Color Temperature

Lightroom Color Temperature (or Colour Temperature if you spell correctly!)

“Andy – why the heck is Lightrooms temperature slider the wrong way around?”

That’s a question that I used to get asked quite a lot, and it’s started again since I mentioned it in passing a couple of posts ago.

The short answer is “IT ISN”T….it’s just you who doesn’t understand what it is and how it functions”.

But in order to give the definitive answer I feel the need to get back to basics though – so here goes.

The Spectrum Locus

Let’s get one thing straight from the start – LOCUS is just a posh word for PATH!

Visible light is just part of the electro-magnetic energy spectrum typically between 380nm (nanometers) and 700nm:

%name Color Temperature

In the first image below is what’s known as the Spectrum Locus – as defined by the CIE (Commission Internationale de l´Eclairage or International Commission on Illumination).

In a nutshell the locus represents the range of colors visible to the human eye – or I should say chromaticities:

1200px CIE1931xy blank Color Temperature

The blue numbers around the locus are simply the nanometer values from that same horizontal scale above. The reasoning behind the unit values of the x and y axis are complex and irrelevant to us in this post, otherwise it’ll go on for ages.

The human eye is a fickle thing.

It will always perceive, say, 255 green as being lighter than 255 red or 255 blue, and 255 blue as being the darkest of the three.  And the same applies to any value of the three primaries, as long as all three are the same.

perception Color Temperature

This stems from the fact that the human eye has around twice the response to green light as it does red or blue – crazy but true.  And that’s why your camera sensor – if it’s a Bayer type – has twice the number of green photosites on it as red or blue.

In rather over-simplified terms the CIE set a standard by which all colors in the visible spectrum could be expressed in terms of ‘chromaticity’ and ‘brightness’.

Brightness can be thought of as a grey ramp from black to white.

Any color space is a 3 dimensional shape with 3 axes x, y and z.

Z is the grey ramp from black to white, and the shape is then defined by the colour positions in terms of their chromaticity on the x and y axes, and their brightness on the z axis:

adobeRGB1998 Color Temperature

But if we just take the chromaticity values of all the colours visible to the human eye we end up with the CIE1931 spectrum locus – a two dimensional plot if you like, of the ‘perceived’ color space of human vision.

Now here’s where the confusion begins for the majority of ‘uneducated photographers’ – and I mean that in the nicest possible way, it’s not a dig!

Below is the same spectrum locus with an addition:

PlanckianLocus Color Temperature

This additional TcK curve is called the Planckian Locus, or dark body locus.  Now please don’t give up here folks, after all you’ve got this far, but it’ll get worse before it gets better!

The Planckian Locus simply represents the color temperature in degrees Kelvin of the colour emitted by a ‘dark body’ – think lump of pure carbon – as it is heated.  Its color temperature begins to visibly rise as its thermal temperature rises.

Up to a certain thermal temperature it’ll stay visibly black, then it will begin to glow a deep red.  Warm it up some more and the red color temperature turns to orange, then yellow and finally it will be what we can call ‘white hot’.

So the Planckian Locus is the 2D chromaticity plot of the colours emitted by a dark body as it is heated.

Here’s point of confusion number 1: do NOT jump to the conclusion that this is in any way a greyscale. “Well it starts off BLACK and ends up WHITE” – I’ve come across dozens of folk who think that – as they say, a little knowledge is a dangerous thing indeed!

What the Planckian Locus IS indicative of though is WHITE POINT.

Our commonly used colour management white points of D65, D55 and D50 all lie along the Planckian Locus, as do all the other CIE standard illumimant types of which there’s more than few.

The standard monitor calibration white point of D65 is actually 6500 Kelvin – it’s a standardized classification for ‘mean Noon Daylight’, and can be found on the Spectrum Locus/Plankckian Locus at 0.31271x, 0.32902y.

D55 or 5500 Kelvin is classed as Mid Morning/Mid Afternoon Daylight and can be found at 0.33242x, 0.34743y.

D50 or 5000 kelvin is classed as Horizon Light with co-ordinates of 0.34567x, 0.35850.

But we can also equate Planckian Locus values to our ‘picture taking’ in the form of white balance.

FACT: The HIGHER the color temperature the BLUER the light, and lower color temperatures shift from blue to yellow, then orange (studio type L photofloods 3200K), then more red (standard incandescent bulb 2400K) down to candle flame at around 1850K).  Sunset and sunrise are typically standardized at 1850K and LPS Sodium street lights can be as low as 1700K.

And a clear polar sky can be upwards of 27,000K – now there’s blue for you!

And here’s where we find confusion point number 2!

Take a look at this shot taken through a Lee Big Stopper:

2 Color Temperature

I’m an idle git and always have my camera set to a white balance of Cloudy B1, and here I’m shooting through a filter that notoriously adds a pretty severe bluish cast to an image anyway.

If you look at the TEMP and TINT sliders you will see Cloudy B1 is interpreted by Lightroom as 5550 Kelvin and a tint of +5 – that’s why the notation is ‘AS SHOT’.

Officially a Cloudy white balance is anywhere between 6000 Kelvin and 10,000 kelvin depending on your definition, and I’ve stuck extra blue in there with the Cloudy B1 setting, which will make the effective temperature go up even higher.

So either way, you can see that Lightrooms idea of 5550 Kelvin is somewhat ‘OFF’ to say the least, but it’s irrelevant at this juncture.

Where the real confusion sets in is shown in the image below:

1 Color Temperature

“Andy, now you’ve de-blued the shot why is the TEMP slider value saying 8387 Kelvin ? Surely it should be showing a value LOWER than 5550K – after all, tungsten is warm and 3200K”….

How right you are…..and wrong at the same time!

What Lightroom is saying is that I’ve added YELLOW to the tune of 8387-5550 or 2837.

FACT – the color temperature controls in Lightroom DO NOT work by adjusting the Planckian or black body temperature of light in our image.  They are used to COMPENSATE for the recorded Planckian/black body temperature.

If you load in image in the develop module of Lightroom and use any of the preset values, the value itself is ball park correct(ish).

The Daylight preset loads values of 5500K and +10. The Shade preset will jump to 7500K and +10, and Tungsten will drop to 2850K and +/-0.

But the Tungsten preset puts the TEMP slider in the BLUE part of the slider Blue/Yellow graduated scale, and the Shade preset puts the slider in the YELLOW side of the scale, thus leading millions of people into mistakenly thinking that 7500K is warmer/yellower than 2850K when it most definitely is NOT!

This kind of self-induced bad learning leaves people wide open to all sorts of misunderstandings when it comes to other aspects of color theory and color management.

My advice has always been the same, just ignore the numbers in Lightroom and do your adjustments subjectively – do what looks right!

But for heaven sake don’t try and build an understanding of color temperature based on the color balance control values in Lightroom – otherwise you get in one heck of a mess.

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