The Color of Light

I know it has been awhile since my last post but I have been busy (ok obsessed) with various projects I have been working on. In a recent phone conversation I had with Joy Meadows it became apparent that an updated post was in order. While it could have been the fact that she said “Tullio we need another post for the blog”. It was actually a question she received from a hobbyist that got me going.

Considering the question, and the fact that she knew what my response would be. I can only conclude that I have been set up in yet another one of Joy’s uncanny plots to coerce me into getting something done. Right here is where I should say “but seriously”. The funny truth is, I am serious. Anyway I would like you to consider this question that I will eventually address while I rant and rave about an issue that has plagued our hobby for a long time.

The hobbyist question was; “how blue is the Reef Brite 20.000k metal halide lamp compared to a Radium lamp”? What is really disturbing is that considering the hype that usually surrounds aquarium lighting this is a very legitimate question. Quite often aquarium lamps are given exotic names like “White House white; or “Blueberry Blue”; which tell us nothing about the light they actually produce. Ironically, while I was trying to drive home a point with this exaggeration. These fictitious names that I pulled out of thin air at least give us some idea what the light produced would actually look like.

The problem describing the color of light goes way beyond our industry. After all if you compared two different 20,000k lamps they should look the same shouldn’t they? If not, then why bother giving lamps Kelvin ratings at all? The truth is describing and labeling the color of light can be very elusive, even to a lighting professional like myself. I realized how serious this problem had become when I came across two hobbyist hurling turbo snails at each other in a heated debate about which lighting was best for a reef aquarium.

I was about to intercede (after I put the turbo snails back in the aquarium) when the question came up about the color comparison between a Reef Brite 20,000K metal halide lamp and a Radium. Before we continue it is important to note that no turbo snails were harmed during the writing of this post. While I made up the story about the dueling hobbyist. The problem describing the color of light, and the hobbyist question are real.

Currently there are a few methods to describe the color of light. These are standards used by the lighting industry by which the color of light emitted by a light source is measured or plotted. I could stick my foot in my mouth and explain why most of these are technically irrelevant to the aquarium lighting. But that would just spoil all the fun and make most of this post irrelevant as well. The first measurement we will discuss is Kelvin which most of you are probably familiar with.

Kelvin is used to describe the color temperature of a light source. The word temperature is important when discussing Kelvin because it refers to the temperature of a theoretical black body or emitting body of light in degrees Celsius. Most of us are familiar with an incandescent lamp which has a filament (theoretical black body) made of tungsten. As current begins to pass through the filament it will begin to heat up. As the amount of electrons (current) passing through the filament increases it will start to glow producing light. If you were to measure the temperature of the filament and note the color it is glowing, you now have a fundamental idea of how color temperature (Kelvin) works.

As a piece of metal is heated it will change color from red to bluish white depending on what temperature you heat the metal to. The color of light produced is determined by the temperature of the metal, hence the term color temperature. A fun little side note. A toaster has filaments which operate the same way as the filament in a light bulb. The next time you go shopping for a toaster. Be sure to ask your helpful salesperson what the color temperature is on the model you are purchasing. While I will leave the problem of the color temperature of a toaster for you to figure out. The color temperature of other light sources is well documented.

Most incandescent lamps are in the 2700k – 3200k range producing a yellowish orange light commonly known as warm white. Now initially the term warm white makes sense since it is in the lower end of the Kelvin temperature range. This often confuses people at first. Especially since the term warm white has nothing to do with temperature. Actually the term warm white is based on the way objects appear to the human eye. In actuality it is not white at all by nature of its color appearance. The term warm white with the emphasis on warm will become more apparent later on when we discuss C.R.I. (color rendering index).

Direct sunlight is 5300k and daylight is 6000k and above. Northern daylight is 7500k and will vary slightly by season depending on the earth’s orientation to the sun. The reason for this (aside from the change in angle of incident) is because daylight is direct sunlight mixed with blue from the sky. As you get to 10,000k here is where things get really interesting. At 10,000k according to the way Kelvin or color temperature works light essentially becomes blue. Hold on for a minute! We have been taught as aquarist that 10K is White!

Before I start this next paragraph there is something nagging at me that I really need to get off my chest. White is not a color! White is often defined as a color without hue. Basically that is like saying it is a color without color. Others categorize white as a shade. That would make pure white a shade without shade? For the purpose of this post, from this point on I will refer to white as neutral white. The reason for this will become apparent when we start discussing color correlated temperatures (CCT). Hopefully hobbyist will gain a new appreciation for full spectrum white light and how difficult it can be to accurately produce. So let’s finish up with Kelvin so we can move on.

What you need to understand is that Kelvin was derived as a means to gauge the color of light when lighting was still in the infancy. With the advent of LEDs and other lighting technologies the lighting industry as a whole has had to totally rethink the way we measure the color and output of light sources. For example most light meters including PAR meters will provide you with correction factors because they are inaccurate when measuring the light output of LEDs.

The reason Kelvin does not work when measuring the light output of let’s say an LED is because aside from incandescent and halogen lamps that use a theoretical black body (filament) no other technology uses a black body to emit light. Now we all know how inefficient incandescent lamps are at producing light but there is a physical reason why filament based technology is impractical, even if it could produce light more efficiently.

Since T5 lamps are so popular in our industry I will use these as an example. I am sure most hobbyists do not realize that when the tube (glass) temperature of a T5 lamp exceeds 40 degrees Celsius the light output and subsequently the efficiency of the lamp drops significantly. The higher the temperature the greater the ratio of light loss. So if you are operating a T5 fixture or retro-fit; and the temperature of the fixture or retro-fit is exceeding this threshold; the light output will be drastically reduced.

It is simple physics. The temperature of the surface of the Sun is 6000K. Since the temperature of a lamp (light/heat source) cannot be less than the ambient temperature of their environment, imagine if a T5 lamp used a filament (theoretical black body) to emit the light that it produces (which it doesn’t); and that filament actually reached a temperature of 10,000K (which it couldn’t). You would no longer have to worry about light loss, because there would be no lamp left. While your lamp was vaporizing, your fixture would be going up in smoke as well.

While the arc temperature (plasma arc inside the arc tube) of metal halide lamps can reach thousands of degrees. If the arc temperature reached 20,000 degrees (20k), the pressure created inside the arc tube would cause the lamp to explode. By now I think you get the picture. At some point I will be writing a piece on how T5 lamps, metal halide lamps, and plasma lamps produce light. It is really quite fascinating.

Since it is pretty clear why Kelvin is not how the color temperature of most light sources is determined. Then how is it that we use Kelvin to denote the color produced by aquarium lamps? It is called color correlated temperature (CCT) and is also expressed in Kelvin based on black body curve. In short, if you could heat a black body to a given temperature; say 20k (without your lamp exploding). The color produced should probably look like this. If you are getting the feeling this sounds pretty vague you are correct.

As I explain how color correlated temperature is derived you will understand why. It will also become apparent why pure white (neutral white) is such an interesting and elusive color of light. I know white is not a color. I was just testing you. To properly understand color correlated temperature we need to go over a few basics. While I could wow you (or bore you) with a whole bunch of scientific mumbo jumbo about the black body curve and the use of X, Y coordinates. In truth it is actually pretty simple.

It is common knowledge, that you can create any color imaginable (up to 16,000,000 to be exact) with three simple ingredients. Red, Green and Blue. It is actually pretty amazing when you think about it. Now imagine the black body chart as a grid with separate areas of red, green and blue. To create a color correlated color temperature on a black body chart I would need to explain X,Y function and slopes and frankly this post has already gotten way out of hand.

Besides I have a better way to help you visualize this and all you need is a paper and pen. Who could have imagined a relatively simple question from a hobbyist could get out of hand like this? I think we know the answer to that, right Joy! Okay back to our quasi color correlated black body chart. First I want you to draw a triangle on a piece of paper. Do not worry about the size or how perfect it is, any triangle will do.

At the bottom left hand corner of the triangle label that corner Blue. Label the top of the triangle Green, and the remaining right hand corner Red. If you have any corners left over, just ignore them. You may also want to call a close friend and ask them how to draw a proper triangle. Now I want you to draw a small circle in the center of the triangle; and in the center of that circle put a dot. Since it is impossible to put anything past you guys (gals). It should be clear that this is a very important dot.

Okay so the suspense is killing you and right now you are at the edge of your seat just dying to know what this dot represents? You do realize that if this were television series, this would be the point where the announcer says “tune in next week”….. The dot represents neutral white. White light in its purest form! What is so amazing about that? We will get to that in a bit. The circle that this amazing dot is located in the center of represents white or to more correctly put it, the many variations of white light.

If you were to move the dot in any direction within the circle the corresponding color correlated temperature would be white but the appearance to the human eye would change dramatically. Right here would be the perfect point to segway into C.R.I. (color rendering index). But before we do there are a few details about color correlated temperatures that aquarium hobbyist might find quite interesting.

So far we have covered white light in the 3000k-10,000k regions. But what about 14,000k and 20,000k which are very popular in our hobby? And what is so unique about neutral white (pure white light)? Neutral white as I have stated is pure white light. It is where Red, Green and Blue converge at a point where colors no longer exist. Since light is essentially a transparent medium the colors Red, Green, Blue would continue to cancel each other out to a point where they simply vanish.

While this may freak some people out, it may comfort you to know that the dominant wavelengths of these primary colors still lives on. Essentially the colors are still there. You just can’t see them. This phenomenon is quite fascinating if you give it some thought. If you could manipulate the light projected onto an object (scientist have already done this with small objects). You could theoretically mute or cancel out its perceived color and therefore its shape. This would render the object invisible to the human eye. Since we have a hobbyist patiently waiting for an answer to his question let’s get back to things we can see.

As for 14,000k if you used the triangle we created it would be on the outer fringe of the circle we drew down near the blue region of the triangle. Incidentally; if we were trying to locate 14,000k on the actual black body curve it would be on the outer end near infinity in the blue region. And what about 20K? Are you ready for this? According to the black body curve it does not exist! I discussed this many years ago at a lecture I did on lighting at MACNA. Thankfully there were company representatives from Ushio lighting present who conferred with my statement or I might have started a full blown riot.

Simply put 20,000K is past infinity on the black body curve so theoretically it does not exist. When Jack the owner of Reef Brite and I were developing what is now known as the Reef Brite 20k metal halide lamp we struggled for a long time trying to come up with an appropriate name for the lamp. But after feedback from distributors, vendors and hobbyist we simply had no choice. To call the lamp anything else would be too confusing for the market so we finally had to concede and call it a 20,000K lamp. One of the original names based on the lamps concept was Marine blue. We have had numerous comments on how favorably the lamp performs which is how this whole mess with the hobbyist question started in the first place.

By the way I still have not answered the question yet have I? Why don’t we continue for now with C.R.I. and see how things turn out. Color rendering index or C.R.I represents the ability of a light source to render color when compared to a standard light source. The light emitted by a source is then categorized and rated by its ability to not only render color, but whether it gives objects a warm (warm white) or cool (cool white) appearance when viewed by a human observer. While this last statement might appear painfully obvious to some (I sincerely doubt your fish are reading this) many organisms perceive light differently than we do and have their own lighting standards. If there are any fish reading this post (some people have really smart fish) please except my humble apology.

Both Daylight and any black body that emits light are considered standard light sources. Are you thinking what I am thinking? Daylight, and the light produced by a black body are very different from one another in terms of color appearance and color correlated temperature. Yet by definition both daylight and the light produced by any incandescent source (including halogen lamps) have a CRI of 100! At this point if you are ready to rip the monitor off your desk (or laptop) you are not alone. There is some good news at the end of all of this so for the time being put the monitor down so we can continue.

In the case of warm lamps a high CRI indicates the lamps ability to produce light comparative to an incandescent source. The CRI of a cool lamp represents its ability to replicate daylight. These would be full spectrum lamps which work very well for planted and reef aquaria. By definition actinic lamps would have a very low CRI. As would many of the lamps used in our hobby that are 10k and above.

Plasma lamps are often touted as having a high CRI but there is one small catch. AT 6000K (daylight) a plasma lamp would have a high CRI. But most reef hobbyist do not like the appearance of 6000K over there aquariums. As soon as you attempt to dim a plasma lamp (or use a different plasma altogether) to raise its color correlated temperature towards the blue region. The light output, CRI, and subsequently its PAR values, all drop significantly.

Now for you skeptics, I do not want you to take my word for it. The next time you witness a plasma system being demonstrated ask them to dim the unit until the light produced is suitable to your taste. Then ask them to show you the light readings. Honestly, Plasma technology has a lot of potential in our hobby when used correctly. Although for half of the cost you could have excellent color rendering (CRI) coral growth (PAR), and fluorescence by purchasing the LED hybrid system I designed for Reef Brite and incorporating their 6700K metal halide lamp.

The next standard commonly used for gauging the color produced by a light source is known as the Chromaticity diagram. It is based on a concept very similar to the triangle we drew earlier. By mixing three hypothetical primaries (not necessarily colors) you can create the sensation of any color to the human eye. The value of the three primaries X, Y and Z will always add up to 100. Since the value of these three primaries must always add up to 100 the value of Z can be implied allowing you to plot coordinates on a two dimensional plane (black body curve).

In this way all colors can be represented. Like our quasi triangle the outer regions or corners would represent spectrally pure colors (Red, Green, and Blue). There is one small problem. As with color correlated temperature; according to the Chromaticity diagram two light sources can have the same X,Y coordinates; one above the black body curve; and one below it and still have identical color correlated temperatures. Theoretically it is irrelevant except for one small detail. The two light sources would appear completely different from one another to the human eye.

Last but not least we have Spectral Power Distribution. What makes SPD so useful to aquarist? It represents the spectral composition or energy of a light source via bands of wavelengths in the visible region of light where photosynthesis primarily occurs. Seeing as how photosynthetically active radiation (PAR) is a measurement of the velocity (energy) of the photons in specific spectral regions of visible light. It seems we maybe on to something here.

I told you we were going somewhere with all of this. While I am considered eccentric by most of my colleagues (ok everyone that meets me). I am not cruel and would not have you suffer through this entire post for nothing. While SPD and PAR theoretically work hand in hand with each other. It is important not to get their purpose confused. SPD is often used by lighting specialist like myself to describe the color composition of a light source and its ability to render color.

Unlike Kelvin, CCT, CRI, and CIE (chromaticity diagram). Spectral power distribution is absolute in that it does not base its self on a theoretical black body or black body curve. It is very much like color mixing or painting with light. Just as you would use various pigments when creating a paint color. By mixing bands (wavelengths of light) in the visible region and their amounts you can manipulate the color of light produced.

This is essentially how RGB led systems work. In any case, while PAR gives you the amount of photosynthetically active radiation produced. Spectral power distribution or even better spectral intensity, tells you where it is coming from. This is key in determining the ability or suitability of a light source in promoting photosynthesis in specific organisms. While I would love to get into the various types of zooxanthellae and chlorophyll found in corals, invertebrates, and plants and their PAR absorption rates at specific wavelengths.

The bottom line is, while a light source may render a high PAR reading. Its suitability for reef aquarium use is still not certain. The main problem with PAR meters is that they measure photosynthetic energy in specific areas of the red, green, and blue regions simultaneously and do not discern between them. In my opinion the performance and quality of a reef lighting product should never be based on color, brightness or even PAR. Spectral power distribution/spectral intensity completes the circle and when used in conjunction with PAR gives you an accurate picture of the light you are placing over your reef or planted aquaria.

So there you have it. Everything you did not want to know about the color of light. So what about that hobbyist question that started this whole mess in the first place? How does the blue light produced by a Reef Brite 20k metal halide lamp compare to a Radium lamp? The answer actually lies in the eye of the beholder.

Like fingerprints that make us each unique. So does the construction of our eyes and their ability to perceive color and light. The perception of color and light also changes as we age. In this sense; our Scotopic (low light) and Photopic vision (normal to intense light levels) is almost as unique per individual as fingerprints themselves. Is there greater spectral energy in the blue region in the Reef Brite 20k? That I can answer factually because Jack and I put it there. While the spectral composition of the lamps Radium vs. the Reef Brite 20K is literally almost identical, the primary difference is that there is a very large peak in the 420nm (Nanometer) region of the Reef Brite 20k that does not exist in Radium lamps. Until next time, happy Reefing.