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Color Temperature


The colour temperature in the Kelvin scale is the handiest way to express the content of the colour of a light source.
People that have been occupied with photography, even for a short period of time, must have encountered references to this term. However, there is often a confusion with regard to this term and its concept.
Why do we use the term "temperature" ? How can temperature and colour relate ? On the other hand, the knowledge and understanding of it are necessary for a good photograph. For the understanding of this term, one has almost obligatorily to refer to theory, that is, to certain terms and concepts in Physics. Like the one of the Black Body, of the Absolute Temperature etc, known to us from our school years.
From Physics, we know that when we bombard atoms with any kind of energy whatsoever, then the electrons around the nucleus jump to bigger orbits, that is as we feed them with energy they are transposed to larger distances from the nucleus. Yet, they cannot maintain this orbit for a long time and they return to lower orbits around the nucleus. When this happens, they emit the excessive energy they accepted under the form of energy particles, called photons.
The larger the amount of energy provided, the higher is the frequency of the emission of this radiation, and the shorter the wave length. This, in general, is the quantum theory first expressed by Planck, and developed later by Einstein and Bohr.
The simplest way to activate the electrons of an atom is to warm them. When a piece of iron is heated, we observe that at the beginning it takes a dull red colour, and when it will get warmer, it will have a blue/white colour.
This, initially, shows the relation between temperature and colour. As it is heated this piece will become white, then violet blue and finally it will emit the invisible ultra violet radiation.
When we stop heating the piece, the cooling will begin and then the piece will turn to yellow, then orange and red. Finally, it will emit the invisible infra-red radiation. Before we proceed further into the definition of the colour tempereture, we will examine two more of its terms. Black Body and Kelvin scale temperature.
Black Body : This is an object totally absorbing all the incident radiation. It is a theoretical body. In the laboratory it is achieved with great approximation, when we consider a spherical cavity with a small hole on its surface. This is a perfect black body. From the walls of the cavity no radiation is comming in. If radiation passes through the hole, then, because of the form of the cavity, it is reflected many times and it is diffused.Thus, the radiation is entirely absorbed by the cavity, regardless of its wave length, and consequently it behaves like a perfect black body. When this cavity is heated, then,there is a emitted a radiation from the small hole, the Black Body radiation. This radiation depends solely on the temperature of the cavity, while the nature of the walls does not play any part.
The Absolute temperature or the Kelvin scale is defined by the formula K=273, 16+C. It is equal to the sum of the Celsius temperature with the addition of the 273,16 constant. From this formula, we conclude that the Absolute Zero K=0 equals to -273C. At this temperature all movements of the molecules at the inner side of the matter, stop.
In view of the above, colour temperature is defined as follows : Colour temperature in the Kelvin scale (K) of a light source is the temperature in the Absolute Scale (273+ C), to which a perfect Black Body must be heated in order for it to emit a light of the same quality with the light source under control. So, when we defined that a Tungsten - halogen lamp has 3200K colour temperature, that means, that it emits a light whose quality is the one of a Black Body, when this is heated at 2927C (3200-273).
I would like us to examine the way of distribution of emission of power from a black body at each temperature, as well as its colour behaviour.
The more the temperature of a Black Body rises, the more the emitted radiation wave length falls. This, in photography, means that at higher temperatures we have a shift of the colour performance to the blue area (smaller wave length), while at lower temperatures, to the area of red/yellow. So, we see that in photography, we mistakenly refer to cold colours (blue-violet etc.) and warm (red, yellow, orange). In reality, it is exactly the opposite, because blue colour (called cold photographic colour) corresponds to high colour temperatures,while yellow (warm) to low temperatures.

THE SUN AS A BLACK BODY

The sun behaves as an Absolute Black Body. The temperature on its surface is 5800K, while inside and especially in the centre, it reaches millions of degrees. However, the light that it produces must firstly pass through its atmosphere and then through the atmosphere of the earth before it reaches earth's surface.
So daylight consists of a mixture of the light that comes directly from the Sun with the indirect light that comes from diffusion and reflection, caused by earth's atmosphere and clouds. This makes spectre distribution very variable and the respective colour temperature alternates from 2000K to 20000K. In the table, there are some indicative temperatures that show its alteration, depending on time and atmospheric conditions.
.................................................. Colour Temperature(K)
Daylight condition.......................................................................Lowest.............Highest
_________________________________________________________________
In shade ( light only from Blue sky )............................................12000.............20000
In shade ( light only from hazy sky )............................................. 7500...............8400
Sunlight+light from clear sky, midday............................................6000.............. 6500
Sunlight+light from clear sky, midday morning/evening.................5700.............. 6200
Sunlight+light from hazy sky..........................................................5700...............5900
Direct sun alone, midday............................................................Average.............5400
Direct sun alone,morning or evening........................................... 4900............... 5600
Sunlight at sunset..........................................................................1900.............. 2400
The same goes with Tungsten lamps. They also behave like an Absolute Black Body. The spectre distribution by such lamps is the same with a Black Body's whose temperature is approximately 50K higher than the one at the filament of the lamp. Because tungsten melts at 3200K, these lamps have colour temperature approximately 3200K.

BODY COLOUR

When white colour falls on a body, part of the colour is absorbed. This absorbtion explains the colour of different bodies.
Transparent bodies, such as glass, water etc., look colourless because they let all radiation of the white light pass through them.
Non transparent bodies owe their colour to the light that is reflected of diffused by them. If the body absorbs all the radiation of the spectre of the white light, then it looks black.
On the contrary, if all the radiation of the white spectre is reflected by the body then it looks white. Now, if a body absorbs certain radiation, then its colour is defined by the reflected radiation.
The sun's spectre constitutes a small part of the electromagnetic spectre. This part ranges from 4000A to 7000A wave length ( 1A=10-9 m ) and energises the posterior part of our eye. All kinds of irritation are recognised by the brain as colour. Each colour has its own wave lengths.
Wave smaller that 4000A constitute Ultraviolet Radiation. From 4000A to 4500A we have violet, from 4500A to 5000A blue, from 5000A to 5800A green, from 5800A to 6100A yellow,from 6100A to 6500A orange, from 6500A to 7000A red. This distinction is not so obvious but we take it for granted.

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