The human eye has a great ability to adapt to light at different color temperatures, an ability that allows us to 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.

For example, look at the image below, which shows a variety of very different spectral components of light, all of which are considered white, without any special difference, unless they are shown in comparison, of course. For example, sunlight (from the sun) has a continuous spectrum, generally in the blue range, while fluorescent lamps emit a “stripe” spectrum, and our eyes still think it is white because all of their receptors are stimulated.

 

1. LIGHT SPECTRA OF DIFFERENT WHITE-PERCEIVED LIGHTS

However, it goes without saying that film and digital sensors are much more stringent than our eyes, so the differences captured can only be clearly noticed when we observe the results copied on paper, screen or monitor.

 

Therefore, when we take a picture or take a scene, especially if we use artificial lighting instead of natural sunlight, we can’t rely on our eyes at all. Instead, we have to make an objective evaluation with the help of a spectrometer and accurately measure the composition of the light, so that we can make corresponding correction through the adjustment of the filter, camera or light source.

 

Photography, film and modern digital cameras are designed to respond to a given type of “white light”, that is, they are calibrated to a given color temperature. The advantage of digital cameras is that this calibration is usually modifiable.

 

Hot and Cold Light

 

Each light source has its own characteristic spectrum, which determines a hotter or colder light, or the “temperature” of light. This variable, measured in Kelvin (k), refers to the temperature at which an ideal uncolored (black) object emits the same hue of radiation. It should be emphasized that “warm” light corresponds to lower temperature, not higher temperature, because our synaesthesia is inversely proportional to temperature. For example, a red hot object tends to be red (warm), and when heated further, it tends to be blue (cold).

 

The wavelength of light is represented by its spectral power distribution measurement system, and its measurement unit is nanometer (nm). Low temperatures match orange or yellow, and at lower levels, there are red and infrared (invisible to the human eye). By increasing the degrees on the Kelvin scale, the light changes from white to blue, purple, and finally ultraviolet (invisible to human eyes).

 

The two classic temperatures used in photography are:

3200K, or “tungsten”, equivalent to the light of tungsten wire, with light yellow and orange (warm light)

5600k, or “Sunlight”, corresponding to sunlight, light blue (cold light)

 

 

1. SUBJECT LIT BY A 5600K LIGHT SOURCE
2. SUBJECT LIT BY A 3200K LIGHT SOURCE

These are two basic temperature values corresponding to your camera to get a good balance ‘white’, no blue or yellow dominant, depending on the type of lamp used. For example, the white light from neon lights tends to be green, and although the eyes will feel it is white, the camera will show its true color.

 

 

 

 

 

 

 

WAVELENGTHS EXPRESSED IN NANOMETERS, WITH VISIBLE LIGHT RANGING FROM 380 NM TO 740 NM.
1. ULTRAVIOLET
2. VISIBLE LIGHT
3. INFRARED
4. DEEP INFRARED

 

Color temperature differences

The minimum perceptible color difference between two light sources depends on the difference of reciprocal temperature rather than the difference of temperature itself. In other words, the change of color temperature does not produce the same color perception change.

 

Let’s imagine two options as the starting point, 5600k of daylight and 3200K tungsten. If we add both to 100k, it is clear that the result will be 5700k of sunlight and 3300K tungsten.

 

 

 

 

 

 

 

 

 

 

 

1. DAYLIGHT CONVERSION
2. TUNGSTEN CONVERSION

 

If we do a field experiment, we will clearly notice the difference of tungsten light, although it is difficult to detect in sunlight. In short, if the light is warm, our 100k difference will be more obvious, on the contrary, if the light is cold, it is less obvious.

That’s why in 1932, Owen g. Prester proposed the concept of “deep depression”, that is, microscopic reciprocity, to measure the smallest perceptible difference. This measurement system is used to calculate the amount of color correction needed to achieve the desired color temperature. The mud value (m) is calculated as follows:

 

M = 1,000,000 / T

where T is the color temperature in Kelvin.

To illustrate this principle more clearly with an example, if we take the initial t of 5500k, it will correspond to 181 mireds (1000000 divided by 5500). Now, if we take the final color temperature of 3200K, we get 312 bogs. The last value subtracts the starting value to get 131 mireds (312-181), which is equivalent to a sun 85 filter.

Most filters and gel manufacturers’ websites show data of all the corresponding relationships between filters and mireds. Most modern spectrometers can provide us with accurate filter usage models, because internally, they have the list of the most common filter manufacturers. In any case, it’s worth noting that a positive value indicates that the filter tends to yellow, while a negative value indicates a blue filter. Then, in order to evaluate the final result of using multiple filters together, the notch values of various filters can be summarized.

Daylight and Tungsten light

Why are lights mainly used in the temperature of these two colors? The answer lies in the calibration of the first films. Outdoors, we used to rely on the sun as the key light, so we made a special film for it. Tungsten lamps are used indoors and another type of film is made for this purpose. Therefore, the use of these two standard temperatures is a legacy of this traditional 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 studio and outdoor settings. Another possibility is to focus on correcting the color temperature of the film itself or in a digital camera.

For example, for film, you can use a yellow or blue filter, depending on the type of film you use. The most common filter for this conversion is the 85 (Orange) or 80 (blue) filter. There are also two types of 80A and 80B on the market. The packaging of the film should indicate what kind of filter should be installed before simulating static or motion picture camera. However, with digital media, this correction can be done in real time from the setup panel without using any filters. Color correction can also be carried out in post production, but it can only be carried out when shooting in the original image format, which is equivalent to the digital format of negative film and contains more information than the viewer can see.

The last suggestion is to use tungsten film. When you add filters in the sun or in the HMI light, you should also use UV filters because the tungsten film does not work well with UV.