In the world of machine vision and optical testing, few tools are as universally recognized as the USAF1951 resolution test target. Whether you're calibrating a new lens, comparing optical performance, or troubleshooting image quality issues, this standardized pattern has been the gold standard for decades. But what exactly is this pattern, and how do you use it correctly?
What is a USAF1951 Test Target?
A USAF1951 test target (United States Air Force 1951 resolution target) is a standardized pattern used to measure the resolving power of optical imaging systems. It consists of groups of three horizontal and vertical bars arranged in decreasing size, allowing engineers to determine the resolution limit of cameras, lenses, and complete imaging systems.
The target was originally developed in 1951 by the United States Air Force to test the resolution of aerial photography equipment, hence its name. Today, it remains the most widely used resolution target in machine vision, microscopy, and general optical testing applications.
The pattern is typically printed or etched onto glass, chrome, or film substrates, with various sizes and contrast ratios available for different applications.
How to Read a USAF1951 Target?
Reading a USAF1951 target involves identifying the smallest group and element where you can clearly distinguish all three bars in both horizontal and vertical orientations. Each group contains six elements, and each element consists of three bars. The group and element numbers are used in a formula to calculate the resolution in line pairs per millimeter (lp/mm).
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Here's a step-by-step guide to reading the target:
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Find the smallest discernible pattern - Look for the smallest set of bars where you can clearly see all three lines with space between them, in both horizontal and vertical orientations.
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Identify the group and element numbers - Each pattern is labeled with two numbers: the group number and the element number (1-6). The numbers may appear directly on the target or require a reference chart.
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Calculate the resolution - Use this formula to determine the resolution in line pairs per millimeter:
Resolution (lp/mm) = 2^(Group + (Element-1)/6)
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Convert to other measurements if needed - For example, to find the resolving power in microns, use:
Resolving power (μm) = 1000 / Resolution (lp/mm)
Let's walk through a practical example. If you can resolve Group 4, Element 2, your resolution would be:
Resolution = 2^(4 + (2-1)/6) = 2^(4 + 1/6) = 2^4.167 ≈ 17.96 lp/mm
This means your system can resolve details as small as approximately 18 line pairs per millimeter.
Types of USAF1951 Targets Available
Several variations of the USAF1951 target exist, each designed for specific testing scenarios:
1. Positive vs. Negative Targets
- Positive targets feature black lines on a clear background, ideal for transmission measurements and backlit applications.
- Negative targets have clear lines on a black background, preferred for reflective measurements and front-lit setups.
2. High vs. Low Contrast Targets
- High contrast targets (typically >90:1) are used for standard resolution testing.
- Low contrast targets (often 5:1 or 10:1) help evaluate system performance under challenging lighting conditions.
3. Extended Range Targets
Some targets feature additional groups (from negative groups up to higher positive numbers) to accommodate very low or very high resolution systems:
- Negative groups (e.g., -2, -1) for testing very low resolution systems
- Extended positive groups (e.g., up to Group 9) for evaluating microscope objectives and high-magnification systems
4. Substrate Options
USAF1951 patterns can be found on various substrates:
- Chrome on glass - Highest durability and precision
- Film-based - More economical but less durable
- Reflective substrates - For testing systems that capture reflected light
Practical Applications of USAF1951 Targets
Camera and Lens Testing
The primary use of USAF1951 targets is determining the resolution capabilities of imaging systems. This allows engineers to:
- Compare the performance of different lenses
- Verify that a camera meets its specified resolution
- Measure the impact of aperture settings on resolution
- Evaluate depth of field by measuring resolution at different distances
System Calibration
Beyond basic resolution testing, these targets help in:
- Measuring and correcting optical distortion
- Calculating precise magnification
- Calibrating pixel-to-real-world measurement systems
- Assessing optical aberrations
Quality Control in Manufacturing
In production environments, USAF1951 targets are used to:
- Ensure consistent performance across manufactured batches of cameras or lenses
- Verify that vision systems meet minimum resolution requirements
- Document system performance for ISO compliance and quality assurance
Common Challenges and Best Practices
Proper Target Illumination
The quality of illumination significantly impacts resolution measurements. For accurate results:
- Ensure even illumination across the entire target
- Use diffuse lighting to minimize glare and reflections
- Match the color temperature of your light source to your application
Target Positioning
For reliable measurements:
- Position the target perpendicular to the optical axis
- Place the target at your system's working distance
- Test resolution at multiple positions in the field of view, not just the center
Interpreting Borderline Results
When examining targets, you'll often find borderline cases where bars are partially resolved. Follow these guidelines:
- Count only patterns where all three bars are clearly distinguishable
- Both horizontal and vertical bars must be resolved to count an element
- In automated analysis, set consistent contrast thresholds
Digital Analysis of USAF1951 Targets
Modern image analysis allows for more precise resolution measurements than visual inspection alone:
MTF Calculation
Modulation Transfer Function (MTF) provides a more comprehensive resolution assessment:
- Measures contrast preservation at different spatial frequencies
- Quantifies not just if bars are visible, but how well their contrast is maintained
- Can be automatically calculated from images of USAF targets
Automated Resolution Determination
Software tools can:
- Automatically identify the smallest resolved element
- Calculate precise lp/mm values
- Generate reports for documentation and comparison
- Provide more objective results than visual inspection
Alternatives to USAF1951 Targets
While the USAF1951 standard remains widely used, other resolution targets offer specific advantages:
ISO 12233 Chart
- Designed specifically for digital still cameras
- Includes slanted edge patterns for SFR/MTF measurement
- Better suited for evaluating modern digital imaging systems
Siemens Star
- Provides continuous resolution measurement (not discrete steps)
- Allows evaluation of resolution in all directions, not just horizontal and vertical
- Useful for detecting astigmatism and directional resolution differences
Ronchi Rulings
- Consist of parallel lines with precise spacing
- Simpler than USAF targets but limited to one direction
- Useful for quick verification of specific resolution values
Choosing the Right USAF1951 Target for Your Application
Selecting the appropriate target depends on your specific testing needs:
Resolution Range
- First, determine the expected resolution range of your system
- Choose a target with groups that extend beyond your expected resolution in both directions
- For microscopy and high-magnification systems, select targets with extended high-resolution elements
Physical Size
- Consider your field of view
- Larger targets are easier to work with but may not fit in microscope stages
- Smaller targets are suitable for high-magnification applications
Substrate and Durability
- For laboratory environments, chrome on glass offers highest precision and longevity
- For field use, more durable and portable options may be preferred
- Consider cleanability and resistance to environmental factors
Maintaining and Storing Your Target
To ensure accurate measurements over time:
- Store targets in protective cases when not in use
- Handle only by the edges to prevent fingerprints
- Clean according to manufacturer specifications
- Periodically inspect for damage or contamination
- Recalibrate or replace if accuracy is questioned
Using USAF1951 Targets with OKlab Products
The USAF1951 test target is an essential companion to many of the machine vision components available through OKlab. When used with OKlab's FA lenses, cameras, and lighting solutions, these targets allow you to:
- Verify the performance of newly purchased optical components
- Ensure your system meets resolution requirements for specific applications
- Compare different lens options objectively
- Document system capabilities for clients or quality assurance
OKlab offers a variety of USAF1951 test targets to meet different application needs, including:
- High-precision chrome on glass targets for laboratory and production testing
- Positive and negative variants for different lighting setups
- Multiple size options to accommodate various fields of view
- Extended range targets for high-resolution systems
By incorporating regular resolution testing into your machine vision workflow, you can catch potential issues before they affect production and maintain consistent imaging performance over time.
Conclusion: The Enduring Value of USAF1951 Targets
Despite being developed over 70 years ago, the USAF1951 resolution target remains relevant in today's advanced imaging landscape. Its standardized design, universal recognition, and straightforward interpretation make it an invaluable tool for anyone working with optical systems.
Whether you're developing cutting-edge machine vision applications, troubleshooting image quality issues, or simply verifying that a new camera meets specifications, the humble USAF1951 target provides a reliable benchmark for optical performance.
By understanding how to properly use these targets and interpret their results, you gain a powerful tool for ensuring your imaging systems deliver the resolution needed for your specific applications.
Looking for high-quality USAF1951 test targets for your optical testing needs? Explore OKlab's selection of precision targets designed specifically for machine vision applications.