I hate when the manufacturers still advertise the led's max power which is never reached. At least put on a dimmer, it can't be that expensive to make.
You said it...........couldn't agree more.
But why is 5w diodes run at a lower actual current than 3w diodes, percentage wise?
What I meant is the fact that they either can't take a 50-60% power load, or the light makers are hedging their bets on life span. I'm not saying they don't do the job they are targeted at. If Cree can build chips that run at 100% with the same life span, why can't these do 50-60%? something is fishy when you buy a light with 5 watt chips that are run on 2.1 (-/+ 5%..... ) watts... or they are using chips that should have been binned rejects....
Wish I had a PAR meter. I'd love to do a side by side output test between a 5 watt chip light and a three watt chip light running at the levels used in these grow lights.
Me too. I came across this article
http://www.advancedaquarist.com/2013/2/equipment
Aquarium Equipment: PAR Meters and LEDs - How Accurate are the Measurements? A Comparison of Three Meters and Lux to PAR Conversion Factors for LEDs
By Dana Riddle
How do PAR meters stack up to one another? Dana delves into the data comparing and contrasting three light meters and gives us his take on the results.
Comments
It was 2005 when I last wrote an article presenting results of a comparison between Photosynthetically Active Radiation (PAR) meters, and the lamps used during testing were metal halides of various kelvin ratings (see Riddle, 2007). In those days, the use of light-emitting diodes (LEDs) for aquaria was something discussed by only a few. Nowadays, use of metal halide lamps is much less popular and usually seen over larger aquaria or those of die-hard fans, yet, to my knowledge, there have been no updates on the utility of different brand PAR meters and their responses when judging output of LEDs.
This article will compare the responses of three quantum meters when measuring LED light output. Specifically, these are meters manufactured by Apogee Instruments™ (model QMSW-SS; Logan, Utah), Li-Cor Biosciences™ (LI-1400 datalogger and LI-189 sensor; Lincoln, Nebraska) and Spectrum Technologies™ (FieldScout; Plainfield, Illinois).
Product Details
Li-Cor LI-1400 Quantum Meter and LI-189 Sensor Li-Cor Biosciences (Lincoln, Nebraska, USA) is noted for quality instruments, and their meter/sensor combinations have gained wide acceptance within the scientific community. Quality comes at a price (the referenced combination currently costs more than $3,000). The sensor construction is an intricate one - see Figure 1. In addition, the sensor is relatively large and the cord exits the bottom. These facts restrict its use to larger aquaria.
Figure 1. Typical construction of an expensive PAR sensor, such as Li-Cor's. From Kirk, 2000.
Apogee Quantum Meter Apogee Instruments (Logan, Utah, USA) manufactures entry-level PAR meters and sensors, and many hobbyists have found favor with them due to their affordability. The sensor is relatively small and its cord exits the side making it ideal for use in tight quarters (such as aquaria).
FieldScout Quantum Meter Spectrum Technologies (Plainfield, Illinois, USA) manufacturers a number of products aimed at the agricultural/horticultural markets. Although the meter tested here is the FieldScout Light Meter, the sensors are interchangeable with other Spectrum products (such as their wonderful WatchDog datalogger). The sensor tested here was custom-built for my lab for use when testing artificial light sources. Spectrum does not recommend their quantum sensor for use with LEDs but I wondered just how much of an error there actually is, hence I have included it in this review.
In all fairness, we're comparing an expensive instrument (the Li-Cor setup costing over $3,000) to relatively inexpensive ($300-$400 or so) units. A calibrated light source would be needed to accurately judge the responses of all three meters. This luxury was not available for this review, hence the Li-Cor meter - based on the advertised responses of all three meters - will be considered 'correct'.
There are several things that can affect a quantum meter's reading, these include:
- Spectral sensitivity of the sensor
- Spectral quality of the light
- Sensor Cosine Correction
- Sensor Construction (2 pi or 4 pi)
- Testing medium (air, water, etc.)
- Condition of the sensor (physical damage, age - 'fogging' of optical components, cleanliness)
- Sensor/meter calibration
- Temperature
- Light source used for calibration by the manufacturer
These terms will be used throughout this article:
Glossary Actinity Error: A perfect PAR sensor would be equally responsive to all wavelengths of light between 400nm and 700nm. In practice, this is not possible and response difference between a real sensor and a theoretical one is called the actinity error. Various sensors over- or under-report blue wavelengths while red wavelengths are often under-reported.
Correlated Color Temperature (CCT): is a specification of the color appearance of the light emitted by a lamp relating its color to the color of light from a reference source (a blackbody) when heated to a particular temperature, measured in degrees Kelvin (K). The CCT rating for a lamp is a general "warmth" or "coolness" measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below 3,200 K are usually considered "warm" sources, while those with a CCT above 4,000 K are usually considered "cool" in appearance.
Cosine Correction: A light sensor should be able to accurately measure light at angles to ~90 of normal incidence (0), and a cosine-corrector allows this. Two cosine-correction types exist - one type is a hemispherical plastic diffuser dome (used by Apogee and Spectrum Technologies), while the other is a plastic cylinder (that should rise slightly above its housing in order to properly collect light, which the Li-Cor sensor does).
All sensors are advertised to be cosine-corrected, meaning their response will be the same to a beam of light, regardless of that beam's angle of incidence to the sensor (up to a point. Li-Cor advertises their sensor to be correct for light falling at an 80 angle from normal while Apogee states their sensor is ±1% at a 45 angle (from zenith) and ±5% at a 75 degree angle from zenith).
Full Width Half Maximum (FWHM): This is an important concept in light measurement. It is simple and easily defined. While the spectral width of the light source could extend for some distance, the maximum is easily determined as is the half-maximum. FWHM is generally used to define peaks and half-maxima of relatively narrow bandwidths (such as LEDs and other 'specialty' cases such as fluorescence). See Figure 2.
Figure 2. Full Width Half Maximum (FWHM) is an important concept, especially with narrow bandwidth light sources such as LEDs. In this case, the peak is at 500nm with a FWHM of ~50nm (475-525nm).
FWHM is not used for broadband light sources (such as sunlight and most artificial light sources). Let's take an example of why FWHM is important. See Figure 14 - it is the spectral characteristics of a combination of blue and white LEDs. This example would share the FWHM characteristics of a blue LED while ignoring the full spectrum characteristics.
Immersion Effect: Reflection of light within a sensor immersed in water is less (relative to a measurement made in air) and results in a greater loss of light. This is due to the refractive indices of plastic and air or water. Hence, more expensive devices (such as the Li-Cor) allow for an 'air' or 'water' calibration to overcome the immersion effect. The Apogee and Spectrum Technologies meters do not offer this option.
Integrating Sphere: A device used in measuring light and especially useful when determining flux or spectra of LEDs. Basically, it is a hollow sphere with a diffusive interior coating. Two ports (one for the LED and the other for a light sensor) are at a 90 angle to one another.
Lambertian Reflectance: Diffuse reflectance is that which appears to be of the same brightness regardless of the observer's viewing angle. Labsphere's Spectralon (a fluoropolymer) offers an almost ideal Lambertian surface. Barium sulfate is a less expensive - but less Lambertian - material.
Light-emitting Diode (LED): A light emitting device consisting of a positive/negative junction where a small amount of electrical current excites metallic compounds doped on a small 'cup'.
Photosynthetically Active Radiation (PAR): Light energy powers photosynthesis. This light's bandwidth has been standardized to that electromagnetic energy between 400 and 700nm (violet to red) per area unit (often 1 square meter) per time unit (usually 1 second). PAR is reported as Photosynthetic Photon Flux Density (PPFD) in units of micromole photons per square meter per second (µmol·m²·sec).
Reflectance: The ratio of the total amount of radiation, as of light, reflected by a surface to the total amount of radiation incident on the surface.
Two pi Sensor; Four pi Sensor: Sensors that collect light only from the direction the sensor is pointed is called 2 pi. A scalar sensor collects light from all directions. A 4 pi scalar sensor resembles an incandescent light bulb. See Figure 3.
Figure 3. Two types of Li-Cor PAR sensors. A 2-pi sensor is on the left (like the one used in this report). A 4-pi sensor is to the right.
Spectral Responses of Three PAR Sensors
Understanding the spectral sensitivities of different PAR sensors is helpful in understanding how accurate measurements will be, especially when dealing with narrow bandwidth light sources, such as LEDs. For our purposes, there are two types of sensors - silicon and gallium arsenide phosphide (GaAsP). The Li-Cor sensor is the silicon type, while the Apogee and FieldScout sensors appear to be made of gallium arsenide phosphide. Figure 4 shows the spectral sensitivity of the Apogee meter, Figure 5 the FieldScout's, and Figure 6 that of the Li-Cor. Unfortunately, Spectrum Technologies does not provide the relative ideal response of their sensor and we therefore must make some assumptions about the actinity errors. Figure 7 is a side-by-side comparison of the Apogee and Li-Cor responses.
Figure 4. The Apogee quantum sensor underestimates violet/blue and red wavelengths. Apogee advertises their sensor is responsive to light wavelengths in the range of 409nm to 659nm. After Apogee Instruments' website.
Figure 5. Response of the Field Scout Quantum sensor - it appears to be an unfiltered gallium arsenide phosphide (GaAsP)-based photo-sensor. No ideal response information is available. After data on Spectrum Instruments' website.
The Apogee meter apparently uses a gallium arsenide phosphide (GaAsP) based sensor with a lens/filter in order to slightly correct the sensor's response. However, it is generally agreed that this type of sensor underestimates violet/blue light (400-500nm) and red wavelengths above 650nm.
Figure 6. The Li-Cor quantum sensor underestimates violet (410-420nm) slightly, and red light (690-700nm). This sensor's response is the gold standard in botany/phycology research fields. After data on the Li-Cor website.
Figure 7. A comparison of the Apogee and Li-Cor sensors' responses. The Spectrum meter is not included due to little available information on its spectral response in relation to ideal response.
Effects of Temperature
Apogee's calibrates their quantum sensors at 68F (20C). It reads 0.6 percent high at 50F (10C) and 0.8 percent low at 86F (30C) - see Figure 8. Li-Cor states a change of ± 0.15% per °C (maximum).
Figure 8. Effect of temperature on Apogee PAR measurements. Calibration is done at 68F (therefore, 'zero' error). At the temperature of most tropical reef aquaria, the reading would be 0.4-0.5% low.
Relative Humidity
When making sunlight measurements, the amount of water vapor (humidity) in the atmosphere can cause lower than expected readings. See here for details:
http://clearskycalculator.com/model_accuracyPPF.htm#RH
Note that all reported measurements were made in the air and the impact of the ultimate humidity - water - will impact meters' responses.
'Sun' and 'Electric' Measurements
In the models tested here, Apogee and Spectrum meters offer two measurement modes to overcome deficiencies in the spectral responses of their sensors. Testing revealed that, on average, there is a difference of about 10% between the two modes. However, spectral quality decides which mode is best for a given light source.
Our testing begins with:
Response of Meters to Sunlight
Figures 9 and 10 show the meters' responses to broadband light energy (sunlight, during an overcast morning) and the spectral quality of that light, respectively. As we can see, all meters do a reasonable job of reporting PPFD.
Figure 9. A comparison of the Apogee, FieldScout, and Li-Cor sensors' responses to the light field on a cloudy Hawaiian morning. See spectral characteristics in Figure 10. At this intensity, the Apogee reads ~10% high, and the Field Scout reads ~13% when compared to the Li-Cor measurement." style="margin-right: 0px; margin-left: 0px; padding-right: 0px; padding-left: 0px; outline: none; border-bottom-width: 0.1em; border-bottom-style: solid; border-bottom-color: rgb(204, 204, 204); color: rgb(46, 122, 164) !important;">
Figure 9. A comparison of the Apogee, FieldScout, and Li-Cor sensors' responses to the light field on a cloudy Hawaiian morning. See spectral characteristics in Figure 10. At this intensity, the Apogee reads ~10% high, and the Field Scout reads ~13% when compared to the Li-Cor measurement.
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Figure 10. Sunlight spectral quality on a cloudy Hawaiian morning
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Response of Meters to Individual LEDs
As we have seen, each of the PAR meters have done a reasonable job of reporting PAR values of sunlight, even though their sensors' spectral sensitivities vary dramatically. Results of LED testing will now be presented.
Blue LEDs
Blue LEDs are ubiquitous in lighting designed for reef aquaria and are often combined with LEDs emitting 'white' light ('white' LEDs are blue LEDs to which a phosphor has been added. This phosphor absorbs some of the blue light and fluoresces it in a broad spectrum). Two blue LEDs were examined. See Figures 11 and 12 (notice the differences in the FWHM of the two).
Figure 11. This blue LED's output is maximal at 449nm, with a FWHM of ~430-480nm.
Figure 12. Acan Lighting's blue LED spectral quality (peak emission at 454nm; FWHM=443-467nm). Analysis of Corrected Color Temperature (CCT) revealed these LEDs were at least 50,000 K (measurements bounced between 50,000 and , or 'off the scale').
The following Figure (13) shows the PAR measurements of the Acan blue LED.
Figure 13. Not surprisingly, there are significant differences among the reading of the 3 PAR meters. These are not maximum PAR values.
