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20/03/2014

Test Charts: Simplifying Colour Control In The Digital Age

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Today, digital cameras are smarter than ever and capable of making superb-looking images, so why are test charts necessary? The answer is control, says David Corley, SMPTE Fellow and President of DSC Labs.

Digital cameras can be used to capture beautiful images, but a single scene may look very different when shot with two different cameras or under different lighting conditions. Though the characteristics of images now can be adjusted electronically to address many such differences, this capability can be exceedingly dangerous without a reliable reference standard, namely a precision test chart.
The change of any one colour to a completely different colour will typically affect many other colours in a scene. This is particularly important in the world of advertising, where the familiar colour of a product plays a significant role in branding. The test chart enables the director of photography (DP) to understand the impact of this change.

The Demands of Digital
For at least 100 years, test charts have been used to control picture quality, from motion pictures to printing and publishing. The gray-card and colour bar were essential in producing consistent high-quality images because they provided standard references that were invaluable to laboratory processing and printing of the film.
With film there were relatively few variables. Film sensitivity and colour characteristics were reasonably consistent, as was lighting. Typical sources - daylight, carbon arc, or incandescent - had full-spectrum energy distribution that could be adjusted using simple filters. Viewing conditions were also standardised. With the move to digital, control became more difficult.
In digital production today, colour characteristics depend on much more than colour temperature (3200k, 5000k, 5500k, D65, etc.). Cameras’ spectral sensitivity curves differ significantly, even between cameras of the same make and model, and spectral distribution differs across light sources, which today include daylight, tungsten, HMI, fluorescent, and a hundred different LEDs.
The differing spectral characteristics of the viewing equipment may cause images to look great in the editing suite, but not necessarily so on the home display. Though editors with ‘golden eyeballs’ can make infinite colour adjustment on their consoles, their judgement doesn’t always match the client’s perception or vision. Viewing conditions and eyesight adaption to a particular environment also influence colour characteristics.
Given all these variables, it’s more important than ever to have an on-set reference to indicate how colour should reproduce. If the DP can set up and reproduce a precision test chart accurately, then the rest of the scene should be accurate, too, and the whole process suddenly becomes much simpler. That is the main - and not insignificant - benefit of using accurate test charts. When shooting takes place at more than one location, each with unique variables, colour correction of the reference can quickly bring clips into conformance, allowing shots to be edited together smoothly. In this way, DPs get the looks they want without the time and expense introduced by extensive lab processing.
Electronic test signals may seem a viable alternative to test charts, but they suffice only if the image was originally generated electronically, as in animated movies and cartoons. This approach does not work for real-world images, as electronically generated test patterns on iPods and cell phones tell nothing about the characteristics of the light illuminating the scene, or the variability between electronic displays.

Test Chart Essentials
A test chart used for colour control must provide a truly neutral grayscale, such as the patented spectrophotometrically neutral version in all SMPTE charts from DSC Labs. (Many test charts have a yellow bias that results in undesirably blue-looking images.) The chart’s colour reference chips must provide consistent meaningful electronic data, not just pretty colours, and that data should be easily interpreted on standard electronic equipment such as waveform monitors and vectorscopes. Finally, because nothing lasts forever, the user must have access to a reliable monitoring system that can be used to ensure continued test chart accuracy.
Naturally, test charts must be produced in accordance with current industry standards, including those from SMPTE. In fact, DSC Labs has created three charts specifically for SMPTE, and they are available exclusively from the Society.
The DSC Labs OneShot chart is designed to capture as much camera and scene lighting information in one shot as possible so that dailies look as close to the DP's intent as possible. Unlike most other DSC Labs charts, the OneShot is printed with a matte surface to reduce obvious reflections and thereby help users shoot accurately in high-pressure, time-sensitive shooting environments. Because it’s impossible to create a matte front-lit chart that captures the full dynamic range of a modern camera, the chart gives the colourist solid white and gray references, plus an additional true black so that contrast can be accurately reproduced from scene to scene.
SMPTE’s DSC Labs CamBook 3 is a convenient precision tool that speeds the process of aligning and setting up cameras to REC 709, comparing and matching camera makes and models, and testing lenses for colourimetry and resolution. In production, the CamBook 3 helps users select the best camera for the job and serves as a useful reference both on set and in post for colour correction of images. Last but not least, the DSC Labs CamWhite pocket chart offered by SMPTE serves as a consistent and reliable neutral white reference.
While these and other test charts are valuable in enabling DPs and colourists to achieve the desired look for a motion picture, television program, or ad spot, the fact is that in using this simple yet powerful tool, they also can save an enormous amount of time, work, and money in getting pictures just right.

The article is available in the online edition of RFV here.
(IT/MH)
VMI.TV Ltd

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