CORRECTING FOR THE SPECTRAL ERRORS OF COMMON ELECTRIC LIGHTS
Because 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.
Sufficient 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 capabilities 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 µmol 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. You can also get great information by asking a professional aquarist or by becoming involved in a local reef club.