Color in the Wild

As we discussed in the previous chapter, color in nature is in light, or more precise in the spectral wavelength of light, without light there would be no color. That is even the case when we humans are not observing colors of course.

So to describe color in nature we can use those spectral wavelengths, we don't need any color space, simply because there is no color space in nature, color spaces are all man made.

First, the spectral distributions and intensities of each wavelength are measured with spectroradiometers and spectrophotometers. Then the results are plotted with a graph, showing the spectral wavelength in X and the intensity of those wavelengths in Y. 

The higher a wavelength in the graph, the more intense that color is in the color mix. Two spectra in that graph with the same distribution but different intensities will appear as the same color but with different brightness.

The resulting color of that spectral graph is the sum of all wavelength intensities multiplied.
This allows us to define color with the spectral wavelength of light.

Light sources, materials and even the human response to light and color can be described with those spectral distributions. They all plot the same thing but are named differently for each purpose.
Let’s look at spectral distributions for lights next.

The Color of Daylight and Light Sources

Light sources can be described and compared with so-called Spectral Power Diagrams (SPD). The intensity of the light is plotted in Y and the wavelengths of the spectrum are plotted in X.

The combined spectral power is the color of the light source, in the image above the light source is blue, because it has its highest intensities in the wavelength range of 450 - 520nm.

The distribution of daylight is quite even throughout the spectrum, which makes it very neutral in color with a touch of coolness, as you can tell by its slightly higher blue intensity in the image below. Similarly the Incandescent light source below is also easily identifiable as a warm light source, because it has more red intensity than blue or green

While daylight has an even distribution over the whole spectrum, many light sources don’t. Fluorescent lights and LEDs have spikes and are not very even throughout the spectrum, as you can see in the image above, those light sources are not very good at rendering colors faithfully.

LEDs are used more and more in cinematography and professional LED lights are different from consumer LED lights mostly in their more even spectral power distribution. Since the spikes in consumer led lights can easily create color inconsistencies, unexpected colors or ugly colors, when filmed with a camera.
The spectral power diagram is a key to show how light sources are rendering color, and they are important for cinematographers and gaffers for that reason.

The Color Temperature Scale

There is another way to describe the color of light sources and we probably all are familiar with it when we are using a camera, the color temperature scale. The color temperature scale was introduced as a quick short cut to describe light sources that are emitting light by heat. The scale is based on the color of a black body radiator heated at different temperatures.

Starting with red on the left, with increasing heat the scale transitions from orange and yellow to white and blue. It is measured in kelvin, a tungsten light source has a range of 2500-3200K (kelvin) for example, average daylight is at 5000-6500K.

The camera’s white balance is using the color temperature scale to keep the white in our photos white under changing illumination. A process that is mimicking the behavior of our eyes as we will see in the blog post about human vision.

Light sources like halogen or led lights, that do not create light with heat can just be correlated with the color temperature scale, specified as CCT (correlated color temperature). They are usually better described with a spectral power diagram.


The Color of Objects

The colors of materials or objects can be described with the Spectral Reflectance Diagram (SRD). It works exactly the same as the spectral power diagram of light sources. The reflectance is plotted in percent 1-100% in Y with the wavelength in X.

Since the color of an object depends on the illuminating light source as much as the material, the spectral reflectance diagram of the object has to be multiplied wavelength per wavelength with the spectral power diagram of the light source to determine the color of the object under that light source.

In the image above you can see how our blue light source (SPD), illuminating a yellow object (SRD) changes the appearance of the object to be greenish.


The Color of Gels / Translucent Objects

Gels, used in cinematography to adjust the color of a light source, or translucent materials can be described with the Spectral Transmittance Diagram (STD).

The transmittance is plotted as a percentage in Y with the spectral wavelength in X, as an example below is the spectral transmittance diagram of a Lee 089 gel.

If that Lee 089 gel would be strapped in front of our blue light source from earlier, the SPD of the light source would be multiplied with the STD of the Lee gel to calculate and plot the color of the light. You can see how it would be influencing the blue light in the diagram below.

Summary

There are no colorspaces in nature. Colors in nature can be described with their distribution and intensity over the visible spectrum with spectral diagrams. This can be done for the color of light sources and materials, using spectral power-, spectral reflectance- or spectral transmittance diagrams.

The color of objects/materials depends on the illuminating light source, it is determined by the spectral reflectance diagram of the material multiplied with the spectral power diagram of the light source.

This leaves us with two of the three components needed in color science to specify colors, lights and objects. The last component needed to describe light and color accurately as Goethe realized is us, the observer. Human vision is playing a big role in light and color sensation, so let’s have a look at that next.

As always let me know if you have questions, critique or input please, hello@christophschroeer.com.