Projection Rules

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The Four-Six-Eight Rule -468-

The screen should always fit to the audience, not to the projector! A rule of thumb for screen sizes is the four-six-eight rule -468-. The farthest viewer should be no more than four, six or eight times the image height away from the screen. The three options depend on the following conditions:

-   four times for material with fine details like CAD drawings or other detailed graphics
-   six times for detailed reading (spreadsheets, text, images with text)
-   eight times for watching movies or images

rule of thumb for screen height

CAD: X = 4 * h Powerpoint: X = 6 * h Movies: X = 8 * h

Important: this rule evolved with the majority of projections in the format XGA (1024 x 768). With formats like SXGA, SXGA+ and HD(1080) this changes because the screen height is not built by 768 pixels but by 1024, 1050 and 1080 pixels. This is an increase of about 35 to 40 percent. All details that are presented in native pixel resolution become smaller for 35 to 40 percent !

Also the image height should be within the following range:

Distance of farthest Viewer / 8   <   Image Height   <   Distance of closest Viewer / 2

Text Size vs. Image Resolution

Text size depends on pixel size and image resolution. A Powerpoint Presentation will scale the slide size to the image size but a Excel sheet will be displayed depending on the pixel size.
With higher resolution the presentation should be zoomed in:      or a larger screen could be considered.

WXGA (1280x800) image from an Excel sheet,
text size Arial20

WUXGA (1920x1200) image from the same Excel sheet,
text size Arial20

Higher resolutions always have the disadvantage of smaller text sizes, except the text is zoomed.

1) A often used rule of thumb: the text height should be at least 4mm per one meter of viewing distance. For example: Viewing distance = 10m   =>   the text height should be at least 40mm.

2) The FAA (Federal Aviation Administration, USA) suggests for letters that must be very well readable (security messages) a letter size of at least 20 acrminutes (MOA) based on the human eye's characteristics: FAA Human Factors. 20 arcminutes correspond to 1/200 of the viewing distance.

3) With high resolution projection rule 1 and 2 result in a text height that is too large for a regular Excel display on a regular projection size. For these circumstances a tighter rule can be used: dividing the maximum viewing distance by 500 to get the absolute minimum text height (the absolute minimum for good eyes). It represents a viewing angle of about 7 arcminutes (1 degree = 60 arcminutes).

4) For good readable text with text sizes like 'Arial 10pt' the maximum viewing distance should be divided by 500 to get the necessary text height. But with higher resolutions (HD) this often results in image sizes that are too large to be installed (ceiling height etc.).

Distance Visual Acuity

The distance visual acuity is the visual sharpness or clearness of human eye view. With normal viewing abilities two objects can be just seen individually with an viewing angle of 1 arcminute (MOA). This equals approximately an object distance of 1,5mm at a distance of 5 meters or 1,75mm at a distance of 6 meters (20 feet).

Normal visual ability means that a person who can see this clearly has a Distance Visual Acuity of 5/5 (Europe) or 6/6 (Britain) or 20/20 (US). If a person is tested for a visual acuity of 6/12 (or 20/40), she or he can differentiate two objects only at half distance. The term "6/12" means that two objects with a separation of 1 arcminute at a distance of 12 meters can only be seen separately at a distance of 6 meters, the term "20/40" means equally that two objects with a separation of 1 arcminute at a distance of 40 feet can only be seen separately at a distance of 20 feet.

Visual LossVisual Acuity
no or slight loss6/6 - 6/1820/20 - 20/60
moderate loss6/18 - 6/6020/60 - 20/200
severe loss6/60 and more20/200 and more

Based on the rules of visual acuity letters and numbers should have at least five times the height of the minimum distance between two recognizable objects. This results in a minimum letter and number hight of 5 arcminutes, there as 7 arcminutes are acknowledged for general good readability.

  smallest text size  

Text with a font size of 10 pt (like Arial 10) results on a Windows display in a text height of 10 pixels. With a given text height calculated by the maximum viewing distance (distance/500) and the known number of lines (vertical pixels) of the projected image, a necessary image height can be calculated.

For instance:
a maximum viewing distance of 20m gives a absolute minimum text height of 40mm (20,000 / 500 = 40)
a preferred font size of Arial 10 results in a text height of 10 pixels
an image source (PC) with a format of WXGA (1280x800) results in 800 lines (vertical pixels)
a text height of 40mm (equal to 10 pixels) demands a image height of 3200mm (10 lines = 40mm > one line = 4mm > 800 lines = 3200mm)
the required image height is 3.2m
this matches the rule of thumb that the image height should be "max. viewing distance / 6" = 20m / 6 = 3,33m (depends on PC format) !!

calculate with this Excel file, download here

Font Size in Powerpoint Presentations

Powerpoint and similar presentation tools display letters and numbers in relative size to the entire image area not pixel-dependent. In any other regular PC application letters and numbers are displayed by pixels depending the font size chosen in the application. With a higher display resolution letters become smaller. The same application on a WXGA resolution (for instance 1440x900) looks 'larger' than on a WUXGA resolution (1920x1200), displayed pixel by pixel.
With a Powerpoint application, the actual displayed font size depends only on the relation to the entire screen size, not on the relation to the pixel resolution. In other words, the same Powerpoint presentation displayed on WXGA or on WUXGA 'looks' the same.

The letter size (letter height of a capital B) can be calculated for 19:10 Powerpoint presentations:

Letter Hight h (mm) = Image Height H (m) * 1,63 * Fontsize

Image Height H (m) = Letter Height h (mm) / ( 1,63 * Font Size)

For instance: necessary letter height is 68mm, font size is '16'. What is the needed minimum image height?
H = h / (1,63 * 16) = 68mm / (1,63 * 16) = 2,6m

relation between image height and letter height in a Powerpoint presentation

In a situation with restricted space and a projection display mainly used for Powerpoint presentations a compromise with a smaller image size and a defined minimum font size can be calculated. As long as this font size is guaranteed the 'small' display can still be used satisfactory.

Screen Position

The bottom of the screen should be in a high enough vertical position, allowing those seated in the rear of the audience to see the screen completely. The distance between floor and screen should be about 1.1 to 1.2 meters (43" .. 48").

distance between floor and screen: 0.9 meters (35") - too low distance between floor and screen: 1.2 meters (47")

Viewing Angle

The maximum acceptable viewing angle should be 45 degrees. Beyond that characters and image elements become undecipherable. Ideally, all viewers should be seated within 30 of the projection axis, and never more than 45 off axis.
Screen center should be no more than 20 above the eye level of any viewer.

maximum viewing horizontal angle: 45 degrees on both sides

Keystone Correction and Warping

Using keystone correction and warping always usable light output and usable resolution is increased. The higher the resolution the better both methods work because of smaller irregularities within the resulting image.

projection on axis

projection off axis using warping as correction tool,
resulting in the same image size as with projection on axis,
but over 25% of light output and resolution lost !