Lighting

The purpose of artificial light is to simulate natural sunlight in those places or times when natural light is not available.  Normally this is at night or indoors.  It stands to reason then that we would use natural sunlight as the standard for comparing all artificial lighting systems and 2 factors stand out in this comparison; light intensity and color temperature.  The most common factor we notice when we turn on the lighting in a room is the intensity of the light.  Compared to natural sunlight it is either bright or dim or some level of intensity in between.  But another important factor to consider when setting up an artificial lighting system is the color temperature.  In simplified terms, color temperature is the quality of the light and intensity is the quantity of light. 

Color temperature of light should not be confused with thermal temperature.  There is a relationship between the two but they are very different concepts.  Almost everyone is familiar with thermal temperature.  It is hot in the summer and cold in the winter.  Color temperature, on the other hand, is a very different and more complicated measurement.  The technical definition of color temperature is the thermal temperature in degrees Kelvin to which a theoretical object, referred to as a black body, would need to be heated in order to emit light of that visual coloration.  The best way to describe it is to use natural sunlight and other natural light sources as an example.  At sunrise and sunset, the light from the sun is orange and red and has a very low color temperature of approximately 1,600K.  At noon on a bright day, the light is about 5,200K.  As the sky gets bluer, the color temperature goes up.  For example, a cloud-filled sky will be about 6,500K but a clear deep blue sky will be as high as 20,000K.  Lightning has a very high color temperature of up to 30,000K.  Color temperature of light goes from red at the low end to blue at the high end with white in the middle.  This correlates closely with the thermal temperature at which many objects burn.  For example, wood burns at a fairly low temperature and emits red or orange light.  Most metals burn at a much higher temperature and emit a white or blue light. 

So what does all of this have to do with our normal light bulbs?  Almost everyone who has turned on a light is familiar with the two most common types of artificial light: incandescent lights (the standard screw in the socket light bulb) and discharge lamps (standard fluorescent tubes).  The difference in the light color temperature from these two is very noticeable.  Standard electric light bulbs emit a very orangey-red hue because there is a metal wire in the bulb, usually tungsten, that is heated with electricity until it emits light.  The color temperature of these common light bulbs is usually very low at about 2,800K, similar to the light you would get from a campfire.  Fluorescent tubes, on the other hand, emit a much more noticeably white-blue hue because they operate on a very different principle.  Electricity is passed through a gas inside the tube, usually low-pressure mercury vapor, which causes the gas to emit ultraviolet radiation.  This UV radiation is absorbed by the fluorescent coating on the inside of the glass tube, which converts the UV radiation to visible light.  Fluorescent lighting is usually about 3,400K.  Both incandescent and fluorescent lighting falls short of simulating natural sunlight but often a combination of the two comes very close to meeting the quality of light that we prefer.