As we explain in the article dedicated to theoretical aspects of color, the human eye has a great ability to adapt to light at different color temperatures, an ability that allows us limit the perception of unpleasant shades, even in different lighting situations. As a consequence of this, the chromatic composition of a light is not always perceived for what it really is.
Lights that we perceive as ‘white’, for example, are not always so. Indeed, pure white is the sum of a certain number of the colors in the visible spectrum, and, when we see white, there may be the prevalence of one of these, even if we do not perceive it.
See, for example, the image below, which shows various lights with very different spectral compositions all perceived of as white, without any particular differences, unless, of course, they are shown in comparison right next to each other. Daylight (from the sun), for example, has a continuous spectrum with a prevalence in the blue range, while a fluorescent lights gives off a ‘striped’ spectrum that our eyes still perceive of as white since all their receptors are stimulated.
1. Light spectra of different white-perceived lights
It goes without saying, however, that film and digital sensors are much more strict than our eyes are, therefore capturing differences that can only notice clearly when we observe the result reproduced on paper, a screen or a display.
So when we take a picture or shoot a scene, especially if we are not in the natural light of the sun, but use artificial lighting, we simply cannot rely on our eyes alone, but have to resort, for example, to a spectrometer for an objective assessment, and a precise measurement of the composition of the light, in order to be able to correct it accordingly with filter, camera or light source adjustments.
Photographic, cinematographic and modern digital cameras are designed to respond to a given type of ‘white’ light, i.e. they are calibrated to a given color temperature, the advantage of digital being that such calibration is often modifiable.
Hot and cold light
Each light source has its own characteristic spectrum that determines a warmer or colder light, or light ‘temperature’. This variable is measured in degrees Kelvin (K), referring to the temperature at which an idealized uncolored (black) body emit radiation of the same tonality. It should be emphasized that a ‘warm’ light corresponds to a lower, and not a higher, temperature, due to the fact that our synaesthetic perception is inversely proportional to temperature. For example, a red-hot body that tends to a red color (warm), when heated further, will tend to blue (cold).
The wavelengths of light are expressed in its Spectral Power Distribution measurement system, whose unit of measure is the nanometer (nm). A low color temperature coincides with orange or yellow, and, at an even lower level, there is red, and infrared (invisible to the human eye). By increasing the number of degrees on the Kelvin scale, the light tends to cool from white to blue, purple and finally ultraviolet (also invisible to the human eye).
The two classic temperatures used in photography are:
- 3200K, or ‘tungsten’, which corresponds to the light of a tungsten filament and has yellowish, orangish color (warm light)
- 5600K, or ‘daylight’, which corresponds to light from the sun and has a light bluish color (cold light)
1. Subject lit by a 5600K light source
2. Subject lit by a 3200K light source
These are two fundamental temperatures correspond to values to be set on your camera in order to obtain a well-balanced ‘white’, without a blue or yellow dominance, depending on the type of lamp used. White light from neon, for example, tends towards green, and, although the eye will perceive it as white, the camera will show the color for what it really is.
Wavelengths expressed in nanometers, with visible light ranging from 380 nm to 740 nm.
2. Visible light
4. Deep infrared
Color temperature differences
The minimum perceptible color temperature difference between two light sources depends on the difference of the reciprocals of their temperatures rather than the difference of the temperatures themselves. In other words, a change in color temperature does not generate an equal change in the perception of the color.
Let us imagine having two options as a starting point, daylight at 5600K and tungsten at 3200K. If we add 100K to both, clearly the result would be daylight at 5700K and tungsten at 3300K.
1. Daylight conversion
2. Tungsten conversion
If we did this as a live experiment, we would clearly notice the difference in the tungsten light, though it would be hardly perceptible in the daylight light. In short, our 100K difference is more noticeable if the light is warm, and, conversely, less noticeable if the light is cold.
This is why, in 1932, Irwin G. Priest introduced the concept of ‘Mired’, or Micro Reciprocal Degree, in order to measure minimum perceptible differences. This measurement system is used to calculate the amount of color correction necessary to achieve the color temperature we want to achieve. The formula for calculating the Mired value (M) is:
- M = 1,000,000 / T
where T is the color temperature in Kelvin.
Use an example to illustrate the principle more clearly, if we take an initial T of 5500K, this would correspond to 181 mireds (result of the division of 1,000,000 by 5500). Now, if we take a final color temperature of 3200K, this gives 312 mireds. The final value minus the starting value gives 131 mireds (312-181), which corresponds to a Sun 85 filter.
The websites of most manufacturers of filters and gels show data on all the correspondences between filters and mireds, and most modern spectrometers are able to provide us with the exact model of filter to use, because, internally, they have a list of the manufacturers of the most common filters. In any case, it is useful to note that positive values suggest a filter tending to yellow, while negative values suggest blue filters. Then, in order to assess the final result of multiple filters used together, the Mired values of the various filters can be summed.
Daylight and Tungsten light
Why are mainly lights used at these two color temperatures? The answer lies in the calibration of the very first films. While outdoors, we used to, and still do, rely on the sun as the key light, for which a special ad hoc film was created. Tungsten lights were used in interiors, with another type of film created for that purpose. The use of these two standard temperatures is therefore a legacy of this conventional use.
Film and digital media correction
From what has been said so far, it is clear that correcting color temperature and balancing lighting is fundamental both in the studio and on an outdoor set. Another possibility is to focus on correcting the color temperature in the film itself or in a digital camera.
With film, you could, for example, use a yellow or a blue filter, depending on the specific type of film you are using. The most common filters for this kind of conversion are an 85 (orange) or an 80 (blue) filter. There are also two variants on the market, 80A and 80B, and the packaging of the film should indicate what kind of filter should be mounted in front of the analog still or movie camera. With the digital medium, however, this correction can be done in real time from the settings panel, without having to use any filters. Color correction is also possible in postproduction, but only when shooting in the raw image format, which is the digital equivalent of a negative and contains more information than those visible on the viewer.
A final suggestion concerns the use of tungsten film. When you add a filter in conditions of sunlight or under HMI lights, you should also use an ultraviolet filter, because tungsten film does not work well with UV rays.