One of the most confusing aspects of reef aquariums is the subject of lighting. Much of the problem stems from an insufficient understanding of the basic properties of light. The purpose of this article is to shed some light, so to speak, on light and its properties in the hope of improving aquarists’ understanding of this apparently complicated topic. This article is only meant to be an introduction to the properties of light. More detailed information is available from a good physics textbook.

This first article will cover some of the physical properties of light that have more relevance to reef aquariums.

What is Light?

Light, particularly that from the sun, is one of the most important factors affecting the earth. Almost all life on earth relies directly or indirectly on light (and heat) from the sun. But what is light?

Light is a form of electromagnetic radiation, and includes visible light (and by some definitions) ultraviolet and infrared radiation. Other forms of electromagnetic radiation include gamma rays, x-rays, microwaves and radio waves. Light exhibits both particle-like and wavelike behaviours. The "particles" (while not truly particles) are bundles of electromagnetic energy called photons. A photon is defined as a quantum of electromagnetic radiation. They have zero mass, no electrical charge and an indefinitely long lifetime. Photons always travel in straight lines, unless under the influence of gravity. The effect of gravity is so small that for practical purposes we can assume light travels in a straight line.

Ignoring amplitude (height), waves have two important characteristics: frequency and wavelength and these are in a reciprocal relationship. Increase the frequency and the wavelength gets smaller. Decrease the frequency and the wavelength increases. Waves on the ocean also exhibit this characteristic. The wavelength is the distance between the wave peaks and the frequency is the number of waves peaks that pass in a certain period of time.

While light is not truly made of waves, it can behave like it is. Light may be described as being composed of different wavelengths and frequencies. Light at a wavelength of 555 nm has a frequency of 540 x 1012 hertz. The energy of the photon is based on its frequency. The shorter the wavelength, the higher the frequency and the greater the energy of the photon.

The perceived colour of light is due to energy of the photons. Photoreceptors respond to a narrow range of energy. Energy within the range correctly stimulates the receptor and we perceive that as colour. As the energy level of the photons is based on their frequency and wavelength, colour of light is normally discussed using its wavelength. Light with wavelengths between 400 nm (nanometres) and 700 nm is visible (to human eyes). Light with a wavelength around 400 nm is perceived as blue and at around 700 nm it is perceived as red. Below 380 nm (down to around 100 nm) is ultraviolet radiation. Infrared radiation (which we perceive as heat) ranges from 750 nm to 2500 nm. In the same way as our eyes react to different wavelengths of light, the pigments in plants and algae react to certain frequencies differently than to other frequencies. Chlorophyll-a, for example, has absorption peaks at around 440 nm and 650 nm.

Photons may be reflected and they may be refracted by passing at angles through objects of different densities. Light travels at 3 x 1010 cm.s-1 through a vacuum and nearly as fast through air, however, its velocity is greatly reduced in dense media such as water or glass.

Reflection and Refraction

When light reflects off a mirror or similar surface, the rays reflecting off the surface will exit at the same angle on the other side of line perpendicular to the surface as the incident rays. This is the law of reflection.

Figure 1: Law of Reflection. The angle of reflection equals the angle of incidence.

This type of reflection, where the majority of the rays follow the law of reflection, is known and specular reflection, and is what you normally see from polished aluminium reflectors in lamps. If the surface is particulate, most light rates do not follow the law of reflection and instead are reflected in multiple directions. This is called diffuse reflection as is the characteristic of most painted surfaces. Light reflected off these surfaces appears uniformly bright regardless of the angle of view.

When light passes through materials of different densities, the velocity of the light changes slightly and this causes a bend in the ray at the interface between the two materials. This is known as refraction and is the principle behind lenses and is also why objects appear larger when viewed through a face mask underwater. Refraction is dependent on the differences in densities of the two materials, also called refractive index, and the angle of incidence. Perpendicular rays are not refracted, but as the angle of incidence increases so does the refraction.

Figure 2: Refraction. An example of refraction caused when light
traveling through air hits the water surface at an angle.

Light Intensity

The intensity of light is one of the most important aspects of light relative to photosynthetic organisms. The more intense the light, the more energy is available for photosynthesis. Basically, intensity is the number of photons hitting an area over time.

First, let’s define some units. Don't worry too much about the units themselves, it is the principles that are behind them that are important.

A lumen is a measure of the power of visible light. One lumen is defined as the luminous flux of 1/683 watt (see sidebar) of light at 555 nm. However, a lumen does not measure intensity. It is generally used to measure light output. For example, a 36W fluorescent tube radiates a total of 3250 lumens, but this is the total amount of light being radiated in all directions.

A lux is a measure of illuminance. One lux is defined as the intensity of luminous flux hitting a surface at 1 lumen/square metre. The intensity of the light is dependent on the total amount of luminous flux and the area over which it is spread. The intensity is just how much light reaches a surface. The illuminance of the sun at noon at the equator is over 100,000 lux.

One problem with both lumen and lux is they are weighted to match the human responsiveness to light. Therefore, yellow-green may be overstated, red and particularly blue will be understated.

Photons can be counted (using a quantum meter) and are reported as Einsteins. One Einstein is one mole (see sidebar) of photons. For measuring light intensity, the number of Einsteins hitting a area over time are measured. This is usually seen as E.m-2.s-1 or uE.m-2.s-1. Photosynthetically Available Radiation (PAR) measures all visible light (400 to 700 nm) fairly uniformly and is reported in Einstiens per square metre per second (E.m-2.s-1). In tropical latitudes around noon on a day with no cloud, the average PAR at the sea surface is around 2.5 E.m-2.s-1 (Tomascik et al 1997).

It is important to understand the difference between the light output of a light source and the intensity of the light reaching a subject. It is time for an analogy. If you bought a large bag of sand and placed it in a tank, spreading it evenly on the bottom of the tank, it would be at a certain depth. The total light output of a lamp is equivalent to the volume of sand. The light intensity is the depth of sand. Place the same volume of sand in a smaller tank and the sand would be deeper. Use a larger bag of sand (a brighter lamp) and you will get more depth.

A number of factors affect how much light radiated from a light source actually reaches the subject. The most important factor affecting the light intensity is the distance between the light source and the subject. The rays of light from a point light source are divergent and so the light is spread over a larger area as the subject moves away from the light source. The loss of intensity due to distance is predictable and is known as the inverse square rule. The inverse square rule states that the light intensity will be in inverse proportion to the square of the distance from the light source. That is, if you double the distance from the light source, the intensity will be reduced to 25%. Figure 1 shows the inverse square rule in practice.

Figure 3: Inverse Square Rule.

At 5 units from the light source, the light is spread over an area of pi*22. At 10 units from the light source, the same light is now spread over an area of pi*42. The light intensity at any point on the lower circle will be one quarter of that at any point on the higher circle.

The inverse square rule holds for any light source that approximates to a point and will hold whenever the distance from the light source is more than 5 times the largest diameter of the light source. The sun approximates to a point light source. As it has a diameter of around 1.4 million kilometres and is at a distance of 150 million kilometres, the inverse square rule applies. Of course, the size of the earth prevents us from moving to distances closer to or further from the sun to see the inverse square rule in action. Incandescent and metal halide lamps approximate to point light sources as do some compact fluorescent lamps. Regular fluorescent tubes do not approximate to a point light source over the distances we use them in our aquariums.

The Colour of Light

As explained above, the perception of colour is based on the energy level of the photons reaching the photoreceptors in our retinas. The energy level is related to the wavelength of light and so wavelengths (in nanometres) are used to describe the colour of light. These are the approximate wavelength ranges for the colours we perceive:

Colour Wavelength
Violet 390-450
Blue 450-490
Green 490-570
Yellow 570-590
Orange 590-620
Red 620-770

What we see as "white" light is actually a combination of these wavelengths. If "white" light is projected through a prism, the component wavelengths are split out due to slight variation in velocity of the different wavelengths as they pass through the more dense prism. This is also what causes a rainbow. Sunlight has what is known as a continuous spectrum. It contains a continuous range of wavelengths from below 400 nm to above 700 nm.

The perceived colour of artificial light is based on the relative intensities of the component wavelengths. Many lamp manufacturers publish the spectrums of their lights. Most artificial light sources produce light with an interrupted spectrum. The light is made up of a number of different wavelengths, but not all wavelengths are represented.

Absorption of Light

Water absorbs light and even in clear water about 60 percent of total radiation entering the water is absorbed in the first metre, and around 80 percent is absorbed in the first 10 metres (Gross 1977). Additionally, 3 to 50 percent of incident light is reflected off the water surface depending on the angle of incidence (Tait 1972). At noon, the angle of incidence is small and there is little reflection, but early and late in the day and angle is much greater and much of the light is reflected. Turbid water absorbs and reflects more incident light resulting in even greater attenuation of light.

Water absorbs different wavelengths at different rates. Red light is absorbed by water very quickly and even at a depth of 3m there is a significant loss of red wavelengths. Blue light, however, is absorbed much more slowly and much of the blue light hitting the water surface penetrates to 40m or more. Similarly, UV-A radiation has been shown to penetrate 20m of water or more.

The differential absorption of the wavelengths significantly affects the colour of the light reaching all but shallow depths. Ocean water far from the coast is usually very clean and has few coloured particles or dissolved substances. This water appears deep blue as a result of the scattering of light rays in the water (Gross, 1977).

The next two properties, colour temperature and colour rendition index, are more methods of describing light, particularly artificial light sources, rather than actual physical properties of light.

Colour Temperature

If you turn on an electric stove element you will notice that it radiates both heat and light - it glows. The hotter the element, the brighter it glows. At the range of temperatures you can get from a stove, the colour of the radiated light is red. If you were able to heat up the element further, the colour would change, first becoming orange, then more yellow and eventually what we see as "white" light. This is the principle behind colour temperature.

Colour temperature is based on radiation from a theoretical black body rather than a stove element. As the black body is (theoretically) heated, the colour of the light radiated shifts from the red end (longer wavelength, less energy) to the blue end of the spectrum (shorter wavelength, more energy). The colour temperature of the light produced by the black body is actually the temperature of the body in Kelvin (see sidebar).

The colour temperature really describes the spectrum of the light and the relative quantities of different wavelengths. Here are the black body radiation spectrums for a number of different temperatures (courtesy of Dallas Warren):

Figure 4
Figure 5

Figure 6
Figure 7

Figure 8
Figure 9

Figures 4-9. Black body radiation. Example spectrums of black body
radiation at 4000K, 5000K, 6000K, 7000K, 10000K and 20000K.

Not all light sources necessarily follow the characteristics of the theoretical black body. Our sun, however, is a pretty good match. The Sun itself produces light with a colour temperature at around 5800 K, however, as light from the Sun gets reflected and refracted by the earth's atmosphere, the actual colour temperature of the Sun will vary with different conditions. At noon, on a clear day, the direct light from the Sun alone is around 5500 K, but with the light from the sky included, it is around 6500 K. For this reason 'Daylight' is usually defined as 6500 K. At noon, on a clear day, in shade (so there is no direct light from the sun), the colour temperature may be higher than 20000 K.

Standard incandescent lamps also fairly closely follow the theoretical black body and this is mainly due to the fact that incandescent light produce their light by heating a filament. An incandescent lamp has a colour temperature of around 2300 K.

Fluorescent and gas discharge (e.g. Metal Halide) lamps do not follow the theoretical black body and the rated colour temperature is only an approximation of the colour of the light produced. This is largely because these lamps produce an interrupted spectrum with peaks in some wavelengths while some wavelengths are not radiated at all. However, lamp manufacturers will still publish colour temperature information for their lamps which would be more accurately termed "apparent colour temperature".

Please note that colour temperature can only be applied to "white" light, that is, light that has a mixture of all wavelengths. Actinic lights, for example, do not have a colour temperature as such.

Colour Rendition Index (CRI)

Artificial light sources are also rated with a Colour Rendition Index. This is a indicator of how well colours will be rendered under that light source. A CRI of 100 means colours will be rendered as well as they are under sunlight at noon. Smaller numbers mean the colours will not be rendered accurately. The closer the index is to 100, the more accurate the colours will appear.

In my next article I will cover the biological aspects of light.

If you have any questions about this article, please visit my author forum on Reef Central.


Gross, M. G. 1977. Oceanography: A View of the Earth. Prentice-Hall Inc. Englewood Cliffs, New Jersey. 497pp.

Tait, R. V. 1972. Elements of Marine Ecology: An Introductory Course. Butterworth and Co, London. 314pp.

Tomascik, T, A. J. Mah, A. Nontji and M. K. Moosa 1997. The Ecology of Indonesian Seas. Periplus Editions, Hong Kong. 1388pp,

Further Reading

Ryer A. 1997. Light Measurement Handbook. (

Sidebar: Units

SI (from the French Le Système International d'Unités) is the international system of units.

hertz is the derived SI unit for frequency and is cycles per second.

watt is the derived SI unit for power and radiant flux. It is defined as one joule per second, or in base SI units, one m2·kg·s-3. Don't confuse watts of electrical power with watts of radiant flux, while effectively they are both measuring "power", the efficacy of lamps means that much energy is lost (mostly as heat) in the conversion.

mole is the base SI unit for the amount of a substance and is defined as the number of atoms in 0.012kg (12g) of carbon 12. The number of atoms is 6.0225 × 1023 (Avogadro's number).

Kelvin is the base SI unit for thermodynamic temperature and is an absolute measure of temperature. Note that as Kelvin is absolute, the unit Kelvin (symbol K) should be used instead of degrees Kelvin (symbol °K). This was adopted by the 13th CPGM in 1967. 0 K corresponds to absolute zero which is the point of no thermodynamic energy, 273.16 K to the freezing point of pure water at 1 atmosphere and 373.16 K to the boiling point of pure water at 1 atmosphere. A Kelvin degree (the difference between two points) is the same as a Celsius (or Centigrade) degree, so 273.16 K = 0°;C and 373.16 K = 100°C.

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Properties of Light by Andrew Trevor-Jones -