Physics and photography - white balance
November 13, 2017
Word photography is actually a combination of two words which have roots in Greek language. “Photon” which can be translated to light and “graphia” which means drawing. So, if you combine it together it sounds like “painting with light”. If we deep dive into the first word – photon, we are actually visiting the realm of physics because photon is an electromagnetic wave carrier, a single quantum of the electromagnetic wave. There are many types of electromagnetic (EM) waves which are all part of EM spectrum. Visible light, which is used to create a photograph, is just a small part of the entire spectrum within wavelength range from approximately 400 to 700 nanometers. All the wavelengths which are in that range, our eye and brain can recognize, and we see as the colors of the rainbow, from violet to red. Everything bellow or above we cannot see (infrared and ultraviolet light being the first ones above 700 nm and bellow 400 nm respectively).
So, as we can see, the technical part of creating the photography is all about physics. Everything from your lens abilities and limitation to the characteristics of camera sensor. Almost every term we know from everyday camera use, like exposure, white balance, aperture, etc.… can be explained with physics.
Since all those technical stuff can make you go mad (all the scientist is a little crazy, aren’t they?), you don’t need physics to create a good photograph, just a good “eye”, a little touch of art and some sense of esthetics. So, this text is just for some deeper understanding of words we use often in photography, for more curios folks, and is not in any way intended to make your photographs better. It’s not for the crazy, mad physicist either, since they probably know all of this even being awaken after two hours of sleep after some drinking party.
So, let’s try to explain white balance from the physics point of view. The first thing you hear is about some Kelvin guy and some big numbers. That Kelvin guy actually became an English Lord thanks to his interest in science, mathematics and physics. He was very popular in 19th century in the scientific circle. Well maybe not as popular as movie stars or singers nowadays, but popular enough so that basic unit for temperature was named after him. Yes, scientist measure temperature in degrees Kelvin, and this temperature scale starts from zero Kelvins, called absolute zero, which in our daily life is expressed as -273.15 Celsius.
Now it becomes clear that those numbers we hear when we are talking about white balance are actually degrees in Kelvin scale. But how is this possible? We see very large numbers there, from around 2 000 to 10 000 Kelvins. Is something burning inside the camera?
Of course, nothing is burning or getting hot in the camera, but to explain what those number represent we need to call physics again.
Every object emits EM radiation in according to its temperature. If we are looking at an object which is not reflective, but absorbs 100% of the incoming light we are defining Black body because if no light is escaping, no part of EM radiation is emitted and thus that object has no colour, it appears black. Planck’s Law defines the spectral density and a wavelength with strongest intensity of a black body as a function of its temperature. Simply put, the more temperature the body has, the wavelength of its EM radiation is shorter. And since shorter wavelength corresponds to violet/blue colour, and longer wavelength to orange/yellow colour we can say that the more you heat the body, its colour goes from red to blue. If we measure the temperature when the body looks red it will be around 2000 Kelvins, and if we measure it while it is blue its temperature is then around 8000-1000 Kelvins. From the real-life experience, we know that if we heat the piece of metal gradually, first it will become red, and if we heat it more (raising its temperature), it will be white and finally blue. That is the origin of the numbers in your white balance scale in your camera or in your photo editing software.
So according to Plank’s Law, if you have candle light or tungsten bulb, it is emitting light that corresponds to a yellow colour and your images will usually look yellowish if the camera sets white balance automatically. If you are photographing on a cloudy day, the colours will shift to blue (to higher numbers in Kelvin scale), not because clouds are heated to 9000 Kelvins, but because that blue temperature corresponds to a temperature of a black body heated to 9000 Kelvins. Here is where the physics ends. For the photography use, we just need to memorize that higher the white balance number, bluer will our scene be. To compensate the errors of cameras automatic white balance, we can choose it manually, either by presets given by the camera, or directly changing the numbers on a white balance Kelvin temperature scale. If shooting in RAW, white balance is easily corrected with slider which is inverted if compared to our little story above. The blue is with lower number and yellow is with a higher number because it represents how you need to compensate your initial white balance setting.
So, continue to practice your photography skills and don’t burn anything up trying to achieve cooler white balance ;)