Audio Visual Design Guidelines

BDM only: Sizing the image

9 views November 21, 2018 aetm 0

To specify an image correctly, we require three pieces of information:

  1. Image height (IH)
  2. Farthest viewer (FV); and
  3. Element height (%EH)

A constant called the Acuity Factor (AF) helps simplify our calculation, and this is defined differently for ADM and BDM. AFBDM = 200 and is a result of simplifying quite complex trigonometric calculations.

With any two of the three variables and the acuity factor we can determine the missing variable and adequately size the image. The formula is straightforward, and easy to manipulate to find the missing variable.

To calculate image height, for example:

Rearranging the formula lets us to find the farthest viewer

or the element height



  • Image Height = 1500mm
  • Farthest viewer = 10500mm


  • %EH = 10500 / (1500 x 200) = 0.035 = 3.5%

AVIXA has developed calculators for DISCAS, and these are available free online at


But a percentage means nothing – just how tall should my ‘element’ be?

Good news! You can easily define absolute element height (let’s call it |EH|) by manipulating the same BDM formula.

Consider a presentation compromising a single element occupying the full image height. It follows that:

  1. %EH = 100% (0.01 for the formula)
  2. As the element is full height, then |EH|| = IH

Our formula, then becomes

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