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Ulf Hjorth-Moritzsen 


Generally:
Light is the foundation of Photography, without light, no pictures, and nothing to take pictures of. 
Photography means “to draw with light”. We often say that we “take pictures”, but in fact what we do, is that we capture the reflected light from the motive and transfer it to film or a digital storage medium. Good light ensures good Photos, good light captures mood and emotion, good light creates depth and preserve shapes.
The light from a rising sun is different from the light at noon, and a sunny day is different from an overcast day with respect to light. We all know this, unless we were born blind. Based on this knowledge we can assume that the characteristic of the light at the time when the Picture is being taken will influence the quality of the final picture. 
Parameters like direction and intensity each have effects on the quality of the pictures.
As we continue this argumentation we can assume or better declare that Photographers possessing basic knowledge of light is better prepared to turn ordinary lighting conditions into exceptional pictures because by understanding the nature of light they are better prepared to catch the mood and drama of the light.
So how do you find the best light? 

Before we answer that question we have to study the basic characteristics of light, starting with;

What light is:
Photographers often talk about two types of light, and they are:

1. Available light
2. Artificial light   

Available light is the light that is being reflected from the motive, independent of the origin of the light, whether it being the sun, streetlights, interior lights, the Moon, the Northern light or the stars.
Artificial light is the type of light that is controlled by the Photographer, like flash light, studio lights and other kind of organized light. Light being reflected from a reflector whether it is the sunlight or other kind of organized lights is also artificial light.      
Independent of what kind of light source being used, the Photographer main objective is to create the impression of natural light.

Some Physical Characteristics:
Light is Electro magnetic waves composed of frequencies or wavelengths that is visible to the human eye. The wavelengths lie between about 400 and 700 nanometres. Within this spectrum the color of light varies between violet which represents the shortest wavelengths and red which represents the longest wavelengths. Between these two extremes are all the other visible colors framed. 
Electromagnetic waves with longer wavelengths than red are not visible to the human eye and are called Infrared, Microwaves or Radio waves.
Electromagnetic waves with shorter wavelengths than violet are neither visible to the human eye. They are called Ultraviolet, X-ray or Gamma waves.   

The three most important characteristics of light is:

1. Luminous intensity.
2. Color
3. Temperature 

The measuring unit for Luminous intensity is Candela, which is the unit defining the Luminous intensity from a small source, in a particular direction. The unit was originally based on the light emission from an open flame, a candle light. Later the unit has been defined as the glow from molten platinum and the light intensity is now defined in terms of the unit for radiated power in general, like watt or Joule per second.
The exact definition for one Candela is:
The luminous intensity in a given direction, of a source that emits monochromatic radiation with a frequency of 540x10¹² Hz and a radiant intensity of 1/683 Watt per Steradian in that direction.

Light of a certain color, let say blue, consists of waves with approximately the same wavelength. Light sources with limited wavelength variations are for example road lights from Sodium Mercury Vapor lamps. Other sources are fireworks and neon lights.
Taking Pictures in this kind of light can create unexpected and strange outputs. If a blue object is photographed in the light from a Sodium Mercury Vapor lamp, the object will look black. This is because blue objects do not reflect yellow light, and because the light does not contain any other wavelengths there are nothing to be reflected. The light lacks tonality.

White light is a combination of a lot of wavelengths and it contains all colors, the
tonality is complete. At noon the wavelengths are divided equally between all colors, but during sunrise or sunset the light contains more orange/red wavelengths. The same is the fact for indoor lighting.
If you photograph a white sheet of paper in a light dominated by orange/red wavelength components, the white paper will look yellow unless you take actions to compensate. This is called white balancing. If you look at the paper your eyes will automatically do the correction so that you will see the paper as white. A Photographer has to make the correction while he shoots the picture, or he can do it later in the darkroom or at his Computer. It is possible on some digital cameras to adjust the white balance manually, more about this later.
Color temperature is a simplified way to characterize the spectral properties of a light source. A low color temperature implies warmer light, like yellow or red, while more blue light implies a higher color temperature. The color temperature is measured in degrees Kelvin (˚K).  

It is at this point we stop the theoretical examination and instead start to point out concrete characteristic of light and what conditions that is important for the Photographer. 
And here is something important to put on mind, the Photographic characteristics of the light is namely decided by four conditions:

1. Direction:
The light can come from three angles, from behind, from front (backlight) or sideways.

2. Declination of the sun:
Early in the morning the sun is pale and soft, often pink. At noon it is bright and clear and at sunset it becomes warmer again.

3. Contrast:
Bright sunlight provides the most contrasts between sunny areas and shadow areas.

4. Color:
During daytime the colors will vary considerably. The start and the end of the day provides the warmest colors, while the colors are more cool around noon .
The color temperature is a measurement of the spectral composition of the light. If you imagine a piece of metal being heated in an oven, when the temperature is low the metal is still black. When the temperature rises the metal piece starts to glow with a faint red color. When the temperature raises even more the color changes to lighter red, then yellow and then white (The reflected light contains more wavelengths the hotter the metal is). The same happens with the sunlight. Early in the morning the color temperature is lower than at noon. Early morning and afternoon light has a color temperature between 3000 and 4000 degrees Kelvin. At noon the color temperature is around 6.500 degrees Kelvin.
The light from a blue sky may have even higher color temperatures than the sunlight. A clear blue sky may have a color temperature of 10.000 degrees Kelvin. Clouds have an impact on the color temperature.

A regular daylight film is designed for a color temperature of 5.500 degrees Kelvin. Higher color temperatures will cause the color to turn towards blue, while low color temperatures causes the color to turn towards red. At some digital cameras it is possible to manually adjust the color temperature. In addition it is possible to use filters to correct the colors.
The below table gives some hints about color temperatures during various conditions and what kind of filters that may be used to correct the color temperature. 

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