Light interaction with the environment

For an artist, there is no other subject that could have such significant meaning, than what the behaviour of light and matter can offer. Once you open your eyes and glance around, everything that you see is based upon this.

Understanding the world around us is beneficial for everyone who has interests in science and modern technology, and specially for visual artists. It can offer tremendous practical possibilities on how to approach your work and the choices you make with your materials.

Your paints, charcoals and graphites are all materials that appear on your sight for the same reason. They are all bound by the same laws. Light behaves in spesific and predictable ways in our observable universe. Let`s dive in.

1729 French mathematician, astronomer and geophysicist Pierre Bouguer published his book - Optical treatise on the gradation of light. This is one of the earliest studies on photometry. Bouguer made experiments what happens to the visible intensity of light when it passes thru different mediums such as glass etc. He also compared the apparent brightness of celestial objects to that of a standard flame from a candle. Investigated the absorption of light in the atmosphere and formulated Bouguer`s law. Which is now known as Beer - Lambert law. For these accomplishments he is considered to be the father of photometry. However, when going thru the history of photometry, contributions of Johann Lambert seem to overwhelm Bouguer, even though Lambert several times refers to Bouguer´s discoveries in his book Photometria that was published 1760.

But what photometry actually is ?

In a simple way to put it, photometry studies and measures light behaviour thru space and matter in the visible spectrum. Light is electromagnetic radiation that our eyes can detect. Visible radiation. Light has only few ways to interact in the surrounding environment.

Emission, Transmission, Refraction, Reflection and Absorption.

These interactions happen beyond our visible spectrum and what our eyes are capable to observe. Let´s take a closer look to these to understand what type of interactions they are.

Emission, in a very short way to put it, is when light is being born.

All matter is made of atoms, that consist subparticles such as protons, neutrons and electrons. There are different type of atoms, when they combine and form groups that are chemically bonded together, they are referred to as elements or molecules. ( not as simple but we try to keep this short ) Different elements have different atomic numbers, what this means for example, Oxygen has the atomic number 8 - its atoms contain 8 protons and 8 electrons. Uranium has the atomic number 92 - its atoms contain 92 protons and 92 electrons. Each atom has a set of energy levels associated with it, all of the atoms of particular element have the same set of energy levels, but every element has a unique set of energy levels associated with its atoms. Like a fingerprint.

Electrons exist in the energy levels of the atom, and they always have to occupy one of them, but they can jump back and forth between energy levels. To jump from one level to another, requires exactly equal amount of energy than what is the difference between those energy levels. No more, no less. Once the electron absorbs energy from the surroundings, it will jump to higher energy level turning the atom from ground state to excited stage. But it will not remain in that higher level, it will return to ground state, and as it does it releases that energy it absorbed in a form of light. This process is called emission.

Depending on the amount of energy released, corresponds to a spesific wavelength of light, if it is in our visible spectrum it appears as a spesific color. But more than that, now you understand that light carries energy.

Transmission, happens when matter allows light to travel thru, like window glass.

But we have far more better and fascinating examples of transmission. Red jelly appears red, since it consists of matter that absorbs other wavelengths of light, but not the wavelength of red. It allows red to travel thru. Same way if you place a piece of transparent tape over a flashlight and color it with red marker, you have created a filter. Ink will absorb other wavelenghts but allow red to pass thru and you have made a red flashlight.

Put water in glass and look at it in different colored light, it will allow transmission for every color. However, if we put matter in there, perhaps a fruit, and create orange juice. It turns yellow, but not because of transmission. What happens now, is that the orange absorbs rest of the colors and the juice turns opaque, we can´t see thru it. Instead of allowing the yellow light to pass thru it bounces back, because it is being rejected.

This rejection is called Reflection.

Usually when we think about reflection, things that come to our minds are mirrors, or shiny polished metals, or perhaps the surface of a lake. Solid matters absorb light, and what is not absorbed is being reflected. So in a funny way, the color we see in an object, is not what it actually is. But why this is important to understand if you´re an artist? Let´s take a quick look at this.

In the next image , the red ball is shown as a material absorbing other colors.

Your red paint is material that does the same. Looking at the next image, Blue - complementary color of Orange, we can see what colors it removes in order to appear as cyan.

If you think about these two from the point of mixing your paints, by adding orange and blue paint together, what you are actually doing instead of adding, will result in removal of color. You mixed materials together that now remove all visible range of light as shown in image. Here is why your paint will turn brown, grey, dirty dark color close to black. This phenomenon is Absorption.

Even though at first glance, these science things might feel hard or boring to comprehend, they will have remarkable value in the practical solutions that you apply in your process of creating visual art.

Last one on our list is Refraction.

Typical way of refraction can be seen with rainbow, water droplets kind of “slow down” light and differently with each wavelenght that correspond to colors. So the colors seem to be traveling in slightly different angle to the point they can be observed as single colors. Water also reflects them in different directions, this chaotic bouncing is called scattering. Another way to undertsand it is to look at crystals, or objects made out them such as prisms. When the colors are being separated from the light by the matter as it travels thru it.

Photometry studies and measures what happens to the intensity of the light during these visible interactions. Lamberts Photometria presented significant advancements in this field, but was slightly overlooked, since the main question at the time with optics was : What is the nature of light?

Lambert`s work was not related to this issue at all and it wasn´t incorporated to the mainstream of optical science until nearly a century after it was published, when astronomy and commercial lighting were developed to the point it needed photometry. And guess what,in the modern era ,after computer graphics took over, Lambert´s results were used as the basis for radiosity calculations required to produce renderings.

If you have tried out 3D modeling and rendering, when playing with the materials you might have come across with the term albedo. Lambert was the person who introduced this term to us. It refers to the materials ability to reflect light. Or perhaps you found emission, now you understand that it refers to the object as a source of light in the environment.

Lambert´s cosine law states that the radiant intensity observed from an ideal diffusely reflecting surface is directly proportional to the cosine of the angle between the observers line of sight and the surface normal. Sounds too complicated? In other words, this means that a surface that obeys Lamberts law is said to be Lambertian and it demonstrates Lambertian reflectance. For human eye, that surface has the same brightness regardless of the angle of view. For example white paper is good presentation of something that is very close to being Lambertian surface.

The work of Bouguer and Lambert in developing photometry, became more useful as other fields of science progressed. Bouguer invented the first heliometer, that was later perfected by no other than Joseph von Fraunhofer who invented the first modern spectroscope.

1854 German physicist and chemist, August Beer published his results in his Introduction to the higher optics. His work combined with Bouguer/Lambert law make up what is known as Beer-Lambert law. Basically it is used to determine the concentration of chemical species that absorb light. Put together all this and a new field of astronomical science is born, spectroscopy.

As we earlier went thru how light is being emitted, and how there are almost magical things happening thru the process beyond what we are able to observe with our plain sight, spectroscopy studies and measures these actions that we are not capable of. What is being absorbed, emitted etc. is like a spesific fingerprint of the matter involved in the process. That´s how we know what the light source that we are observing in the distance is made of. What are the planets made of? Does it have an atmosphere, and if so, what it contains? What size they are and how far they are from us? And that´s not all that we can do, it also expanded to our medical healthcare and eventually to what imaging possibilities we have to inspect our bodies, such as MRI. But that is completely different story. However, it all started with Bouguer and Lambert. If this was interesting subject to you, you might want to read my POV how to apply these in your practical solutions with your work in creating visual art.

Light behaves in a spesific and predictable ways in our physical reality.

Sincerely

Mikko