Audio Visual Design Guidelines

Measuring the Contrast Ratio

3 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:

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

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

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