Welcome to the RT Distance Concepts Interactive Module
This module contains 5 exercises that will teach you the fundamentals of Source-to-Object Distance (SOD), Object-to-Film Distance (OFD), and Source-to-Film Distance (SFD).
What you'll do:
1. Learn the Setup — Understand each component and distance
2. Build a Setup — Drag and drop components into position
3. See the Effect — Experiment with a live simulator
4. Calculate Ug — Practice the geometric unsharpness formula
5. Final Quiz — Test your understanding
Important: Please complete all 5 exercises before clicking the "Complete and Continue" button below this module. The exercises are sequential — each one unlocks after you finish the previous one. You'll know you're done when you see the congratulations screen at the end of Exercise 5.
⛶ For the best experience, go full screen first
This module has interactive diagrams, drag-and-drop exercises, and a simulator that work much better with the full screen space.
⛶ For the best experience, open this module in full screen
This module has interactive diagrams, drag-and-drop exercises, and a simulator that work much better with more screen space. Click below to open in a new tab where you can go full screen.
In radiographic testing, we use a radiation source to send X-rays or gamma rays through an object (the part we're inspecting) onto a film (or detector) that captures the image. The distances between these three elements are critical for image quality.
Click on each component to learn what it is and why it matters.
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Radiation Source
Click to learn more
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Test Object
Click to learn more
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Film / Detector
Click to learn more
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The Three Key Distances
Hover over each term to see its definition:
Key Relationship: SFD = SOD + OFD. The Source-to-Film Distance always equals the Source-to-Object Distance plus the Object-to-Film Distance. Click on SOD, OFD, or SFD labels in the diagram above to learn more!
Film in Contact with Object (Ideal Setup)
When the film is placed directly against the object, OFD equals the object thickness and is minimised. This gives the sharpest possible image.
Best Practice: Placing the film directly against the object minimises OFD (it equals just the object thickness). This makes SFD nearly equal to SOD and produces the sharpest image with the least geometric unsharpness.
Exercise 2: Build the Radiographic Setup
Place the components in the correct order from left to right. Remember: radiation travels from the Source → through the Object → to the Film.
Drag each component to its correct position to create a proper radiographic setup.
Position 1 (Leftmost)
Position 2 (Middle)
Position 3 (Rightmost)
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Film
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Source
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Object
Exercise 3: See How Distances Affect Image Quality
Adjust the sliders to move the source, object, and film. Watch how the shadow (image) changes!
Goal: Understand that as
OFDObject-to-Film Distance: The distance between object top and the film. Larger OFD = more geometric unsharpness (blurry edges).
increases, the image becomes blurry (more geometric unsharpness). And as
SODSource-to-Object Distance: The distance from the radiation source to the object. Larger SOD = sharper image (less unsharpness).
increases, the image becomes sharper.
🌟 Challenge 1 of 3
Create the Sharpest Image Possible
Use the sliders below to minimize geometric unsharpness (Ug). Try increasing the distance between the source and the object (SOD), moving the film as close to the object as possible (reduce OFD), and reducing the source size (f). Can you get Ug below 0.05 mm?
Create the Blurriest Image Possible
Now do the opposite. Move the source as close to the object as you can (reduce SOD), move the film far from the object (increase OFD), and increase the source size (f) to its maximum. How high can you get Ug?
Set SFD to Exactly 700 mm with Ug Below 0.5 mm
This time you have a constraint: the total Source-to-Film Distance (SFD) must equal 700 mm. Adjust the source, object, and film positions so that SFD reads 700 mm, while also keeping Ug below 0.5 mm. You may need to adjust the source size too.
SOD: 250 mm
OFD: 300 mm
SFD: 550 mm
Ug: — mm
Image Sharpness:
Moderate
Exercise 4: Calculate Geometric Unsharpness
Use the formula to solve real calculation problems. Type your answer and check it!
Ug = f × OFD / SOD
Where: Ug = Geometric Unsharpness (mm) — the blurriness of the image edges f = Focal spot size of the source (mm) OFD = Object-to-Film Distance (mm) SOD = Source-to-Object Distance (mm)
Problem 1 of 4
A technician uses a source with a focal spot size of 3 mm. The SOD is 600 mm and the OFD is 30 mm.
What is the geometric unsharpness (Ug)?
Ug = mm
Problem 2 of 4
An Ir-192 gamma ray source has a source size of 2.5 mm. The SFD is 800 mm and the OFD is 50 mm.
Hint: First find SOD from SFD and OFD, then calculate Ug.
SOD = mm Ug = mm
Problem 3 of 4
The maximum allowed Ug for a job is 0.5 mm. The source focal spot is 4 mm and the OFD is 40 mm.
What is the minimum SOD required to meet the Ug requirement?
Hint: Rearrange the formula: SOD = f × OFD / Ug
Min SOD = mm
Problem 4 of 4
A Co-60 gamma ray source (source size f = 5 mm) is placed at SOD = 500 mm. The object thickness is 25 mm and the film is placed directly behind the object.
What is the OFD, and what is the Ug?
OFD = mm Ug = mm
0/4
Formula practice complete!
Exercise 5: Final Assessment Quiz
Test your understanding with these scenario-based questions. Choose the best answer for each.
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Congratulations, You've Completed the Module!
You have successfully worked through all 5 exercises covering Source-to-Object Distance (SOD), Object-to-Film Distance (OFD), Source-to-Film Distance (SFD), and the Geometric Unsharpness formula.
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Calculation Score
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Quiz Score
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