Test, QC & Monitoring Global Viewpoint – March 2019

Fine-Tuning a TV Production Set with T&M Gear

The ubiquitous waveform monitor and vectorscope provide a standard technical reference for on-set lighting adjustments and display color correction.

Many highly skilled set-lighting lighting directors, designers and technicians like to say lighting is an art. Many color correction technicians like to say color correction is an art. Both are true. Everyone likes beautiful Hollywood-style lighting and production values, but it can be a bit pricey.

Engineers can operate video cameras, but few are accomplished videographers. While an engineer holding a camera thinks “Wow, look at that!” a real videographer is already shooting it. Engineers can light a set with multiple lights, but most can't do it with the subjective style, bravado and confidence of a professional lighting director.

Broadcast engineers know the objective side of lighting and color correction because it can be scientifically observed and measured on a calibrated waveform monitor and a vectorscope. Not all productions can afford an expensive Lighting Designer or a Hollywood Colorist. Most often, those jobs are done by engineers and directors working with the budget, time and gear they’ve got.

Basic Lighting Calibration

A waveform monitor shows measurable variations in video levels using a graticule calibrated from 0-100 IRE units. Every pixel can have a unique level, but for the purposes of lighting adjustments, we’re interested in controlling luminance variations to make the best pictures technically possible. A waveform monitor displays an analog of how those levels are digitally mapped across a TV display screen.

The least forgiving production sets for lighting variations are a green-screen chroma key wall or an infinity cyclorama (cyc). Issues of uneven chroma key lighting are all too obvious. The same can be said for lighting the talent in front of the infinity cyc or green-screen, but that’s usually more of a creative decision. Flatly-lit people don’t stand out on-screen.

The foundation of a chroma key is a flatly-lit background. To tune a set background for the flattest lighting, lock-down a video camera shot of the set and use a waveform monitor to observe subtle lighting variations across the background. Watch the waveform monitor while physically moving lights, adjusting barn-doors and experimenting with scrims to flatten the lighting.

It’s much easier to detect anomalies on a static shot with waveform monitor than looking at the TV picture. When the waveform monitor sweep is set to Horizontal, the display shows all brightness values from top-to-bottom on the TV screen stacked from left-to-right. When the sweep is set to Vertical, left-to-right on the display is TV all brightness values top-to-bottom. Experiment with the waveform monitor's settings to see the relationships between the TV picture and the waveform display.

When dealing with electronic media, waveform monitors are the only true reference. After flat-lighting calibration, leave further creative lighting decisions to the director. From that point on, the engineer’s job is to ensure that every source the director takes is seamless and stays between 0 and 100 IRE units on the waveform monitor.

Lighting for Video Screens

Chroma keys have been a local TV news and weather set staple for nearly 50 years, with virtual sets being the latest phase. Today’s new trend is replacing TV news and weather set chroma keys with flat panel display walls, some with interactive touchscreens. Huge flat-panel displays on a TV production set brings new dynamics and technological advantages to live show unseen on local TV until the technology became big, bright, and inexpensive enough about a dozen years ago. What once was a green-screen or over-the-shoulder digital squeeze effect is becoming a real, on-set prop that looks better, and itcan be interactive. The engineering challenge is matching the color balance of the video displays and the lighted set.

Changing the color temperature of the stage lights isn’t the answer. Halogen studio lights are usually 3200 K, the preferred color temperature for film. Most digital image sensors are optimized for natural light at 5600 K. Most portable and studio LED lights are 5600 K or are variable between 3200 K to 5600 K.

The color temperature settings of modern flat-panel displays are easily accessed in the display menus. Exact names and numbers will vary by brand, but the typical Cool/Cold setting is approximately 10,000 K, Neutral/Normal at about 9300 K, Warm 1 is about 7500 K, and Warm 2 is about 6500 K. On a set lit by 5600 K studio lights, a 6500 K display will look blue.

Top: Shot with a Canon camera set at 5500K. Below: Same camera with a color-corrected HDMI monitor feed. Note the color consistency of the fruit bowl at the bottom of both photos. Image courtesy Ensemble Designs.

Top: Shot with a Canon camera set at 5500K. Below: Same camera with a color-corrected HDMI monitor feed. Note the color consistency of the fruit bowl at the bottom of both photos. Image courtesy Ensemble Designs.

Essential Kelvin

Kelvin is an absolute thermometric scale, not a degree like Celsius or Fahrenheit. For reference, 0 K is absolute zero, the point where all thermal motion ceases. 0 K = -273.15° C = -459.67° F. The precise Kelvin temperature at which liquid water, solid ice, and water vapor can coexist is 273.1600 K, which equals 0.0100 °C or 32.0180 °F.

The color temperature of a candle is about 1900 K. The effective Kelvin temperature of the surface of sun is 5775 K. Summer shade can be 8000 K or higher. The higher the Kelvin temperature lighting the scene, the “cooler” or more blueish the scene appears on camera until the camera is white-balanced for the higher color temperature.

Matching Kelvins

The no-budget approach to resolving a color temperature discrepancy is to manually adjust the display monitor settings if ‘warm’ or ‘cold’ settings. If those temperature settings are insufficient, individual color channel controls are often found under several different submenu names, depending on the display and manufacturer. Nearly all provide Red Green and Blue gain and black controls. Some include extra individual controls for Cyan, Magenta and Yellow. It's enough to get an amateur without a vectorscope in trouble.

You will need a color bar source, a freshly white-balanced camera on a tripod and a vectorscope. Feed the color bars into the display, shoot the display with the locked-down camera, and observe the camera output on the vectorscope.

Top: Color bars skew blue before color correction. Below: Same camera output with color-correction applied to monitor. Not perfect, but close. Image courtesy Ensemble Designs.

Top: Color bars skew blue before color correction. Below: Same camera output with color-correction applied to monitor. Not perfect, but close. Image courtesy Ensemble Designs.

Different manufacturers make their display adjustments seem tricky, but not so when you have two of the most common studio T&M devices to guide you -- a color bar source and a vectorscope. Feed color bars, perhaps from a 2ndcamera if you don’t have a test signal generator, into the display monitor.

Adjust the display monitor sub-Picture settings to move the camera output color dots on the vectorscope into the same boxes a direct color bar signal belongs. You don’t need to flip an A/B switch to match the camera. The boxes on the vectorscope graticule are your targets and it shows what you are doing to the display monitor, colorby color, on a screen designed to do little else than that. Color shifts will be obvious on the vectorscope.

Once the dots are inside the boxes or as close as possible, write down the new display monitor settings for future reference, because someone with good intentions might mess with them later.

Real Engineering

Everything is better with an adequate budget, and matching color temperatures on a TV news set is no exception. Real engineers need real solutions. The best news is that an affordable display matching solution is available for less than the typical capital expense threshold.

The Ensemble Designs BrightEye BE 72 is one device that's filled with multiple useful features, including built-in test patterns, closed caption decoding, timecode burn-in, audio level meters, H&V shift and horizontal mirror, a safe-zone graticule overlay, and a complete proc amp and RGB color corrector. Control software for it runs on a PC or Mac connected by USB.

What's useful to help tune a production set is that the built-in color corrector can color can balance any HDMI monitor without having to touch a single control or setting on the display monitor itself. A recent Ensemble Designs white paper describes color correcting on-set monitors with a BE 72. Comparable alternative test gear can provide roughly similar results if the corrections are tuned and applied in the display settings.

“Step 1. Select the 20/80 Test Pattern in BE 72. The 20% Gray and black regions will be used to set black balance. The 80% Gray (nearly white) is used to adjust the gain of each of the Red, Green, and Blue channels.

Step 2. With the camera, compose a shot of the monitor with the 20% region filling the screen. If you are working with small monitors the BE 72 has full screen 20 and 80 percent patterns, too.

Step 3. View the camera output on a vectorscope. Adjust the RGB offsets to collapse the chroma to a single dot. NOTE: This is best done using just Red and Blue, leaving Green as the reference. The controls are all accessible through the BE 72 front panel using the On-Screen Display, or with BrightEye Mac or PC software to connect to the USB port.

Step 4. Frame the shot on the 80% segment. Repeat the adjustment procedure to collapse the chroma, but this time use the RGB Gain controls.

Step 5. Select the 10-step grayscale chip chart, or SMPTE Bars on the BE 72 internal TSG. While viewing the monitor through the camera, use The BE72's Proc Amp controls (Brightness, Contrast, Chroma) to achieve proper exposure.

Remember: The face of the monitor is an actual source of light, while everything else in the set is reflecting light from the lighting instruments. It is easy to make the monitor too bright. If you hold a white card in front of the monitor, next to the 100% white flag in color bars, you can adjust the monitor until they match.

Extra Credit: You can check the results by comparing the vector presentation of color bars.

The white paper ends with a disclaimer that bears repeating: “Lining up cameras and monitors is more Art than Science. We are design engineers and we do not presume to dictate how people with far more hands-on expertise than we have use this equipment. Certainly, there is more than one way to achieve good results. Our job is to give you the tools to do it. Each operator will develop their own, preferred technique.”

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