Still- it's one we get on a fairly consistent basis so, in the interest of helping out all the noobs in the house, we put together a concise description and a few photos that should clear up where in the world the first foot of your R/F (or rad room) tape measure went.
So, you went to calculate the thickness of your subject (distance to tabletop minus distance to subject) and found the tape measure on your collimator looking pretty different from the one in your toolbox at home. Specifically: it starts at 12".
There's a very good reason for this and, if it's something you've forgotten or somehow missed, it's also very understandable that you would be confused by it. You see, most measuring tapes start at 0" because they are intended to measure the distance between two points that will be determined by the user on a use-by-use basis. In the case of the collimator tape measure, one of those points remains constant and (herein lies the confusion) it is NOT the bottom of the collimator.
The real starting point for your collimator-mounted tape measure is actually about one foot higher, at the focal spot of your tube. In other words, the measurements you're really looking for are the distances to your tabletop and your subject from the X-ray source, not the beam-limiting device. With this in mind, your collimator's manufacturer went ahead and built those inches right in.
In case you're wondering exactly where your focal spot is, it's most often behind a cooling fan and/or the tube mount controls (left). When these items are not obscuring it, you can see the spot marked with a decal (right).
If there are other R/F room questions on your mind, our service team can help you with those too. We love to see imaging facilities enjoying maximum equipment uptime and we love helping to make it happen with service support, field engineering, and replacement parts. Contact us to find out what we can do for your R/F system, or take a peek at some of our other R/F resources to learn more.