Aquarium lighting is an essential component of reefkeeping. If you’re just getting started with this endeavor, you may be curious about the best lighting for your livestock. It can be challenging to find the right light spectrum and intensity, but aquarium lighting technology has improved by leaps and bounds over the course of the past decade. Here, we share insight into how light is evaluated in freshwater planted and saltwater reef aquariums.

Measuring Light Spectrum with the Kelvin Scale

The visual appearance of a light spectrum is often referred to as the “temperature” of light, and is measured in degrees Kelvin. The Kelvin scale operates a bit differently than Fahrenheit or Celsius measurements: this unit of measurement evaluates the color of a light modeled by a “blackbody.” To visualize a blackbody, think of a piece of metal that glows when heated. As it heats up, the color emitted transforms from longer red wavelengths to shorter blue wavelengths. The Kelvin temperature scale is similar to Celsius, and goes to absolute zero. At 0 degrees Kelvin, there is no energy and no light emitted. As the hypothetical blackbody continues to heat up, light wavelengths change color to yellow, green, blue, and finally, violet. Humans are capable of seeing light in the range of 2,000-7,000 degrees Kelvin. As a reference point, daylight falls around 4,500 degrees Kelvin. It is common to see a Kelvin rating on bulbs or LEDs, especially CFLs and T-5 fluorescent bulbs. The higher the temperature, the “bluer” the light. Blue wavelengths penetrate water very efficiently. That is why a white-bottom pool appears blue – the light can make it through the water, bounce off the bottom of the pool and come back. By comparison, the majority of the red light will not make it back to the viewer’s eyes to see. Aquatic organisms that use light favor chlorophylls that use blue more efficiently.

Kelvin is a good scale for understanding the appearance of a light source, but cannot be used alone to decide whether the light is right for your livestock.

Light Intensity Measurements

The intensity of light is measured in two ways, which we’ll outline below:

  • Watts: The intensity of light at the source has historically been measured in watts. This is one of the metrics that causes a great deal of confusion, especially for fluorescent and LED lights. A watt is one joule per second, a measure of the electrical current supplied to a light. When all lights were filament-style bulbs, this was a good proxy for the intensity of a light – more electricity in, more light out. This is still the case for metal halide bulbs – a 400W bulb will be almost twice as intense as a 250W bulbs. For LEDs and fluorescent bulbs, the relationship breaks down a bit. These technologies are much more efficient at producing light from a given amount of current, so cannot be compared to incandescent or metal halide bulbs based on wattage. This is the reason behind the “equivalent to” language on the packaging of high efficiency bulbs produced today – consumers are so accustomed to the standard wattages that vendors needed to coach them through the shift to more efficient tech.
  • Lux: Lux is an international metric unit of measurement that refers to the intensity of light measured at the surface it impacts. The base unit is a lumen, and a lux is equal to one lumen per square meter. For example, the brightness of a candle that’s held one foot away is equal to 10.7 lux, while the intensity of the sun’s rays on the surface area of water can reach or exceed 120,000 lux. This is a much better metric for reef and plant lighting comparisons.
  • PAR: Photosynthetically active radiation, or PAR, is a measurement of the number of photons that hit a square meter per second. PAR is the most important unit of measurement for reefkeepers, and a PAR meter is especially helpful to have on hand. A note about PAR – it is constrained to the wavelengths from 300 to 700, which are visible to the human eye. Ultraviolet light, for example, can be very powerful but still contribute very little to PAR, because a lot of the light produced will be outside of the visible range.

Both light spectrum and intensity will have a significant impact on the health of your reef. You’ll need to find the right balance for your aquarium, and it may take some experimentation to perfect the formula. If you’re on the search for an advanced reef lighting system that’s easy to monitor and make changes with, browse our two generations of LED lighting systems. Our reef lighting systems are designed to help corals thrive, and the accompanying app can be used remotely to keep a close eye on your reef. When you’re ready to learn more about our inventory, please visit us online.

Reef Output Plot