Television engineers tend to think of the world as entirely digital these days. CCDs for image acquisition (digital capture), digital video recorders, video servers, archive systems, digital transmission systems and, of course, DTV have convinced us technological solutions in the industry have become wholly digital.
This impression is only partially correct. Digital images begin as analog signals on (largely) CCD imagers. The individual pixels in a CCD actually store an analog representation of the scene as focused by a lens. The stored charges in these analog cells are converted to digital representations of optical flux, and with appropriate infrastructure can remain digital all the way to the consumer.
But it is important to remember that the imaging process begins with the original analog medium, light. A lens focuses the image of a scene from the analog flux of visible light reflected from the subject. Although industry physicists remind us that light has a distinctly "digital" character at the quantum level, it suffices for our purposes to view light as an analog medium that must be managed in a manner no less critical to the digital imaging process than CCDs and VTRs.
Any number of processes generate light. In our industry two essential processes are used. The most widely used technology is the process of heating a filament to a temperature hot enough to release black body radiation. Pretty crude stuff, and a process that has been in use for over 100 years in consumer and commercial lighting. Specialized sources can produce light whose spectra are broadband and applicable to high-quality reproduction of complex colorimetry. The tri-stimulus sensors used in cameras need a broad range of spectral content to permit the image in the camera to replicate the visual image our eyes (also analog sensors) perceive. This process has been thought through by several generations of imaging engineers and scientists and has reached a high state of art. Camera sensors and processing hardware and software accommodate the unique characteristics of the physics involved and the response of the human visual system to the same stimuli. Taken as a complete system the net effect is to produce accurate and repeatable imaging.
A second, and considerably newer, technology leverages another consumer lighting technology. We all have fluorescent lighting in our homes, offices and even flashlights. The basic physics in this case utilizes an energized plasma that pumps electrons to a (usually) cylindrical surface coated with phosphorescing chemicals. The energy efficiency is greatly enhanced because a filament does not need to be heated to produce black body radiation. The nature of the emitting elements produces light output per unit of energy input, but for most of the history of this technology the light has been concentrated in narrow spectral bands. While acceptable for consumer lighting for decades, this technology did not produce acceptable colorimetry for television imaging.
In the last decade the technology has been updated to permit colorimetry that works. However, the light source is still an extended surface and requires a more technically complex system. Which brings us to the second half of the physics lesson as it applies to imaging.
Our visual system creates a huge database in our brains of what natural images should look like. We "automatically" adjust contrast range and color locus without thinking about it. But much more complex information about the image is stored as a representation that can be thought of as digital memories of what an image should look like. For instance, we know what a night scene should look like. The memory of the image includes the colorimetry, the contrast range, range of image lighting sources and shadows applicable to a night scene. Similarly we have memorized models of many natural scenes that allow us to interpret the image content without actively thinking about the details. We are more used to single primary light sources for daylight scenes, and multiple light sources for indoor scenes. We recalculate the image expectations without much thought.
These natural image expectations include the type of light source. Think of the sun as a point source. Conversely the light of the sun falling on a white tent canopy would produce a diffuse light source from under the canopy. In a material way the point source is the equivalent of incandescent light instruments, and as a generalized rule, now regularly broken, the fluorescent source is closer to the tent surface.
If you want to produce images our minds will recognize from our inventory of natural image types you must carefully pick the type of light source and the instruments that project that light on the image you wish to represent. Doing so will produce an image that meets our expectation, and ignoring this question will create a scene that is quickly tagged by the mind as fantasy. This is not a problem, indeed it is technique well understood by video and film professionals for generations, used to translate the artistic intent of the producer.
Controlling lighting is much more than picking the type of light source. Filtering the light, intentionally diffusing the light and changing the intensity of the source are common processes for lighting and imaging professionals. The extent of the light source for a fluorescent (in our industry usually a compact fluorescent source) makes focusing it more difficult. The heat generated by a black body incandescent source makes filtering requirements different. Incandescent sources are easier to modulate for intensity, though dimmable compact fluorescent has been available for several years. The trick in both cases is to modulate the intensity without modulating the spectral content. This allows the light source to be dimmed without the color changing in the camera.
Lighting technology choices must be made with careful consideration of issues like size, weight and energy consumption. Power consumption might lead to a choice of compact fluorescent sources. However, if the lighting instrument needs to be mounted on top of an ENG camera, a small black body instrument may be the only practical choice despite the fact that it will consume a considerably more power. In the end the technology will supply a range of solutions with tradeoffs in portability and power requirements for the professional to choose the best solution.
All of this is background, and perhaps may not appear very useful to the professionals on the firing line each day. Rest assured the designers of the hardware used by everyone in the industry must have a thorough understanding of the issues so that the engineering solutions they pick will produce the desired effect when used by the videographer on the job. What is important for the professional on the street is to understand that the tools used have a basis in underlying science that affect how they should be chosen.
