For inspiration: a great macro anemone shot.

The image above is from the Reef2Reef archives courtesy of @maidia ©2019, All Rights Reserved.

Successful reef-keeping is very much a numbers game. Specific gravity, pH, temperature, calcium & magnesium concentrations are important to know. Light intensity should also be monitored, and all values recorded for reference purposes. This short article will examine why measuring light intensity is important, and how to do it correctly.

Years ago, in the hobby, light intensity was referred to in vague terms such as ‘low’ or ‘high.’ These terms are so vague and subjective that they are virtually useless.

A leap forward came when lux meters with submersible sensors were introduced to the hobby. Although the numbers are of some use, and better than nothing at all, lux are weighted towards the visual perception of the human eye and report wavelengths towards the green portion of the spectrum (important blue and red light are under-reported.) Hence, the human eye is a poor judge of light intensity, especially when attempting to estimate blue and red light.

A quantum leap (pardon the pun) came when Apogee Instruments (Logan, Utah, USA) introduced a low-cost quantum meter in the late 1990’s. Before this, quantum meters were expensive (starting at $1,500 and up.)

A quantum (or PAR) meter reports Photosynthetically Active Radiation (PAR) in units called microMol per square meter per second (µmol˖m²˖sec). This is a count of photons (wavelengths of 400 nm to 700nm--violet to red--falling upon a square meter every second) and the metric is Photosynthetic Photon Flux Density (PPFD.) As a reference, sunlight PPFD can be as high as about 2,100 µmol˖m²˖sec during the summer at noon on a cloudless day.

Our understanding of Symbiodinium (zooxanthellae) found in tissues of healthy photosynthetic corals and clams has greatly increased since the mid-1990’s. At that time, many in the hobby believed that corals/zooxanthellae had unlimited adaptive capabilities to light intensity--the ‘more is better’ concept. We now know that this is not true, and too much light can be just as harmful as too little.

Generally, PPFD in a coral reef aquarium should be about 150 to 200 or so µmol˖m²˖sec at the bottom of the aquarium. There is little reason to exceed 500 µmol˖m²˖sec even when trying to make the coral produce colorful non-fluorescent proteins. Many fluorescent proteins will bleach when exposed to too much light, so it is advantageous to have high- and low-light zones within an aquarium.

It should be apparent that utilizing a quality device to measure light intensity is important. These are things to look for when buying a quantum (PAR) meter:

· The quantum sensor should be able to accurately report violet to red wavelengths. Spectral response of the sensor should be available.

· It should be cosine-corrected--if not, it is critical that the sensor is held exactly level under the light source.

· The sensor should be small.

· The sensor should be designed to correct for the immersion effect. If not, the manufacturer should provide correction factors. Without corrections for the immersion effect, readings could be off as much as 30%, perhaps more.

· Meters having data-logging capabilities is a plus. This is especially important when using LED lighting where intensity is programmed by the user, or in situations where lighting is incrementally increased and decreased during the photoperiod. See Figure 1.

· Use a submersible wand to avoid light reflected from your hand or arm. These are commercially available for the Apogee sensors but can be made from CPVC pipe (due to its smaller diameter.)


The image above is courtesy of @Dana Riddle ©2019, All Rights Reserved.

Figure 1. Light intensity of a 12-hour photoperiod.

As mentioned earlier, PPFD at the bottom of the aquarium should be about 150-200 µmol˖m²˖sec. All light sources degrade over time. This is generally recognized with ‘conventional’ light sources such as metal halide lamps and fluorescent tubes (T5, T8, T12, etc.) What is not as commonly known is that LEDs can also lose intensity in a time much less than their estimated useful life. This can be due to stresses on the diode due to age, heat, humidity, and other factors.

Knowing the light intensity a coral is conditioned to is important as well. When making a purchase at a local fish store, a quantum meter makes quick work of knowing how much light is required (especially when considering the light over a period of time, something allowed through use of data -logging.) Some on-line vendors can supply the light intensity in their grow-out tanks.

To recap, light intensity should be monitored in order to make sure minimum (and maximum) light intensities are met. All light sources are subject to degradation in output, and monitoring will ensure that replacement of the light source is done in a timely manner.

Quality meters are not inexpensive but considering the cost of some small coral fragments and the potential of preventing potentially lethal stress due to light shock, the purchase of a quantum meter should be considered an investment, not an expense. If this purchase is outside of the means of the reef-keeper, consider joining a local fish club where resources are pooled, and the purchase of a quantum meter is made for use by members.

For situations where monitoring light at extremely low or high intensities (<1 or >4,000 µmol˖m²˖sec), my instrument of choice is the Li-Cor quantum meter and submersible sensor. For almost all aquarium situations, I prefer Apogee Instruments’ MQ-510 meter.


About the Author: Dana Riddle

Dana Riddle is Reef2Reef's resident lighting expert, and he has his own lighting forum here. He has been keeping saltwater livestock since he was a child the 1960's.

He has published over 250 articles and a book called The Captive Reef. He regularly speaks at aquatic clubs and conferences, and was MASNA's Aquarist of the Year in 2011. Dana has a very sophisticated laboratory at home that includes an analytical balance, centrifuge, spectrometer, colorimeter, data loggers, Ocean Optics spectrometers for analyses of light, two PAM fluorometers, drying oven, incubators, water bath, chlorophyll meters, and electronic water velocity meter. His research into aquaria lighting is ongoing.


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