Radiometric & Photometric Comparison

Since as far back as I can remember wattage is often used when considering the output or suitability of a light source for aquarium use. As a result our hobby has become fixated on wattage when comparing the output of light sources including LED’s. The bottom line is, while wattage is used to express the radiometric output of a light source, it is meant to represent the total amount of radiant energy produced. Coincidentally the amount of radiant energy produced is equivalent to the amount of electrical energy consumed, also express in watts.

Have you noticed anything yet? In neither of these statements has there been any mention about how much light is being produced. This is because while wattage indicates the energy produced (or consumed) by a light source only a certain percentage of that energy is in the visible region where human vision and photosynthesis occurs.

The rest of the radiant energy produced is UV, infra red and heat. This is why aquarium lamps, and yes even LED’s can produce so much heat during operation. The percentage of UV, visible light, infra red and heat radiated can vary greatly depending on the light source. So as you can see, the radiometric output or wattage of a light source gives us little information about the amount of visible light produced or its usefulness for photosynthesis.

Before we move on I would like to point out a couple more things regarding wattage and light sources. The output of any light source including LED’s is solely dependent on the efficiency of the fixture system it is housed in and the control gear (ballast and other electronics) used for its operation. The efficiency of the fixture will determine the amount of light produced versus the amount of light and energy lost. A simple rule of thumb, the hotter a fixture system including LED’s the less efficient it is.

Remember we discussed energy produced by a light source? Heat means loss whether it is from light that is getting trapped in the fixture itself or excess heat from the control gear or light source itself. Consider fans like a cold remedy. They help the symptoms (heat) but do not eliminate the cause of the problem. Should they fail the results are usually catastrophic.

Lastly before we move on it seems that the fixation on wattage has reared its head once again regarding the output of LED’s. While we have already covered the principals of wattage and light, wattage of an LED only indicates the amount of power an LED and safely handle before permanent damage or complete failure will occur. It is NOT an indicator of light produced when compared to other LED’s.

The amount of light produced is determined by the LED’s overall efficiency as well as the efficiency of the fixture system it is housed in. But for now we need to get back to the other half of this post which is Photometric output. Simply put the photometric output of a light source is the amount of visible light it produces. As a reef aquarium (or planted aquaria) hobbyist your main concern is the amount of visible light produced by a light source as this is where photosynthesis primarily occurs.

With the growing popularity of LED’s some of you may have heard the term “lumen” used when expressing the output of an LED. This is a photometric measurement of light and is typically used to express the visible intensity of light produced by all light sources not just LED’s.

The next time you are at a home improvement store go to the isle that sells lamps and somewhere on the box of very lamp you should see its output expressed in lumens. Now, while lumen gives us the visible intensity of light some might ask why we have we not been using lumens all along. Considering what we have already discussed about radiometric output, using lumens would be a lot better than watts but this still doesn’t give us the complete picture.

Many corals and invertebrates need very specific wavelengths of light to thrive, grow and to reproduce. Light energy is absorbed in different amounts at varying wavelengths, and other wavelengths act as triggers for various biological and chemical processes. So as you can see we still need more information than visible light intensity (lumen) alone.

Other lighting terms you might be familiar with such as Kelvin and C.R.I. also tell us little about the spectral composition of light as they pertain primarily to the color of light this is being generated. When it comes to the photometric output of light I could go on and on listing the various measurements used to measure and categorize light but for our purposes the two primary measurements that will probably be most useful are lumens, and PAR.

Of all the lighting terms we have discussed so far PAR (photosythetically active radiation) is by in large the most useful as it measures light intensity at specific wavelengths of light in the visible region that are used in the process of Photosynthesis. Of course there is a way more scientific definition of PAR but for our purposes here it would just be a fancy way of saying the same thing. For you techies out there PAR can also be expressed as PPFD (photosynthetic photon flux density) which is photon (basic unit of light) irradiance expressed in moles per square meter and per second. Another way to express PAR is w/m 2 (watts per meter square) which is important in energy balance considerations for photosynthetic organisms.

In closing I would like to add that while PAR is probably the most popular measurement of light in our industry, it is far from perfect. This is because a PAR meter measures all light in the visible region whether it technically promotes photosynthesis or not and cannot discriminate between the wavelengths of light produced or the spectral composition of the light source. In any case, while PAR gives you the amount of photosynthetically active radiation produced in the visible region. Spectral power distribution or spectral intensity, tells you where it is coming from.

Response curve of an Apogee Quantum (PAR) sensor

A simple example would be if you took a high pressure sodium lamp used for horticulture and measured the PAR you would obviously get a very impressive PAR reading but it does not mean you would want to put that lamp over your reef aquarium. This is because the spectral power distribution of the source is totally different than a lamp or light source used for aquarium use.

The only way to accurately determine or compare the suitability of a light source for reef aquarium use is with the use of a Spectrometer. A spectral distribution chart is a visual representation of the light spectrum produced by a light source. It is a graph showing the relative intensities of a light source at each wavelength. You can use these charts to compare the energy levels of various light sources used for coral or plant growth. As you will see, it is the most practical way to compare the quality of light created by different light sources used for reef aquaria. The chart shows exactly which wavelengths of light (measured in nanometers) the corals are receiving. The highest energy output of the light source is plotted as 100% Relative Energy. The 100% peak is used to compare the energy levels of all other wavelengths of light produced by that light source.


The bottom of the chart shows all of the wavelengths of visible light energy that the light source produces. For example, if a wavelength is at 50% relative energy, that peak has half the energy when compared to the 100% peak. Scaling each chart to 100% relative energy allows side by side comparison of light sources with different lumen ratings (intensity) or wattages.

This is key in determining the ability or suitability of a light source in promoting photosynthesis in specific organisms. The bottom line is, while a light source may render a high PAR reading. Its suitability for reef aquarium use is not certain.

By now hopefully you have a clearer understanding of the differences between radiometric output and photometric output. In my next post maybe I will clean up the mess I started here and explain in detail some of the areas we glanced over like Kelvin, CRI and how they play a role in aquarium lighting. Maybe we will discuss the spectral components of sunlight and the different types of daylight that can be found around the globe. Maybe some of you have some suggestions of your own of topics you would like to cover.

Until next time happy reefing.