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Colour Temperature

Colour Temperature and Visual Acuity. Salty Shines a Bright Light on the Facts.

Your friendly amphibious denizen of dirt and sea has just returned from a swim through another manufacturer’s swamp and Salty is, well, a bit salty. Far from being a fierce competitor, old Salty is usually a pretty laid-back reptile, but on this occasion he’s a bit snappy.

What has inspired your crocodilian correspondent to put his claws to the keyboard? Well, the not so illuminating claim that the warmer colour light of halogen is better for visual acuity than LED. ‘Wassat’ I hear you splutter through your morning foureggs? Yep, visual acuity, the ability of your eyes to make out the shape and detail of the things they see. Obviously, this is an important thing when you’re barrelling down your favourite country road at night, hatchlings in the back and Mrs Salty strapped in next to you. The claim by our fellow light lover is that the lower wavelength of halogen light, around 3000 kelvin, is actually better for your ability to see what’s up the road in front of you. Now Salty knows a thing or two about seeing in the dark, so that claim needs to have a bit of light shone on it. Salty’s eyes are yellow after all, a bit like halogen light. The research is done and the results are in: the science shows that our fellow light lovers have their facts back to front. Sit back and grab a coldy, this is going to be a long one.

Yellow Lights 1


Lord Lucas likes yellow lights too.

The Good Lord Nelson

Let’s start with a definition of Kelvin because, well because it’s a good place to start and we need to understand what it is before we tackle the topic of how it affects visual acuity.  Kelvin is a measure of temperature, named for Brit scientist William Thomson, later Lord Kelvin, which sounds like the name of a pub if you ask me but I digress. Based on the concept of infinite cold, or absolute zero, being about -273 degrees (Salty’s reptilian soul shivers at the thought), Lord Kelvin proposed a ‘thermodynamic’ temperature scale which was independent of the temperatures of the freezing and melting points of fluids, which change depending on environmental factors like pressure.  Absolute zero, -273.16 kelvin, is based on the negative reciprocal of 0.00366, which is the accepted expansion coefficient of gas per degree Celsius relative to the ice point. Scientists like to use the kelvin scale in their experiments because it’s based on the physical properties of any gas, and as such can be calibrated precisely anywhere in the world.

So how do we get from temperature to light? Ole Lord Kelvin wasn’t thinking about light when he developed his temperature scale, so why do we attach so much importance to it when we argue about what is and what isn’t a good colour temperature for spotties with our mates at the pub? Well, temperature is a measure of the average kinetic energy, the energy of a mass in motion, of a substance’s molecules. Keep that in mind, and allow Salty to introduce another scientific concept, that of a ‘black body’. “Jeez Salty, you’re losing me” I hear you complaining. Hang in there folks, I’m getting to the point.  

A black body is a theoretical object that absorbs 100% of the radiation that hits it, so it reflects nothing and appears perfectly black. Now this black body only exists at an absolute temperature of 0 degrees as defined by Lord Kelvin’s scale, and is the point at which all molecular motion stops and where any radiation is totally absorbed by the ‘body’.

Kelvin’s experiments used carbon to find this absolute zero black body, which is convenient, because by running a current through a metallic alloy of carbon and tungsten, we can finally find a correlation between the temperature in kelvin of the black body and the colour produced as it heats.  As current flows through the ‘black body’ alloy it heats up, and at various temperatures in degrees kelvin we can observe different colours, from shades of red, to orange, to yellow, and then through the colour spectrum to green and blue and beyond.

So to wrap all that up, the colour temperature of light is defined as “the absolute temperature expressed in degrees Kelvin of a theoretical black body whose chromaticity most nearly resembles that of the light source”. Phew, are we there yet? Nearly. For incandescent and halogen lamps with a colour rendering index (CRI) of one, the colour temperature is a true value. For LED and other lamp types like high intensity discharge (HID), the CRI value is approximate, and is referred to as the correlated colour temperature, or CCT. Both terms refer to the colour temperature so are used interchangeably.

CCT and Eyeballs

The CCT rating is an indication of how ‘warm’ or ‘cool’ the light source appears. The higher the temperature, the cooler the colour will appear. The lower the temperature, the warmer the colour will appear. Lights with a colour temperature of 3500K or less, like halogen, have a warm or yellowish white appearance because the light is saturated with longer red and orange wavelengths which bring out the warmer colours such as red and orange more richly. Lamps with higher colour temperatures above 4100K are saturated with shorter wavelength green and blue, which bring out cooler object colours like…. blue and green.

Now, back to eyeballs and visual acuity, and a few more gems of scientific phraseology: photopic, scotopic and mesopic. These terms relate to the primary use of the cones, rods, and other light-sensitive cells on the retina of the eye, and they’re important to understand when discussing visual acuity.

Photopic refers to cone vision. There are three kinds of cones and they used to be called the red, green and blue cones. Science now refers to them as the long-wavelength, medium-wavelength and short-wavelength cones. Together, the three types of cones allow humans to see in colour, a bit like Salty’s old Rank Arena TV did back in the 70s. The cone cells are most active at medium and high light levels. As the light level drops the cones become less effective and it becomes hard to discern fine details and colours, which is why you can’t make out much colour in the middle of a paddock on a moonless night…. or on an outback highway at night with crappy lights.

The cones cover most of the eye’s retina, but their greatest concentration is at the fovea, which is at back of the eye in the centre. Loosely, this is the circular black spot in the eye, and back there the cones are very closely packed, which gives fine visual acuity when you look directly at something.

Fun fact: did you know that we salties have a horizontal streak of receptors in our fovea instead of the circular spot you humans have? This gives us the handy ability to scan the shoreline of our favourite swamp for a snack without moving our heads. Clever aye?  

The cones become less dense away from the eye’s fovea, resulting in less precise peripheral vision, which is the job of the rods and where they take over. Now Salty is not talking about the rods you fellas use to snitch his barra snacks, I’m referring to scotopic or rod vision. Rod vision provides peripheral vision and information about contrast and movement.  Finally, mesopic, or twilight vision is the term that refers to a range of human vision with both the rods and cones active. For most night-time applications vision is in the mesopic range, with the peak being somewhere between yellow-green and blue-green light.

Wrapping it Up

Better get another beer because this is the important bit when it comes to the colour temperature of driving lights, as Salty will now explain. Finally.kelvin

Summer sunlight at noon on a clear day has a colour temperature of about 5500K, which just happens to be about where Salty’s bosses at Dirt and Sea tune the CCT of their driving lights and light bars. There are a couple of reasons for this. Visible light is the spectrum of light between a wavelength of around 400nm, the blue end of the spectrum, and around 700nm which is at the red end. Roughly, these wavelengths correspond to colour temperatures of around 10,000k and 1000k respectively, with 5500k landing somewhere in the middle and appearing to us as natural white. White light is actually the whole spectrum of colour between 400 and 700nm, but where we tune the CCT of our lights is important because research has shown that white light that has more energy in the blue-violet short-wavelength part of the spectrum and appears to be brighter than white light that has more energy in the orange-red longer part of the spectrum.

Scotopic or rod vision is greatest at a wavelength of 507 nm. 507nm is firmly in the blue-green part of the spectrum, and it’s here that our peripheral vision peaks too, which is pretty important when driving at night. The combined peak sensitivity of our photopic or cone vision is highest at a wavelength of 555 nm, in the yellow-green part of the visible spectrum, so it’s at this wavelength where our ability to discern fine details and colour is greatest. Taking this into account, light with a colour temperature in the 5000k to 6000k range, straddling the yellow-green and green-blue part of the spectrum, gives the best of both worlds: fine visual acuity and better peripheral vision out wide where the light level drops off. All the better to spot that stray kangaroo, cow, camel or goat lurking in the bush. Salty is a WA boy and has never had to dodge koalas or water buffalo.

Research has proven that lighting in the higher colour temperatures results in the highest visual acuity when measuring symbol identification and colour recognition, both pretty important psychological effects when driving at night. Experiments comparing LED relative to fluorescent lighting showed this effect was greatest at the highest colour temperatures. In terms of psychological and cognitive performance, the research subjects fatigued more when performing tasks under fluorescent lights relative to LED. More importantly when it comes to driving, they also showed slower response times on tasks measuring spatial and verbal memory when measured under the warmer fluorescent light, that is at lower kelvin temperatures. Remember now, your average fluorescent light is a higher kelvin temperature than a halogen spotlight!

Even more research has shown that light at around 5000k is best for alertness and comfort when compared to levels of 2500k or 8000k, another good reason to aim for a CCT in the 5-6000k range.

No wonder staring off into the distance of the warm yellow glow of halogen headlamps can make you sleepy, which is how your old mate Salty feels after all this research. As for maintaining the best visual acuity, stick to light around daylight temperature, 5500k.

Back to the swamp.

Yours in dirt and sea,

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