Thursday, 29 May 2014

Colour - White Balance, A closer look

Pimp my Grey Card!

I decided to create a little mobile laboratory to delve more deeply into the complexities of light.  
More of that in a moment.  First a quick recap on white balance.

The Colour of Light

Sunlight is at it's purest when the sun is highest in the sky.  As the sun approaches the horizon, the colour of sunlight changes from white, through yellow near the horizon, to reddish right on the horizon.  
The reason for this is simple.  Light is scattered by the atmosphere and the denser the atmosphere the greater the scattering of light.  The atmosphere between the observer and the horizon is many times thicker than it is between the observer and zenith (looking straight up).  This causes drastically more scattering of sunlight when the sun is at or near the horizon.  Atmospheric pollution including smoke and ash only serves to increase this scattering affect.  The bluer, shorter wavelength portion of light is scattered the most by the atmosphere, deflected by particles and molecules in the air.  

We observe this phenomena, termed RAYLEIGH SCATTERING, in two ways.  Firstly we observe the sky as blue.  This is the shorter, mainly blue light being scattered in all directions, including towards the earth and out into space.  In effect, the blue sky canopy is a blue light source capable of lighting in all directions, including into deep shadows.  Secondly, we observe the sun and the light coming from it as appearing progressively yellower, then redder, as the sun nears the horizon.  This is just the same phenomenon that gives us the blue sky, except in reverse.  As the blue portion has been progressively scattered, the light that remains appears progressively redder.  

Here I have created a representation showing the sun's position at high noon in Ireland on both the summer and winter solstice.  The difference in the quality of light is remarkable.  Note the density of the atmosphere at the horizon relative to zenith - it is minimum 37.5 times denser, and a lot more if the air is polluted!

My animated gif below gives a representation of the changing nature of daylight over the course of 24 hours here in Ireland during both the summer and winter solstice.  A number of things are particularly striking.

(1) Light temperature is ever-changing during the winter solstice.  It never properly stabilises.
(2) The purity of daylight is already much better come mid-morning in mid-summer than it is at any time during a winter's day, simply because the sun never gets high enough above the horizon in winter.
(3) Colour temperature is very stable for long periods of the day in mid-summer.  While this may seem like an advantage, a high, bright sun brings it's own challenges both for bird observation and photography.

Colour Temperature and Black Bodies

We have a scale for measuring the colour of light.  It is the Kelvin scale.  So, what has a temperature scale got to do with light one might ask?  Well it turns out that the colour appearance of light closely matches the colour appearance of objects (specifically black bodies) as they are heated.  If you heat a pot on an oven it glows red as it gets hot.  If you put it in a furnace you will note the pot's colour change from red to yellow to white and eventually to blue.  As these colours mirror the colours of daylight they represent a useful comparative scale.  It also happens that artificial light sources from candles to light bulbs produces light colours that can be measured along the same scale.  Thus, the Kelvin scale has become the standard benchmark for studying and measuring the colour of light.  It is referred to as COLOUR TEMPERATURE and here is the scale as it is normally displayed.  The numbers are temperatures in degrees Kelvin.

You will notice that the scale contains no violet or magenta, though in my gif above I have included these.  The fact is that light colour varies quite a bit as the sun approaches and drops below the horizon.  There is often a magenta glow at sunset, called ALPENGLOW.  At twilight, as the light fades away the sky is bathed in light of increasingly short wavelength from blue, through indigo, violet, ending in ultra-violet light which is invisible to us.

Colour Temperature and Colour Theory

Colour theory describes all colours in terms of three colour channels, Red, Green and Blue.  Cyan is the opposite of Red, Magenta the opposite of Green and Yellow the opposite of Blue.  You may have noticed that the Kelvin scale does not contain Green or Magenta.  Effectively one could argue the green channel is unaffected by the position of the sun in the sky, except around dusk - the famous green flash and alpenglow (see above).  This has an important bearing when it comes to white balance correcting tools as I will explain in a moment.

White Balance

Why bother with white balance?  Well white balance is not simply an invention of the camera world, it is an intrinsic part of the human visual system, and presumably that of other animals that see in colour.
As an adaptation it makes sense.  If we are on the look out for colourful fruits and hazardous plants marked by different colours, it helps that our brain adjusts our visual senses to cater for ever-changing light temperature.  

The reason we correct white balance in photographs is mainly for aesthetic reasons, to mimic what the human brain sees.  In terms of bird identification it also serves a useful purpose in allowing us to appreciate colours as they might look under more ideal lighting.

Most manual white balance tools only consider the blue and red channels and only make corrections aligned with the Kelvin colour scale.  So if there are other colour "contaminants" in an environment (eg. a vivid green cast created by a foliage canopy (see HERE), or say a bright magenta evening sky) manual white balance correction tools will not solve those lighting problems.  This is where the use of manual white balance tools, based solely on the Kelvin scale fall short.  Not only does the human visual system cater for colour casts along the Kelvin scale, our brain can adjust to any colour cast, including a green or magenta cast.

For instance in MS Office 2010 the Color Tone scale used for manual white balance image correction only utilises the Kelvin scale so it cannot make a colour correction in the green channel.

Grey Card White Balancing

One of the key advantages of using a grey card for white balancing is that it works in all three channels.
The colours in the scene will be normalised, at least in so far as removing any colour casts that influence the grey card.  It is important that the grey card is positioned properly to reflect the lighting conditions on the subject. If the subject is in shade but the grey card is in the sun the white balance correction will not accurately reflect the lighting on the subject.

Pimp My Grey Card

While the grey card and colorchecker are fantastic tools they don't address all of my questions around the strange, changing behaviour of light and how it interacts in our environment.  I decided to bolt on some additional instruments to my colorchecker to gather more information for analysis.  The rig, and these different instruments are explained below.  

Various tests involving this rig are presented HERE.

For a look at manual white balance correction see HERE.


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