Proper lighting is one of the most important aspects of successfully maintaining a reef aquarium, yet many hobbyists leave it to chance. I’ve pondered why this is for years--perhaps it is because it can be a complicated subject with many variables involved (compare this to water chemistry where there are rigid numbers such as calcium and magnesium at 400-450 and ~1,500 parts per million, respectively.) It could be that light meters tend to be expensive, and once light patterns in an aquarium are established, a meter can sit on a shelf collecting dust. Whatever the case, measurement of light is still critical to the husbandry of a coral reef tank.

During a recent visit to the garden section of a home improvement store (Lowe’s) I saw a meter that claimed to measure light intensity. It also reportedly measures soil pH and moisture (the latter being the reason I purchased it.) This meter is imported by Panacea Products, Columbus, Ohio and is sold by the name of HoldAll Moisture, Light and pH meter (Model 60182L: See Figure 1.) After testing for soil moisture content of the carnivorous plants I grow and being generally satisfied with the results, my attention turned to the light meter. Could it possibly be of any use for aquarium use? This thing sells for $9.99, and it isn’t water proof. I wasn’t optimistic but decided to test it against a $1,500 Li-Cor quantum meter.
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Figure 1. The HoldAll device features a light meter.

In this initial test, I mounted a jig holding the Li-Cor quantum sensor (calibrated for ‘air’ measurements) beneath a dimmable LED fixture rated at 20,000K manufactured by BuildMyLED. After making the PAR measurements at the full range offered by the dimmer (12.5% to 87.5%), the procedure was repeated with the HoldAll sensor in the same position as the Li-Cor’s. Results are shown in Figures 2 and 3.
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Figure 2. Light intensity (PPFD or Photosynthetic Photon Flux Density) generated by a dimmable LED strip light rated at 20,000K.


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Figure 3. Light intensity as reported by the HoldAll device.
The HoldAll meter reports light intensity in arbitrary units (not lux or PPFD) on a scale of 0-2,000 and its width is about ¾”, which makes estimating the exact reading difficult. When dividing the HoldAll’s measurements by PPFD determined by the Li-Cor meter, the divisors ranged from 1.96 to 2.81 (with an average of 2.43.)

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Figure 4. A closeup of the light scale on the HoldAll device. It is less than 1” wide.
The HoldAll meter cannot be made waterproof without an unreasonable amount of effort, hence measurements made in air is realistically the best it can do. But that is not to say that it doesn’t have utility. Use this procedure to estimate light intensities at depth:

Take a measurement directly beneath your light source and as close to the water surface was possible. Divide the reading by 2.43 – this will provide a rough estimate of PPFD (or PAR, if you will.) Then multiply this estimate by these factors to obtain light a rough estimate of light intensity at various depths:
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Example: The HoldAll device reports a light measurement of 1800 directly below the light source and as close to the water’s surface as possible. Divide 1800 by 2.43 to obtain an estimate of PPFD. In this case, PPFD is 741 µmol·m²·sec. If you want an estimate of PPFD at a depth of 18 inches, multiply 741 by the coefficient of 0.139. Hence, PPFD at 18” is 102 µmol·m²·sec.

This procedure is not without fault. It will work best with line sources of light such as T5 (or other) fluorescent lamps, LED arrays with multiple lights (such as Orphek, ReefBreeders, etc.) It will work with point sources (such as metal halides and ‘puck’ LEDs such as Noopysche, Radions, etc.) if PPFD estimates are needed directly below the light source.

On the upside, a light measuring device for less than $10 is an incredible value.

I’ll try to look at how spectral qualities affect the HoldAll’s readings soon.

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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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