Audio Visual Design Guidelines

Calculating the System Contrast Ratio

11 views November 21, 2018 aetm 0

In order to design a compliant system we must be able to calculate the ‘brightness’ required to achieve the specified System Contrast Ratio.

Projection

Gather the following information:

  • Ambient light on screen (L) from all natural and artificial sources. This is realistically your ‘black’ level, as the system cannot achieve lower. Measured in lux.
  • Target contrast ratio (C)
  • Image area (A), which you can easily derive from the Image Size calculations you have prepared. Measured in m
  • Screen gain (Sg).
    • If you are using a typical vinyl or fibreglass projection surface, the screen should be unity gain (1.0) but maybe slightly above or below
    • Specialist screens – particularly ambient light rejection types will usually offer higher gain but have a smaller field of view
    • In all cases – ask the screen manufacturer for data if you are unsure.
  • Lens derating factor (Dl). It is often difficult to get accurate lens data so the following assumptions are made:
    • A ‘standard’ lens is one with throw ratio around 2.0 (in practice often 1.7-2.3) and which has the least complex light path; i.e.: fewest glass elements. Dl = 1.0
    • Short- and long-throw lenses generally contain more glass elements and are expected to lose 20% of the light. Dl = 0.8
  • Projector derating value (Dr) is applied so we achieve the required System Contrast Ratio on average across the life of the projector.
    • A new lamp has 100% output, and end-of-life is 50% brightness then the calculation point is at 75% brightness. Dr = 0.75

Once you have the requisite information, apply this formula to determine the minimum required projector brightness (Luminous flux, measured in lumens):

Example

  • Screen:
    • 120” 16:9 (2655 x 1495 = 3.97m2)
    • Standard vinyl; gain = 1.0
  • Ambient light on screen = 80 lux
  • Viewing category is BDM, target contrast ratio = 15:1
  • Standard lens

Then

= 6352 lumens

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