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Hi everyone!

Thank you for taking a look on the forum at the Adelson illusion (Adelson 1993) shown in Figure 1 (left)! It created some great discussion that caused me to redraft the entire follow-up article!

Indeed, we did couch the question differently than Edward Adelson. We asked the following question: “Which square, A or B, do you think is receiving more light or more intensity?” We edited that question quickly--really to match the topic of the article. Which is--how do you know where your tank receives more light?

As it turns out, your eyes are not very good at being a physical light meter.

So, how do you figure out where most of the light is in your tank? Most of us have been doing this for many years by using our eyes. But, is that good enough?

The Adelson illusion actually illustrates two points, as noted in the discussion.

1. The original question was, “which square is brighter, A or B?” Asked that way, my answer was at first “B”. Indeed, the two boxes, A and B, are exactly the same color (Figure 1, right). The point of this illusion was to emphasize that context and contrast influence our judgement of intensity.

2. Interestingly, if you rephrase the question as “which is receiving more light, A or B?”--most of us will come to a different conclusion. The shadow illustrated in the figure convinces us to cognitively incorporate the laws of physics to conclude that A receives more light, despite the fact that A and B are the same color and intensity and despite the illusion that “A” appears perceptually darker than “B”. This is actually consistent with a 19th century hypothesis of von Helmholtz which suggested that we could unconsciously come to conclusions utilizing visual cues (such as shading) despite the brightness or luminance of objects within the scene (von Helmholtz 1924).

A complete discussion of the Adelson illusion can be found on the web pages here:

Shadows are not a figment of your imagination!

If you were tasked with finding all of the shady regions in your tank, I’m sure you could find them. It is indeed that experience which influences your interpretation of the Adelson illusion.

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Figure 2 was created and provided by Greg Gdowski, ©2019, All Rights Reserved.

If you can see a shadow in your tank, so will your corals in terms of growth. Figure 2 is a photo of a red/orange Montipora monasteriata in my tank. When that coral grew out over the last year, it encountered a shady region created by the rocks above. What is remarkable is that the shape of the edge of the coral nicely followed the contour of the shade. As it encountered less light, it stopped growing in that direction.​

While shady regions can be readily identified in your tank, how can we assess the amount of light reaching the rest of the tank? For example, is the rest of that Montipora receiving the same amount of light? Let’s try another test to see how good your eyes are at addressing that question.

Look at the left panel in Figure 3. Are the grey boxes to the left (A) and right (B) the same color, or are they different? Is the color uniform in both A and B? This particular illusion is known as the Craik O’Brien Cornsweet illusion.

The only difference between the left and right images is the inclusion of the white square with black lines. See text for discussion.

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When confronted with this illusion, we often perceive A and B to have the same uniform color. Indeed, A and B are the same (Figure 3, right). However, the intensity or brightness linearly increases from left to right in both A and B. This is easier to visualize when you can see the edge between the two boxes. There have been many variations on this illusion, see Purves et al. for a further discussion of the Cornsweet effect (1999). My main point in providing this illusion was to demonstrate how hard it can be to differentiate differences in light intensity.

The next time you look at your tank, think of this article and the two illusions that were illustrated. You will quickly realize that your ability to perceive light intensity differences in your tank--is not very good. I will go out on a limb and say that your ability to estimate the PAR intensity level in your tank is most likely NIL.

A common question I see on Reef2Reef and FB is a picture accompanied by the statement “Is this enough light for my tank?”. I hate answering this question because it is nearly impossible to answer. Any answer that doesn’t include encouraging the person to use a PAR meter is merely reinforcing the use of guesswork.

If you have read any of my other articles, one of the common themes is that I am not a big fan of guesswork. (Perhaps, because of my repeated failures). If you buy a PAR meter, you will at least shift the odds of success more in your favor. I can’t overstate the need for utilizing a PAR meter to assess the lighting in your tank. Here is why:

The problems of relying on the manufacturer’s light distribution specifications in lighting your reef.

There are many companies and papers that provide light distribution maps for fixtures that convey a sense of where the light will go in your tank. While these are useful, they really only provide a first approximation of what you can expect in your unique tank. In general, light intensity decreases with both depth and the radial distance from the LED or bulb.

However, many of those tests are done with the lights centered over tanks without rocks. In your case, your bulbs may be offset in your fixture (e.g. many fixtures have LEDs that are not located in the center). We like to think that long T5 tubes provide linear lighting (similar intensity) along the length of your tank. However, many factors can impact the path of light propagation in your tank. Your front glass may reflect light, while your rocks may refract light. Consequently, linear lighting in a tank, especially along rocks where the corals reside, may be further from reality than most light manufacturers would prefer to have you believe. I won't even mention that how clear or turbid (not clear) your water is also affects how much light reaches different depths.

The problems in using your eyes to assess your reef lighting.

There are many things you cannot assess by eye with reliability. The output of all light fixtures changes over time in both in their spectrum and intensity (Clark, 2017). It is well known that chromaticity stability (shifts in spectrum) of metal halides and power compact bulbs necessitated their replacement over time. It is thought that the spectral shift in LED bulbs is slower than prior technologies, but it cannot (and should not) be ignored in a reef that is dependent on lighting with specific spectral content.

Stop guessing! Understanding of your lighting will help guide your selection and placement of corals in your tank!

You can now purchase sensors that report PAR (photosynthetic active radiation) values, which are a measure of light intensity that corals consume and use for photosynthesis. The sensors integrate the number of photons of light over a specified spectral range of wavelengths. The energy source, PAR, is expressed in Photosynthetic Photon Flux Density (PPFD) units, µmol m-2 s-1. The units for PPFD are often referred to as either PAR or µmol m-2 s-1.

There are a number of companies that supply PAR meters (Seneye, Neptune, and Apogee Instruments.) The Neptune PAR meter appears to utilize an Apogee Instruments SQ-420 sensor. I use a full spectrum smart quantum LED par sensor made by Apogee Instruments (SQ-520). This sensor integrates over a wider range of wavelengths (389-693 nm) in comparison to the SQ-420 sensor (410-655nm).

Theoretically, that should capture more of the ultraviolet (<400 nm) and red (>635 nm) spectral components that are produced by my LED lights (AI Prime HD). I couldn’t find the specifications for the Seneye sensor for comparison. A number of groups, including Bulk Reef Supply have done side-by-side comparisons. One nice feature of the Apogee Instruments system is that they provide a mechanism for compensating for water immersion that allows them to be readily used in underwater applications.

A nice overview of how these sensors work can be found on the Apogee Instruments website.

The cost of the SQ-520 sensor is not cheap at $345. In comparison, the SQ-420 is $227. The Seneye is cheaper ($200) and records additional parameters. Regardless of which system you choose, it is a worthwhile investment when you consider how much lighting and corals cost.

Bulk Reef Supply also now offers a service for renting these Apogee PAR meters ($50/2 weeks).

I use my sensor for a number of reasons. It allows me to assess the state of my light fixture(s) over time. I also use it for directing and adjusting the intensity of my fixtures. And I use the sensor to determine where the light is going, and where it is most intense within my tank.

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Figure 4 was created and provided by Greg Gdowski, ©2019, All Rights Reserved.

Occasionally, I will measure the PAR level with the Apogee SQ-520 placed just above or around each coral. Figure 4 illustrates how this was done with the Montipora monasteriata shown in Figure 2. The left two photos in Figure 4 show how the SQ-520 was positioned in different places around the coral. The right photo in Figure 4 is a top-down photo of the Montipora monasteriata with the experimentally measured PAR values shown superimposed around its perimeter. This illustrates how difficult it is to assess PAR levels by eye.

While the photos in Figure 2 and Figure 4 appear rather uniform in lighting intensity, the PAR values around the perimeter obtained with the SQ-520 show that they are not uniform. You can see why shadows are easy to pinpoint by eye. The PAR value within the shadow discussed earlier was only 7 µmol m-2 s-1, while just adjacent to it the intensity was 93 µmol m-2 s-1. Near the fixture’s center of projection, the intensity reached a value of 128 µmol m-2 s-1. As you move towards the front edge of the tank (i.e. farther away from the center of projection) the PAR value dropped off (104 and 105 µmol m-2 s-1) as expected.

Finally, another shadow was strategically created by me, by placing another coral above it, limiting the light on the far edge of the Montipora to 23 µmol m-2 s-1. This shadow limits growth of the Montipora towards the middle of the tank. My expectations are that this coral will continue to grow towards the left side of the tank (especially near the 128 µmol m-2 s-1 zone), and it will eventually require fragmenting as it progressively shades the corals below it.

More recently, I have started using this technique to help me place new corals in the tank. Many coral supply companies (e.g. Vivid Aquariums) provide suggested PAR levels for corals they sell. This really takes the guesswork out of lighting your tank and serves as a guide in placing your corals.

Happy reefing!




"SQ-520: Full-Spectrum Smart Quantum Sensor (USB)." Apogee Instruments, from

Adelson, E. H. (1993). "Perceptual organization and the judgment of brightness." Science 262(5142): 2042-2044.

Clark, T., Davsi, L., Duffy, M. Gaines, J., Hansen, M., Haugaard, E., Paolini, S., Pattison, M., Robinson, C., Sarraf, S., Soer, W., van Driel, P. (2017). LED Luminaire Reliability: Impact of Color Shift. U. S. D. o. Energy. Next Generation Lighting Industry Alliance, LED Systems Reliability Consortium.

Purves, D., A. Shimpi and R. B. Lotto (1999). "An empirical explanation of the cornsweet effect." J Neurosci 19(19): 8542-8551.

von Helmholtz, H. (1924). Helmholtz's treatise on physiological optics (Trans. from the 3rd German ed.) Rochester, NY, The Optics Society of America.


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Author Profile: Greg Gdowski, Ph.D.

Greg has 20 years of aquarium experience, and he has been keeping reef aquariums for the past 10 years. He and his wife are also both dog lovers and have two special-needs Vizslas at home.

Greg is also the Executive Director of the Center for Medical Technology and Innovation and Associate Professor in the Department of Biomedical Engineering at University of Rochester.