Understanding light

“Life is a long, serene river” is the poetic title of a 1988 French comedy (La vie est un long fleuve tranquil) directed by Étienne Chatiliez. Somehow that seems to fit what I do – watch the day go by as I work with cameras and fish for light. No dramas as such. The sun comes up and goes down, the light meanders, and I watch the ebbs and flows, waiting for my opportunity. At least I used to.

In these hurried times, exposure and the color of light are now adjusted automatically by the cameras (and cell phones) which work very well. But if you are working with cameras professionally, it is good to understand what is going on.

Take electric light for example. When incandescent light bulbs were invented around 1878, a wire filament in a vacuum heated by electricity produced a welcome warm glow – much better than the blue light from the gas lamps. And soon, bulbs were being created to satisfy all kinds of needs: to create ambience, to imitate daylight, help plants grow, for precision color matching. 

Today we can shape and control light as we wish. But what exactly is light?

What is light?

Simply put, light is energy, radiated energy or electromagnetic radiation, and best understood as a stream of mass-less particles (photons) traveling in a wave-like pattern at the speed of light.

The visible light we humans see consists only of a tiny tranche of the electromagnetic spectrum of radiated energy which, beginning with the shortest wavelength, divides into Gamma rays, X Rays, UV, Visible light, Infrared, Microwaves, and long TV and Radio waves.

The shorter the wavelength, the more quickly it oscillates, and the more heat and energy it creates. So long wavelengths have low energy, and low temperatures.

The spectrum of visible light

At one end of the spectrum, our eyes see the waves of photons oscillating at high frequency reflected as violet. At the other end, we see the long, less-energetic wavelengths reflected as red. And the different colors we see in between are simply different wavelengths of reflected light.

In other words, the different colors we see are determined only by the wavelength of the light that is reflected off the objects we are looking at. For example, a red ball will reflect only red light to our eyes, a blue ball will reflect only blue light. And a white paper will reflect all the colors in the spectrum so making it appear white.

So reflected light, and emitted light from a light source are quite different. Because it is the makeup of an object that determines the color of light that it reflects.

A light source can include all the colors. The sun emIts wavelengths of radiation ranging from ultraviolet, all the way through visible to infrared light. But the color of emitted sunlight can change too.

Measuring the temperature of visible light

In 1842, Lord Kelvin proposed a new thermometric scale for scientists to measure temperature where 0° degrees K corresponded to absolute zero (minus 273° Celsius). And today we measure the temperature of emitted light in degrees Kelvin. For photographic purposes, only the range 1,200°K to 9,000°K is used.

The higher the K reading is, the bluer and whiter the light will be, because light sources get whiter as they get hotter emitting the full spectrum of visible light. Thus the lower the degree on the K scale, the redder the light will be. Red hot is cooler than white-hot. See the diagram above.

For example, the red in a wood fire might be 1500°K. And a regular tungsten bulb will emit a yellow/orange light from 2,700°–3,300°K – a soft welcoming color, flattering like sunlight in the late afternoon. But daylight approximates at 5,600° K, and the light temperature at midday with an overhead sun at 6,500°K.

Lighting for video and photography

In video production, bright Halogen lights are sometimes used although the bulbs get very hot and use a lot of power, and the light they produce  is a warmish 3,200°K. Fluorescent tubes can be a better choice because they give off very little heat, use less power and provide a soft, even light. They are available at two different color temperatures from  3,200°K to a coolish 5,500°K (N.B. These are special tubes should be marked with a Color Rendition Index – CRI – close to 100).

Photographers’ strobes, studio flashes are mostly rated at 5,500°K, close to the 5,600°K temperature of daylight.

When mixing light – for example daylight and incandescent light – it is easy change your electric light to match the blueish daylight coming in with a CTB (color-temperature-blue) gel in front of the light(s) you are using.

Personally I use LED (Light Emitting Diodes) lights. They are inexpensive, portable and use little power. And unlike old-fashioned bulbs, they have no filaments to burn out since they work by exciting electrons in a semiconductor. And better still, LEDS can be temperature matched and temperature variable.

However, if you have really deep pockets, and don’t care how hot lights get, how much power they use or how large they are, the best way to light a studio is still with mercury vapor HMI lights, high power medium arc-length lamps made specifically for the film and entertainment industries.

The bottom line, is that even if your camera will do much of the work for you, understanding the color of light is fundamental to film-making and photography – and there is still a place for color temperature meters and light meters to help judge exposure.