As discussed, the majority of spaces in contemporary institutions will be categorised as BDM; and to specify images we must determine the size of the smallest element you wish to be able to resolve under typical conditions. This is called the element height and is the height of the element expressed as a percentage of the image height (%EH).
We cannot control the quality of user-authored presentations, so each institution must determine an appropriate image height through observation and investigation. The %EH may differ between institutions, and will be partially informed by less tangible parameters:
- If instructors typically display complex diagrams, images or text you might oversize images to improve readability
- Viewers may be able to assimilate very complex materials given sufficient time and the ability to focus – but they are also listening to the instructor.
- Existing buildings may have been designed decades before the advent of presentation technologies, and the local architecture may constrain the size and location of screens
A good starting point is a minimum element height of 3%, which fits neatly with the old ‘rule of thumb’ that called for farthest viewer distance to be no more than 6x image height.
For those institutions wishing to retain a 5.3IH multiplier, %EH = 2.65
Note that for calculation of image size, only two categories are considered:
- ADM: In those areas where pixel-level detail must be resolved (e.g: some fine arts, medical imaging, engineering spaces.)
- BDM: Everywhere else… most learning and teaching spaces are categorised as BDM
A space may be categorised as both ADM and BDM, though the compliant viewing areas will differ.
An element can be graphical or text, but many designers want to translate it to font sizes as a frame of reference. This section is not an authoritative discussion of typography – any font is characterised as much by the white space as the black as well as several other artistic decisions – but provides a basic understanding of text construction which may be useful.
The minimum typical electronic character is defined within a 9 x 7 grid of pixels; e.g.:
The smallest possible character, then, is a lower case ‘e’ which only requires five lines of pixels:
When considering element height, consider how your user will resolve unscaled text (next chapter) that may only be 5 pixels tall.
Always remember that the human eye can generally resolve only down to one minute of arc (one sixtieth of one degree)