Lighting

Bring your own light

By Bob Newman, first published February 2013

One of the topics that this series of articles returns to often is exposure. Remember that exposure has essentially two elements, one is luminance of the subject, the other is the proportion of that luminance transmitted to the image plane, which is usually expressed as the exposure value – which in turn is determined by the f-number and the shutter speed. Thus it is that most photographers learn to control exposure by controlling the exposure value. However, another possibility exists, which is to control the luminance of the subject, which is what this month’s article is about. Generally the luminance of the subject may be controlled by varying the illuminance – the amount of light shining on it. So, to control this, it means that the photographer must provide a light source of some variety. From flash powder at the turn of the twentieth century to LED panels available now, a significant part of the photographer’s arsenal has been photographic lighting. Here we will be examining the different technologies available today, how they work and what that means for the photographer.

Physical principles

Essentially all light sources work on the same basic principle – if surplus energy is imparted to an object, the energy will be emitted as electromagnetic energy in order for the object to regain equilibrium with its environment. Electromagnetic energy includes radio waves, microwaves, x-rays and most importantly for our purposes, light. So, essentially what is required to produce artificial light is to load a suitable object with enough energy that it emits light. The loading energy generally is in the form of electrical energy, although in the past in flashbulbs and the aforementioned flash powder, chemical energy was used – and as an aside, the electrical energy used in modern cameras is generated by a chemical reaction. So, what are the mechanisms by which substances loaded with energy generate light?

The first is black body radiation. When a body has an elevated temperature, it must lose that some of that energy to the environment in order to regain its thermal equilibrium. To do that it will emit photons, which carry away the surplus energy. It is this phenomenon that causes hot objects to emit light, and the colour of the light depends on the temperature of the body – we are used to the terms ‘red hot’, meaning that an object is hot enough to emit red light, and ‘white hot’, meaning that it gives out white light. In fact, by its nature black body radiation never gives out a pure colour. The energy of the photons, and therefore their colour is randomly distributed, but the average colour moves from red to violet as the body gets hotter. Figure 1a shows the colour distribution of black bodies of different temperatures (in Kelvin, or degrees above absolute zero) while Figure 1b gives the path of those distributions on the CIE colour chart. Notice as the body gets hotter, the colour balance moves from red to blue. This is the basis of the term ’colour temperature’ applied to a light source – simply it is the temperature of a black body which would give that colour balance.

Figure 1 (a) Black body radiation as produced by incandescent lamps, and in part by flash tudes, produces a continuous spectrum of light, particularly attractive for colour photographic use.

Figure 1(b) The hotter an incandescent lamp runs, the higher its colour temperature, and the more efficient it is.

The second mechanism by which photons can be generated involves imparting excess energy to an electron which is confined in an orbit of an atom, molecule, or more recently and artificial nano-scale structure such as a quantum dot. The energy causes the electron to move to a more energetic orbit. Eventually it will revert back to the original orbit, dumping the excess energy as a photon. The energy of the photon, and therefore the colour of the light produced depends on the energy difference between the two orbits, and therefore this mechanism will produce very pure single colours, rather than the spread of whitish light produced by black body radiation. Some molecules have a large number of available electron orbitals, so can produce light that is a mixture of a number of different pure colours.

One variation or another of these two principles is used for every artificial light source available to photographers. The way in which it is done is a matter of technology.

Technology

From the above, it would appear to be the case that black body radiation is the light source best suited to photographic use since it provides a broad even spread of light, and that is indeed the case, except for one major drawback, which we will come to later. To produce black body radiation all that is required is to make something very hot. The first way of doing this was by use of chemical energy. By burning a metal such as magnesium or aluminium in an oxidizer, an intense heat is produced, which heats the combustion products to sufficient temperature to produce the required intensity and spectrum of light. The original embodiment of this principle was flash powder, magnesium powder mixed with as solid oxidizer such as potassium nitrate. A further refinement was the flash bulb, in which the magnesium wire was placed in a glass bulb filled with an oxygen atmosphere. The wire would be heated by an electric current, causing it to ignite and produce a flash of intense light. Flash bulbs carried their energy source with them, but were by nature a single use device. If an external source of electrical energy were available, then it could be used to heat a metal wire directly by resistive heating to produce the required temperature. This is the formerly ubiquitous incandescent light bulb. It produces a continuous even spread of light, and would be an ideal light source, except for the problem referred to earlier. Looking again at figure 1a, we can see that a black body radiates in all wavelengths including those far to long to be visible, known as ‘infra red’. Thus a lot of the radiation energy is wasted producing non-visible light, which just becomes converted to heat. Incandescent lighting is therefore inefficient, and runs very hot. The situation is made better by running the lamp hotter, which means that more of the energy is converted to light in the visible spectrum, but the temperatures required will melt most available metals, even tungsten, the high melting-point metal used for lamp filaments. There are work arounds, for instance filling the lamp envelope with a halogen gas, which sets up a chemical cycle which continually redeposits evaporated tungsten back on the filament, allowing it to last longer at the elevated temperatures required. This produces a ‘tungsten halogen lamp’ which is the type used in most incandescent continuous photographic lamps.

Another solution is to heat a gas rather than a metal. If a gas is ionized (electrons added or taken away) – which can be done by an intense electric field – then it will conduct, and will be heated by a current passing through it. This is an arc lamp, and can produce an intense light at a higher working temperature than can an incandescent lamp. For photographic use, these arc lamps are used in the form of a flash tube, producing short individual bursts of light using the electrical energy stored in a capacitor. This allows the available energy to be concentrated during the exposure itself, and thus drastically reduces the amount of energy needed. The gas used should be as chemically unreactive as possible, otherwise it will destroy the glass container and electrodes at the elevated temperatures used. Typically the inert gas xenon is used. Arc lamps don’t produce pure back body radiation, a proportion of the light comes using the second mechanism,

However, continuous lighting has many advantages, allowing easier composition and focusing and being essential for video. To produce the greater efficiency that allows this use can be made of the other light generation principle, the light produced as electrons move between orbits. The technological key to this is use of a phenomenon called fluorescence, in which light radiation energises the electrons of particular materials, and is then re-emitted as a different, cooler colour. These materials are called phosphors and can be engineered to produce a large number of individual emission colours, which closely simulate the visual properties of the continuous black body radiation. Suppose an arc light is made that runs so hot that it produces light in the ultraviolet (too energetic to be visible) – it will be much more efficient than a cool running one, the only drawback being that the light is invisible. If it is coated with a suitable phosphor, then that light can be converted to useful visible light, while retaining the efficiency. This is the operating principle of fluorescent lighting, including the ‘high efficiency’ lamps used in typical fluorescent photographic lights. Phosphors also make possible the highest efficiency form of lighting currently available, the white light emitting diode (LED). LEDs work on the second light emitting principle, with electric energy directly exciting the electrons in a semiconductor material, which then revert to their original state, emitting photons with a very pure colour – unsuitable for photographic use. In a white LED a blue or violet LED is coated with a phosphor which re-emits the light produced across a wider, more useful range of colours.