Center Resolution

How sharply an image-intensifier tube can separate fine detail at the center of its image—and why the largest number on the sheet does not automatically win.

What Center Resolution Measures

Center resolution describes how finely the tube can distinguish alternating light and dark detail near the center of the image.

It is normally reported in line pairs per millimeter:

lp/mm

One line pair consists of one light line and one dark line. As the pattern becomes smaller and more tightly spaced, the tube eventually reaches a point where the lines can no longer be distinguished as separate.

That limiting point becomes the reported resolution value under the applicable test method.

Commonly seen values include:

• 64 lp/mm
• 72 lp/mm
• 81 lp/mm

These are tube-resolution measurements. They are not the same thing as digital pixels, display resolution, field of view, magnification, or the resolving power of the complete assembled device.

"Resolution tells you how fine a pattern the tube resolved during a test. It does not certify your lenses, your focus, your eyesight, or your ability to read a license plate through three pine trees."

Why It Is Called Center Resolution

The measurement is taken near the center of the tube's image because that area provides a standardized reference point.

Center resolution does not describe sharpness across the entire field of view. It does not reveal:

• Edge distortion
• Peripheral sharpness
• Optical aberrations from the complete system
• Tube cosmetics
• Fixed-pattern noise
• Halo around bright sources
• Focus quality
• Depth of field
• Image performance during movement

A tube can have excellent center resolution while the complete system still appears soft because of poor focus, dirty optics, incorrect diopter adjustment, damaged lenses, or assembly problems.

How the Test Works

L3Harris describes resolution testing using a USAF 1951 tri-bar target and a 10× magnifying optic. The evaluator identifies the smallest pattern whose light and dark bars remain distinguishable.

The reported number represents the limiting resolution reached under that test.

L3Harris explains that its U.S. Army specification method reports standardized values such as 64, 72, and 81 lp/mm. Tester experience and eye fatigue can affect evaluation, which is one reason the procedure and reporting steps must remain controlled.

Do not assume every manufacturer, contract, or tube sheet used an identical procedure. If two sheets report values differently, verify the manufacturer's test method before making a direct comparison.

Do not average or invent an intermediate value.

Reading Line Pairs Per Millimeter

A higher lp/mm value means the tube resolved a finer test pattern under the stated conditions.

For example:

• 64 lp/mm resolved the applicable 64-level pattern
• 72 lp/mm resolved a finer pattern
• 81 lp/mm resolved a finer pattern still

This does not mean a 72 lp/mm tube looks exactly 12.5 percent sharper than a 64 lp/mm tube to the user. The mathematical difference between the numbers does not translate directly into an equal percentage of perceived field improvement.

What the observer sees also depends on:

• Signal-to-noise ratio
• Scene contrast
• Available illumination
• Objective and eyepiece quality
• Focus and diopter adjustment
• Gain and screen brightness
• Halo and optical scatter
• Movement
• The observer's eyesight

The specification is real. The oversimplified conclusion is usually where things go sideways.

Resolution and SNR Work Together

Resolution describes the tube's ability to separate fine detail. Signal-to-noise ratio describes how well useful information stands above image noise.

A tube can have high reported center resolution but lose practical detail when a dark scene becomes heavily obscured by scintillation.

Example:

Tube A has the higher center-resolution value. Tube B has the higher SNR and FOM. Neither short comparison proves universal superiority.

Tube A may hold an advantage when the scene provides enough light and contrast to use the extra measured resolution. Tube B may produce a cleaner image as illumination decreases.

The application and the rest of the specifications decide which difference matters.

Tube Resolution Versus System Resolution

The tube is only one part of the optical chain:

Scene → objective lens → tube → eyepiece → eye

The complete system cannot deliver detail that has already been lost elsewhere in that chain.

Dirty glass, internal contamination, incorrect assembly, poor optical alignment, scratched lenses, or bad focus can erase the practical advantage of a higher-resolution tube.

Low-Light and High-Light Resolution

Some data sheets or manufacturer materials may distinguish between low-light and high-light resolution.

These are not interchangeable measurements.

L3Harris describes high-light resolution as limiting center resolution measured at an illumination level closer to dusk conditions. Low-light tube performance places greater emphasis on what the tube preserves when available illumination is severely limited.

Before comparing two resolution values, determine:

• Whether both are center-resolution measurements
• Whether both used comparable illumination
• Whether both used the same testing standard
• Whether both are actual measurements or minimum requirements
• Whether both manufacturers report results in the same increments

A number taken from a different test condition should not be dropped into a comparison as though it means the same thing.

Field Scenarios

Common Myths

Comparing Resolution Correctly

When comparing tube sheets:

1. Confirm both sheets match the tube serial numbers.
2. Identify actual measurements versus minimum ratings.
3. Confirm both values represent center resolution.
4. Check the units and testing method.
5. Compare SNR and calculate FOM.
6. Review EBI, halo, gain, and screen quality.
7. Evaluate the complete devices with matched focus and lighting.
8. Do not assign a false level of precision that the test did not report.

Lesson Takeaway

Center resolution tells you how fine a standardized pattern the tube resolved near the center of its image under a defined test.

It is a legitimate and useful specification. It is not a complete verdict on the tube, the optics, or the assembled device.

Read resolution beside SNR, FOM, EBI, halo, gain, cosmetics, and the intended environment.