We have discussed that the closest viewer in a BDM space should not angle their head more than 30° above horizontal to view the top of the image, so can apply the standard trigonometry tangent function to determine the closest viewer distance; i.e. the length of the adjacent side of the triangle.
In this example
- Angle (α) is 30°; and
- The length of the opposite side is the sum of the height of the image (IH) and its offset with relation to standard eyeline (IO).
Substituting for our terminology and rearranging delivers a dramatically simpler formula:
- Image Height = 1500
- Bottom of image = 1400; and
- Standard eyeline = 1200
- IO = (1400 – 1200) = 200
- CV = 1.732 x (1500 + 200) = 2944mm
Width of the CV row
It is useful to also calculate the maximum width of that front row, which is naturally constrained by our horizontal viewing limits of 60°. To avoid ambiguity, many users abbreviate this term to CVROW.
Using our example above for a 16:9 image,
- IH + IO = 1500 + 200 = 1700
- IW = 16/9 x 1500 = 2667
- So CVROW = 6 x 1700 – 2667 = 7533
You should use this value to check your work when plotting the viewing area – if it doesn’t match what you’ve drawn you’ve made an error!
TIP: AVIXA publishes calculators online at https://www.avixa.org/standards/discas-calculators/