Pixel pitch and pixel spacing

In “MTF and high resolution sensors” I discussed the impact of pixel pitch on resolution. Having received a question on this, I’d like to explain briefly why its pixel pitch that matters and not pixel size (although they are of course related,) and tell you where to find the information.

The size of a pixel gives you an indication of its light-receiving area. However, in discussing resolution it’s the spacing between the pixels that matters. The equation is:

Fmax-practical = 1/(4 x pixel pitch)

Where Fmax-practical is the maximum line-pair frequency that the sensor can resolve.

I gave the example of the 5Mp Sony ICX625 CCD which has pixels of 3.45 microns. However, the pixel pitch is around 4 microns. Why is the pitch greater than the pixel size? Well not all of the silicon collects light. In effect, there’s a border around each pixel, or to put it another way, there’s a space between neighboring pixels.

To determine the pitch you need the dimensions of the active region of the CCD, and for that you need to sensor’s spec sheet. Strangely, I couldn’t find it on Sony’s web site – they seem to want to keep it secret – but it's out there if you look hard enough. Once you've tracked it down you'll learn that the ICX625 measures 9.93 mm in the horizontal direction, and there are 2448 pixels in that length, which is how we get the pixel pitch of 4 microns. As a sanity check, you could do the following: given that the sensor is a 2/3” format with a 4:3 aspect ratio and a diagonal of 11.016mm, work out the length of the sides and calculate the pitch from there.

Hope that makes things clear. And by the way, I do welcome comments, so if you’ve got questions, complaints or anything you want to share, please make use of the “Comment” function.