This next set of postings will deal with the temperature of light. This is a rather broad subject and really warrants some additional reading, but I will attempt to offer up a very basic background of the physics and touch on the more common issues with how the temperature of artificial light affects photography.
In the first post of this series on light I touched briefly on what light is. In this last section we are going to be discussing an aspect of light that is very dear to photographers, namely the temperature of light. This is sometimes referred to as the color of light, but it more properly refers to the spectrum of light.
Any light source, with the exception of a laser, will emit a variety of colors, some more than others. When the source approximates the spectrum emitted by the sun, it is said to be a full spectrum light source. The sun emits light in all of the visible colors, but not all of the colors are emitted at the same intensity. Sunlight is also affected by the earth's atmosphere, so the light that we receive on earth is certainly not the same as the sunlight in space, nor is it consistent from day to day. In fact, here in the Seattle area it is not consistent from minute to minute due to our ever changing weather. However, on clear days, the emission of light from the sun is relatively consistent, and so this is what gets used as our standard for a model of sunlight emissions.
The Wikipedia tells us that “Color temperature is based upon the principle that a black body radiator emits light whose color depends on the temperature of the radiator. Black bodies with temperatures below about 4000 K appear reddish whereas those above about 7500 K appear bluish.” This reference is based on the kelvin temperature scale and is commonly used to describe light for photographic applications. Essentially, you heat up a black body and at some point it starts radiating in the visible light spectrum, starting with red. The more it is heated, the more the emissions spread toward blue, violet, ultraviolet, and beyond. If we heat it to 6000 K, then we get light emissions that approximate noon day sunlight. Go a littler hotter, say 7500 K and the light gets a somewhat blue. Go a little lower, say 4500 K and the light gets somewhat reddish. Do note that in spite of the temperature, blue is considered a cold color and red is considered a warm color. This comes from the mood of the color and has nothing to do with its kelvin temperature. For more information on color temperature, visit the Wikipedia or do a Google search.
Here is a chart showing the emissions of sunlight (approximate) for a cloudless day. Note that its emissions extend into the ultraviolet on the left, and far into the infrared at the right, but the majority of its spectral intensity is concentrated within the visible spectrum.
Light sources are rated as either continuous or non-continuous. A continuous light source will reproduce light across the entire range of visible light, from red to violet. A non-continuous light will produce only some of the colors from the visible range. An example of continuous light is a standard incandescent light bulb. While it is very weak in blue light, it does contain enough light across the visible range that our eyes are able to properly identify colors when illuminated under this lighting. An example of non-continuous light is a sodium vapor lamp, which produces mostly an orange/yellow color and has virtually no blue color. If you view a color chart under a sodium vapor lamp, you will see a very poor rendering of color, such that everything will have a distinct orange tint and many colors will not be discernible.
When we consider light sources for photography, we obviously want to use a continuous light source so all of the visible colors can be seen. Our eyes are optimized for sunlight and consequently, our digital camera's sensor and image rendering has been optimized to mimic the way our eyes see. Because of this, we would like a light source that closely matches the emissions of the sun, within the visible light spectrum. Ideally our light source will contain little or no ultraviolet and little or no infrared. The camera's sensor is sensitive to both infrared and ultraviolet. Most digital camera sensors use filters and coatings to reduce the absorption of light outside the visible spectrum, but the effectiveness of these filters vary and there are trade offs with light loss and cost.
In the next post we will take a look at some common light sources and discuss their characteristics and suitability for photography.