Audio Visual Design Guidelines

Measuring the Contrast Ratio

365 views November 22, 2018 aetm 0

We don’t often measure System Contrast Ratio following a new build – our design calculations and display settings should ensure compliance by a substantial margin. When evaluating/qualifying an existing room, however, a quick and reliable method of measuring is required.

Full details are contained in ANSI/InfoComm 3M, but the general process is simple and described here. It requires only two items of test gear:

  • A test pattern which you can build yourself in PowerPoint – simply 16 equal rectangles and alternately filled white or black.
  • A spot photometer/luminance meter with a narrow (<2°) acceptance angle. We want to measure reflected light (what the eye sees) and the narrow acceptance angle allows us to target an area right in the centre of each box.
    • Several devices are available, from hobbyist/photographer level to those designed for scientific use. For a typical institution AV team, one of the photographer-level devices offers the best value for money e.g. Sekonic L858 series.
      Ensure your meter provides measurements in lux – many speak a language only photographers understand
    • Ensure any test equipment is calibrated regularly – most standards require calibration at no more than two-year intervals.

The actual procedure is straightforward:

1 Let your projector/display warm up for a few minutes so it is operating in its usual temperature range
2 Load the test pattern on a handy computer and display it full screen
3 Set room lighting and blinds/drapes to the normal default setting for projection
4 At each measuring location (refer standard) use your meter to measure the luminance in the centre of each of the rectangles. Record the results.
5 When measurements are complete,

·        Calculate the average of all the WHITE measurements

·        Calculate the average of all the BLACK measurements

·        Divide average WHITE by average BLACK to derive the contrast ratio at that position

6 Repeat this procedure for different lighting presets in a multipurpose space (e.g.: general teaching and ‘screening’ modes)

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