Light: Part 2


A useful rule of thumb is that for a planted freshwater aquarium of one of the standard dimensions the minimum amount of lighting required is 0.5 watts of fluorescent tubes per every liter of tank capacity. Because of the lower efficiency of halogen and incandescent lights, this corresponds to about 1.5 watts per liter of halogen lighting, or 2 watts per liter of incandescent lighting. Marine tanks with common sea anemones need about this much light as well, but tanks that maintain stony corals require double this amount.

Remember that these are minimum values.


We measure the intensity of light in various ways, but ultimately, the important factor is light flux of visible radiation (measured in lumens) per unit surface area (measured in square meters). This value is called “illuminance”, measured in lux. One lux is one lumen per square meter.

There is also something called “irradiance”. This is radiant power (expressed in watts) per unit surface area. It can also be expressed as number of photons travelling through a square meter per second, or rather, Einsteins per square meter per second, where an Einstein is a mole of photons.

Sometimes you’ll see irradiance and illuminance used almost interchangeably, but irradiance includes the spectrally useless infrared and ultraviolet radiation, not just the visible stuff, so be careful how you use these terms. But if you are talking about a solar type spectrum, the practical differences between the two is minimal, since almost all of the Sun’s light is visible.

Here’s a way you can estimate the illuminance of your aquarium hood. All you need is a 35mm camera with a built-in light meter, or more specifically, one of the “older” 35mm cameras that doesn’t do everything for you. You have to be able to manually set the shutter speed, film speed, and the f-stops.

Set your camera to a film speed of 25 and 1/60 second shutter speed. Aim and focus the camera on a white card illuminated by your aquarium lights. You want the card to fill the viewer. Adjust the f-stop until the needle indicates the correct exposure. To convert the f-stop to lux, use the following table:



2 1000
2.8 2000
4 4000
5.6 8000
8 16000
11 32000
16 64000

Let’s do some quick calculations. My 450 liter tank has a surface area of 0.74 square meters, and it is lit by four daylight fluorescent bulbs rated at 3050 lumens and two “grow and show” tubes rated at 1000 lumens. The wattage of all six bulbs is 40 watts; giving me the “rule of thumb” 0.5 watts per liter. Ignoring for the moment that light lost in my less than perfect reflector, the surface of my aquarium would receive

(2(1000 lumens) + 4(3050 lumens))/0.74 m2 = 19000 lux

Assuming a reflector efficiency of about 70%, we find that the surface should receive about 13000 lux.

For the record, I found the light in my tank to be 16000 lux. The noon equatorial sun, on the other hand, creates almost 100,000 lux. It is therefore evident that even our apparently well-lit tanks are much dimmer than natural waters, and it is only the adaptability of plant species that allows them to grow in our aquaria at all.

However, the measured lux and lumens of an aquarium lighting system are misleading. As discussed last month, the eye is most sensitive to yellow and green light while plants absorb most efficiently red and blue light. The lumen rating relates to eye sensitivity, not chlorophyll absorption, so you can have a high lumen rating but still have little light usable by plants, which is the case if you use cool whit fluorescent tubes exclusively. You may have wondered why I chose the two seemingly inefficient 1000 lumen “grow and show” bulbs when the daylight tube are rated for three times their brightness. But the grow and show bulbs are designed to match (as well as economically possible) the absorption spectrum of chlorophyll, and thus produce a lot of red and blue light but comparatively little green light. They therefore produce a lot of “photosythetically available radiation” (PAR), and also help bring out the red and blue colors of the fish a little better. It would be nice if manufacturers also listed their bulbs’ PAR output as well as their lumen output. PAR meters are available from scientific supply firms but there is no economically feasible way for an aquarist to measure his tanks’ PAR.


It is often written in aquarium literature that light follows the inverse square law, so that if you double the distance from the bulb you decrease the intensity of light by a factor of four.

This isn’t true.

Not for any real light source anyway. It only holds true for a point-source of light that radiates light in all directions equally. If you direct the beam with a reflector, or if you build a light that is not infinitesimally small, the inverse square law does not apply.

In fact, if you are less than one tube length away from the middle of a fluorescent tube, light drops off at an approximately linear rate. That is, if you are two feet away from a 4-foot tube, you receive half (not one quarter) the light that you would receive if you are just one foot away.

And if you are in front of the middle of a whole bank of six or more fluorescent tubes, the light intensity at two feet away is almost the same as it is only one foot away! Especially if the bank of lights is in front of a flat reflector. I won’t bother going into the proof of this (it requires some second year calculus) but think about it for a while and you will realize that as you move away from the center of the reflector, you receive more light from the sides, and this compensates for the increasing distance. This works as long as you are still closer to the center of the reflector than the reflector is wide.

So a multiple tube fluorescent light fixture (with a flat reflector) produces a very even light throughout the depth of a tank. If you want to defeat the inverse square law, get more tubes.

Another way to defeat the inverse square law is with a curved reflector. A curved reflector can reflect the light from the bulb as parallel rays. The best shape for this is a parabolic reflector, like the ones used in searchlights. They produce a parallel light beam that does not dim appreciably with distance, giving lights equipped with parabolic reflectors a very long range. Parabolic reflectors are also used in aquaria, mostly with metal halide bulbs. Such a combination produces an intense light, but unfortunately the light doesn’t spread over the tank bottom very evenly. So a modified parabolic reflector is often used instead, so that the light does spread enough to cover the bottom but not so much that it gets wasted.


Another affect is also important in getting light to the bottom of deep tanks, and that is total internal reflection. Water is considerably denser than air and also has a higher “index of refraction”. This means that a light ray is bent (refracted) when it crosses from water to air. Total internal reflection occurs when a light ray travels from water to air at such a shallow angle that the ray is bent back away from the air, that is, it is reflected. This effect is what allows light to follow a fiber optic cable as it twists and turns. It also traps light in an aquarium.

The light travels down through the aquarium from the lights above, but, unless the light is a perfectly parallel beam from a parabolic reflector, the light also spreads out so that it hits the sides of the aquarium. But this spreading angle is low enough that the light undergoes total internal reflection at the vertical sides of the aquarium. To see this affect, look at a table next to an empty aquarium that has an operating light fixture. Note how brightly lit the table is next to the tank. Then fill the tank with clear water, and note how much dimmer the table is, and, more importantly, how much brighter the bottom of the aquarium is.

However, real aquaria invariably gather algae and other assorted crud on the tank walls, so that rather than reflecting the light back into the water, the tank walls simply absorb the light. If you want to maximize the light levels in your tank, keep the walls shiny and clean.


The one thing that struck me on my last trip to the Vancouver Aquarium was the ripple lines on their tank bottoms. These moving bands of light and shadow are quite natural looking (anyone who has ever snorkeled over a sandy bottom is thoroughly familiar with them) but they are rarely seen in home aquaria. They can however be created by using a compact light source (such as a metal halide light), a parabolic reflector, and a small wave generator.

The parabolic reflector creates parallel light rays, just like the Sun’s rays. These are focused/defocused by the wave ridges and troughs made by the wave generator, creating the moving bands of light and shadow.

There has been speculation that photosynthetic corals and clams may need ripple lines and their bursts of super-bright light in order to thrive. As of yet, I’ve seen no presented evidence of this. And given that ripple lines are never seen in water more than a few meters deep, the presence of corals and giant clams at depths of > 10m would make such a hypothesis dubious.

All in all, ripple lines are probably just a very nice visual effect, but they are expensive to achieve in the home aquarium.


Myth: The inverse square law applies to aquarium lights.

Fact: Only the light from a compact bulb with no reflector drops off as per the inverse square law. The illuminance of a single (or dual) fluorescent fixture drops of approximately linearly with distance. And the illuminance of a multi-tube fluorescent fixture is almost independent of distance. Furthermore, the light from a compact bulb in a parabolic reflector is nearly independent of distance as well.

Myth 2: Deeper tanks need more lights.

Fact: An aquarium with lights over its surface is effectively a light pipe. Just as light will travel through a fiber optic cable without losing intensity, light will travel through an aquarium with clear water and shiny sides without dimming. Only the absorbing effects of algae on the aquarium walls, objects within an aquarium, and the less than perfect clarity of the water cause the light to dim with distance.


So what is the very best lighting for your planted fresh water tank?

If money is no object, get a metal halide light with a daylight (5500K) bulb. Use one 200-watt fixture for every 4 square feet of tank surface. The total cost of the two fixtures needed to light a 4’ x 2’ x 2’ tank is about $600. The cost of the metal reflector (and metal is required because of the heat generated) is about $150. The cost of the two daylight bulbs is $240. Total cost: about a thousand bucks. But all these items can be purchased (usually from stock) from any of the local hydroponics stores. This system provides light at a rate of a little less than 1 watt per liter.

And if you are really splurging, replace the $150 reflector with mirrored parabolic reflectors and add a wave generator to get a pleasing ripple line effect.

Now back to reality: money is an object. Some compromises in performance are required to bring the price down to reality, but an adequate system is still achievable at a much lower cost.

Use one 4-foot dual-bulb fluorescent fixture (cost $24) for every 20cm of tank width. Use a mixture of 40-watt General Electric Ultra-Daylight and 40-watt General Electric Gro & Sho plant bulbs, mixed at a ratio of two daylights for every plant bulb. The cost of the three fixtures required to light a 4’ x 2’ x 2’ tank is $72. The cost of the reflector (which can be wood lined with aluminum foil) is about $30 if you make it yourself. The cost of the six bulbs is $52. The total cost is about $160. And all the required items are available at Revy or Home Depot from stock. This system provides light at a rate of a little more than 0.5 watts per liter, and is the set up I use on my own 4’ x 2’ x 2’ tank.

To lower the cost of this system further, replace the tubes with common cool white fluorescent tubes costing a couple of bucks each. Despite all you may read, the many beautiful plants grown under these lights have proven long ago that they are completely adequate. Be aware however that they are not as effective at growing plants as their apparent brightness would indicate, because they produce little red and blue light.


The future in aquarium lighting probably lies in the next generation of HID lamps. OSRAM and various luxury car manufacturers have developed a HID automobile headlight. Some tricky electronics was required to get rid of both the inordinately long “warm up” time and the hot starting problem of HID lights (since headlights have to click on instantly, hot or cold), as well as the waste heat problem. If this ballast technology trickles down to everyday uses, we won’t want to use anything else.

If however power compact fluorescent lighting becomes significantly cheaper, these lights will overtake the market as the aquarium lighting system of choice.

Exotics such as the sulfur lamp are unlikely to come into their own for at least another decade.


The discussion of aquarium lighting in the typical hobbyist book is cursory, and often used to simply promote the completely inadequate aquarium hoods and lights sold by domestic pet industry manufacturers. The CAS library book The Optimum Aquarium (Horst and Kipper, 1986), does however discuss lighting in more detail, and is by far the best source of lighting information for fresh water tanks. Unfortunately it is biased toward promoting the very high quality, but obscenely priced, Dupla line of products.

The best source of non-commercially biased aquarium lighting information is the Internet.

Eric Olson’s web site, The Krib, contains a list of frequently-asked lighting-related questions at It also contains an archive of the lighting discussion that floated through the plant email list and the *.aquaria newsgroups in the mid 1990’s (

Or you can out George Booth’s page for much of the same material (with some better editing) that’s archived in The Krib, as well as a thorough basic discussion of lighting in planted freshwater tanks. I do however believe that George’s claim that “A metal halide bulb is 2X as bright as the same wattage fluorescent bulb” is in error, as the most efficient small source of broad spectrum light are the new-generation triphosphor fluorescent tubes.

Another good source of aquarium lighting information is the monthly electronic magazine Aquarium Frontiers, located at

Don Klipstein’s Lighting Info Site! at is a good place for general lighting technology information.

The Light Measurement Handbook

, at contains a more physics orientated look at artificial light sources.?