Thursday, 31 January 2008

Gridspot Design Calculator

Not wanting to leave grid design to chance, I decided to come up with some theory on how the beam spread of a snoot or grid could be predicted using a bit of trigonometry. The theory is that the light passes through an angle that is determined by the width of the aperture and its length. So for a given angle of light spread, you can choose to have a short grid with a small aperture, or a long grid with a large aperture.

If you know the angle of spread you want the grid to have and you know the width of the aperture, you can work out the length you need to make the grid, with the following equation.

Length = Aperture/Tan(Angle/2)

Or you may find these useful

Aperture = Length * Tan(Angle/2)
Angle = 2 * Atan(Aperture/Length)

To make life a little easier for you, I have put together a little calculator, that allows you to change any of the variables and will recalculate for you on the fly.

The checkboxes link variables, so that if you change a value in a box, then the one it is linked to, will be re-calculated. Hopefully it will help you avoid making grids that are too long or too short.

Width of Aperture 

Angle of Beam
Spread of Beam

Grid Length

cm per metre


One of the things I have learnt from ths excercise is that I need to get out more and a deep and abiding hatred of javascript and css.

I haven't tested this thoroughly so if you find any bugs or have any suggestions please leave a comment. I'd also like to hear from you if it helped you build your grid


  1. Thanks Paulo, this is great! Cheers!

  2. found your blog on your comment to my strobist post.
    thanks for sharing! great blog.

  3. Thanks for the hard work. I've come back to this several times in planning grids.

  4. Awesome dude! great all you need is "getting the word out" for your calculator.

  5. This is awesome! One question- what's "atan" or the formula for atan?

  6. Its the arctangent