average’ aquarium use: Get a new frosted, incandescent light bulb. Measure the output of this lamp in a dark room, record the results and store the bulb. Measure the lamp output (under standardized conditions) once a year. Return the quantum meter when results are subjectively unacceptable.
In my case, I have the luxury of comparing the Apogee sensor’s response to that of the Li-Cor (which is factory recalibrated every two years). The Apogee hasn’t been calibrated since purchase.
Apogee has cut production costs by utilizing a number of measures – the meter’s plastic housing is not as heavy-duty as the Li-Cor’s. (My first, and incorrect, impression of the Apogee meter was ‘toy-like.’) Battery replacement for the Apogee involves removing two small screws in order to separate the halves of the housing; Li-Cor has a convenient sliding battery hatch. As an aside, the ability to report intensities greater than 1,999 µmol·m²·sec, would be a plus. The sensors also differ. The Apogee contains a less expensive gallium photosensor, and the sensor housing is small and made of plastic (actually an advantage in cramped aquaria).
The Apogee Model QMSW-SS retails for $298.00, plus tax and shipping. I had Apogee build my meter with an option BNC connector, so I could remove the 2 meter cable and sensor for storage. This option was $20 in 1999.
I paid $1,100 for the Li-Cor LI-189 quantum meter, underwater cable (3 meters in length), LI-192SA sensor, and nylon carrying case in 1995.
There are obviously major differences between the two meters in construction. But what about the quality of the actual light measurements? Are they comparable?
A comparison of measurements was made using 5 metal halide lamps ranging in advertised Kelvin ratings of 4,000 to 20,000. This is how it was done:
The comparison protocol was quite simple – a jig was made of degree ” PVC pipe, fittings and acrylic. Rubber bands held the sensors closely together and helped maintain matching vertical and horizontal orientations. The sensors’ cables were attached to the PVC pipe with white nylon cable ties (See Figure 2). In this manner, simultaneous measurements could be made without worry of error due to fluctuating line voltage and brightening/dimming of the lamps’ outputs.
A white plastic bucket was filled with tap water which allowed an effective depth of ~18cm for the immersed sensors during the water comparisons. The PVC pipe created remarkably little water surface agitation; however, the water was given a few seconds to ‘flatten’ before measurements were recorded.
To avoid a major source of error, the sensors were placed parallel to the long axis of the lamps’ arc tube. In this manner, I could minimize the effects of light distribution and resulting radiation gradients. However, the sensors were moved slightly during the course of taking each measurement to determine if gradients were present – No drastic shifts in light intensity were seen.
The Li-Cor meter has calibrations points for ‘air’ and ‘water’ measurements, and the meter was calibrated accordingly. Factory recalibration of the meter, along with sensor rebuild, was performed in November, 2004 – well within the recommended two-year recalibration period
Water and sensor temperatures were monitored with a non-contact, infrared thermometer during testing. Temperatures ranged from 26.6 to 28.3 degree C (well below the maximum temperature ratings of either sensor).
The PAR of 5 metal halide lamps was measured with the Li-Cor meter (appropriately calibrated for ‘air’ or ‘water’) and the Apogee (switched between ‘Sunlight’ and ‘Electric Lamp’ measurements).
I didn’t spend a lot of time investigating how the two brands of sensors respond to light measurements made in air. However, Figures 3 and 4 demonstrate that the two sensors compare favorably under conditions of different spectral quality and intensity in air. But how do the underwater measurements of the two meters compare? We’ll compare results of individual lamps, and then summarize the findings ‘In Closing’.
Underwater Measurements – Philips 4,000K Metal Halide Lamp
Figures 5 and 6 show the response of this lamp (which happens to be the exact lamp used in the PAM experiments reported in the February 2002 Advanced Aquarist. The results of the Aquarium Lighting System – ALS – 12,000K lamp are reported below. This too is the same exact lamp used in the referenced PAM experiments. Both of these lamps have been in storage, and unused, since those experiments).
Underwater Measurements – Iwasaki 6,500K Metal Halide Lamp
Figures 7 and 8 chart the two sensors’ responses for a perennial favorite – the Iwasaki 6,500K ‘Daylight’ Lamp.
Underwater Measurements – Ushio (aka AB/Hamilton) Metal Halide Lamp
The German-made Ushio lamp has been marketed under a number of private labels, and has earned a reputation of quality and reliability. I use the label name of ‘AB 10000K’ and report the results in Figures 9 and 10.
Underwater Measurements – ALS 12,000K Metal Halide Lamp
As mentioned earlier, this is the exact lamp used in the PAM fluorometer article in Advanced Aquarist’s February 2002 issue. It has been archived since use in that experiment.
Underwater Measurements – Coralife 20,000K Metal Halide Lamp
I used this lamp for several reasons. Judged visually, it is the ‘bluest’ metal halide lamp I have in my collection. It has some use on it, and since it is used, represents a blue lamp of depreciated spectral quality. In other words, this lamp is representative of ‘real life’ situations. Theoretically, we should see the largest discrepancies between the competitors in an almost monochromatic blue light source. Incidentally, this is the ‘old style’ Coralife lamp, which was replaced by another model about 5 years ago. See Figures 13 and 14.
The Apogee quantum meter’s measurements were surprisingly close to those obtained with the Li-Cor meter. More surprisingly was that (except for the Philips 4,000K lamp) the Apogee’s measurements were in best agreement with those of the Li-Cor’s when in the ‘Sunlight’ calibration mode.
|Sunlight Calibration||Electric Lamp Calibration|
The actual degree of error between measurements obtained with the two brands of sensors is difficult to describe. Based simply on percent error, these can be misleading, as intensity plays an important part. That is, at low light intensities, a minor difference of even 1 µmol·m²·sec can equate to a profound percentile difference. On the other hand, the difference between actual measurements generally increases with light intensity (and the percent difference generally decreases). It is tempting to describe an average range of light intensity found in aquaria, but this exercise is ultimately an exercise of futility. For this reason, Table 2 reports the average percent difference across the entire range of PAR measurements.
This Table shows the percent difference between the Apogee’s 2 light measurements in its 2 calibration modes when compared to those measurements obtained with the Li-Cor instrument. Generally, the Apogee’s readings were lower than the Li-Cor’s, except for high readings of the Philips 4000K lamp (Sunlight Calibration), and mixed readings (higher and lower, Sunlight Calibration) with the AB (Ushio/Hamilton) 10000K lamp.
|Sunlight Calibration||Electric Lamp Calibration|
|4,000K||8.25% High||1.94% Low|
|6,500K||1.6% Low||11.0% Low|
|12,000K||3.9% Low||7.8% Low|
|20,000K||7.3% Low||12.0% Low|
*Yes, 0.36%, based on measurements that were slightly higher, and slightly lower, across a broad range of intensity than those reported by the Li-Cor LI-189 meter. As expected, the difference in readings was greatest in the almost monochromatic 20,000K lamp.
In my opinion, objections to the validity of the Apogee’s measurements based strictly on the issue of spectral response are unjustly founded. The largest source of error lies squarely (but potentially) with the user of the instrument, and not inherent optical qualities. User error includes sensor orientation, sensor cleanliness, instrument condition (care, maintenance, calibration) etc.
It would seem that any reports of PAR and PPFD measurements should include the instrument’s last calibration date or at least a report of deviation from a standard light source or calibrated quantum meter.
The Li-Cor quantum meter LI-189 has been replaced with a newer model, the LI-250A. I would expect Li-Cor’s same high quality is present in this newer version. The LI-250A includes an ability to toggle between ‘air’ and ‘water’ calibrations with the simple push of switch.
Apogee has upgraded its sensor to include a self-cleaning dome, which also assists in cosine correction. I have been informed (personal communication with Shaun, Apogee Instruments) that they are working on improving the spectral response of their photodiodes.
One may wonder why I have purchased two quantum meters – the Apogee meter was obtained for use in some of the early respirometer work, since its sensor is more compact than that of the Li-Cor, and is especially of use in tight or cramped spots. Later, the Apogee meter would be used for investigative light measurements on Hawaiian reefs. It is inexpensive enough to consider an expendable item should I ever flood my homemade underwater housing.
For further information, see these websites:
- Joe Burger’s Cnidarian Reef: http://www.cnidarianreef.com/lamps.cfm
- Apogee Instruments: http://www.apogee-inst.com
- LI-COR, Inc.: http://www.licor.com
- LI-COR, Inc., 1990. LI-189 Quantum/Radiometer/Photometer Operating Manual. Publication Number 9003-61. Lincoln, Nebraska. 18 pp.
- LI-COR, Inc., 1990. LI-COR Underwater Radiation Sensors, Type SA Instruction Manual. Publication Number 8609-57. Lincoln, Nebraska. 23 pp.
- Kirk, J.T.O., 1994. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, Canberra. 509 pp.