Radiography · Chapter 4
Geometric unsharpness
Why the edges on a radiograph blur, and how to calculate by how much.
Source size drawn larger so you can see it.
Predict, then test
Three distances control the blur
Big F and big OFD blur the edge. Big SOD sharpens it.
Ug0.02″
Film against the part, so OFD = thickness. To scale, except source size and Ug (drawn 40× larger).
The formula
One formula, two jobs
F comes with the source. OFD comes from the part. SOD is the distance you control.
MinUg=F×OFD÷SOD
Ug=F×OFD÷SOD
MinSOD=F×OFD÷Ug
Know the distance? Find Ug.
Min SFD = Min SOD + OFD
F = focal spot size · OFD = object-to-film · SOD = source-to-object · SFD = source-to-film
Where Ug max comes from
The code table
ASME Section V, T-274.2. Look up the material thickness.
| Material thickness | Ug max |
|---|---|
| Under 2″ | 0.020″ |
| 2″ through 3″ | 0.030″ |
| Over 3″ through 4″ | 0.040″ |
| Over 4″ | 0.070″ |
Check the edition and procedure your job is written to.
Practice
Twelve problems, one step at a time
Problem
Wrap-up
Check any setup
- 1A real source has size, so edges get a blurred band: Ug.
- 2Ug = F × OFD ÷ SOD.
- 3Min SOD = F × OFD ÷ Ug max. Round up, never down.
- 4SFD = SOD + OFD. Given SFD? Subtract OFD first.