Published on 01-Nov-2023

Fluorescent Penetrant Testing: The Ultimate Guide

Fluorescent Penetrant Testing: The Ultimate Guide

Table Of Contents

  • Introduction
  • The Importance of Fluorescent Penetrant Testing
  • Selecting the Correct Penetrant Method per AMS 2644
  • Differences Between Sensitivity  
  • Common Terms
  • Conclusion
  • Key Takeaways
  • FAQs


Fluorescent Penetrant Testing (FPT), also known as Dye Penetrant Inspection (DPI) or Liquid Penetrant Testing (LPT).

It is a widely used non-destructive testing (NDT) method used to detect surface defects in various materials, including metals, plastics, and ceramics.

This non-destructive testing method has been a cornerstone in ensuring the integrity of various components and materials. 

Let's delve into the significance of Fluorescent Penetrant Testing and explore how it plays a crucial role in maintaining high-quality standards.

The Importance of Liquid Penetrant Inspection

Fluorescent Penetrant Testing is a crucial Non-destructive Testing Method used in various industries to detect surface defects and discontinuities in materials and components.

This technique involves applying a liquid penetrant to a material's surface, wiping away any excess penetrant, and then applying a developer. 

The penetrant seeps into any surface discontinuities, making them visible for inspection.

Here, we will discuss the importance of Fluorescent Penetrant Testing, the selection of the correct penetrant method according to AMS 2644 standards, differences in sensitivity, and common terms associated with this technique.

1. Defect Detection

Fluorescent Penetrant Testing is highly effective in detecting surface cracks, porosity, weld defects, and other surface irregularities that may compromise the integrity of a material or component. Detecting these issues early helps prevent catastrophic failures.

2. Cost-Effective

Compared to other Non-destructive Testing Method like radiography or Ultrasonic Testing, LPI is generally more cost-effective and can be applied to a wide range of materials.

3. Versatility

Fluorescent Penetrant Testing can be used on various materials, including metals, plastics, ceramics, and composites, making it a versatile method for quality control and inspection.

4. Non-Destructive

Fluorescent Penetrant Testing is non-destructive, meaning it does not harm the inspected part, making it suitable for both production and in-service inspection.

5. Surface and Subsurface Detection

While Fluorescent Penetrant Testing primarily detects surface defects, it can also identify subsurface discontinuities when used in combination with other NDT Techniques.

Selecting the Correct Penetrant Method per AMS 2644

To make sure Fluorescent Penetrant Testing works well and is reliable, AMS 2644 says you need to think about several things when choosing the right penetrant method.

AMS 2644 is a standard that provides guidelines for choosing the appropriate penetrant method based on specific inspection requirements.

Here's a simplified guide to this selection process.

  • Material Type

Determine the type of material being inspected, as this can influence the choice of penetrant.

Some penetrants are better suited for specific materials, such as metals, plastics, ceramics, or composites.

  • Defect Size and Type

Assess the expected size and nature of the defects you need to detect. Larger defects may be visible with standard penetrants, while smaller or more subtle defects may require fluorescent penetrants for enhanced sensitivity.

  • Sensitivity Requirements

Define the level of sensitivity required for your inspection. If you need to detect very fine cracks or defects, a fluorescent penetrant with higher sensitivity may be necessary. For larger, more visible defects, a visible penetrant may suffice.

  • Lighting Conditions

Consider the lighting conditions under which the inspection will be conducted.

If inspections will occur under normal white light, visible penetrants are appropriate.

For inspections conducted under ultraviolet (UV) or black light, fluorescent penetrants are needed.

  • Regulatory or Customer Requirements

Ensure compliance with any specific regulations or customer specifications that dictate the penetrant method to be used.

Some industries or applications may have strict requirements for Non-destructive Testing Method.

  • Compatibility

Confirm that the selected penetrant is compatible with the material being tested and with any subsequent steps in the inspection process, such as the developer and cleaner.

  • Environmental Considerations

Consider environmental factors such as temperature, humidity, and cleanliness of the inspection area, as they can impact the choice of penetrant.

  • Operator Training

Ensure that the inspection personnel are adequately trained and certified to perform LPI using the selected penetrant method.

Types of Penetrants as per AMS 2644:

Based on these considerations, you can choose between two primary types of penetrants as per AMS 2644:

  • Type I (Visible Penetrant)

These penetrants are coloured and provide visible indications on the surface.

They are suitable for detecting larger defects and are used in inspections conducted under normal white light.

  • Type II (Fluorescent Penetrant)

These penetrants contain fluorescent dyes and are used in conjunction with a UV (ultraviolet) light source.

They offer higher sensitivity and can detect smaller defects that may not be visible to the naked eye under standard lighting conditions.

By choosing the right Fluorescent Penetrant Testing method according to AMS 2644, the inspection can be made to fit the material and flaws being looked at.

This gives accurate and reliable results for Non-destructive Testing

Differences Between Sensitivity

The term "sensitivity" in the context of Liquid Penetrant Inspection (LPI) refers to the ability of the inspection method to detect smaller or more subtle defects on the surface of a material or component.

Sensitivity levels can vary depending on the type of penetrant used and the inspection conditions.

Here are the key differences between sensitivity levels in LPI:

1. Visible Penetrant (Type I)

  • Sensitivity:

Visible penetrants, also known as Type I penetrants, have lower sensitivity compared to fluorescent penetrants.

  • Detection Capability:

They are effective in detecting larger defects and surface irregularities that are visible to the naked eye under normal white light.

  • Inspection Lighting:

Visible penetrants are used in inspections conducted under standard white light conditions, which are similar to regular ambient lighting.

  • Indications:

Defect indications created by visible penetrants are typically visible as coloured markings on the surface.

These indications can be seen without the need for special UV lighting.

2. Fluorescent Penetrant (Type II)

  • Sensitivity:

Fluorescent penetrants, or Type II penetrants, offer higher sensitivity compared to visible penetrants.

  • Detection Capability:

They can detect smaller defects and discontinuities that may not be visible to the naked eye. This increased sensitivity makes them suitable for finding finer cracks or flaws.

  • Inspection Lighting:

Fluorescent penetrants require inspection under ultraviolet (UV) or black light conditions.

The fluorescence emitted by the penetrant when exposed to UV light makes defects stand out more clearly.

  • Indications:

Defect indications produced by fluorescent penetrants are typically glowing or brightly coloured when viewed under UV light.

This fluorescence enhances the visibility of defects.

Common Terms in Fluorescent Penetrant Testing

Here are the common terms in Fluorescent Penetrant Testing.

  • Background:

The surface against which the indication is viewed can be the natural surface or the developer coating.

  • Bleedout:

The process where entrapped liquid penetrant rises to form visible indications.

  • Developer:

A material applied to accelerate bleed-out and enhance indication contrast.

  • Developing Time:

The time between developer application and part examination.

  • Dwell Time:

The total time the penetrant is in contact with the test surface, including application and drain time.

  • Inspection:

The visual examination of the test part after liquid penetrant processing.

  • Liquid Penetrant Testing:

Non-destructive Testing using liquid penetrant materials to detect surface discontinuities.

  • Liquid Penetrant:

A solution of dye with the ability to penetrate fine openings.

  • Precleaning:

The removal of surface contaminants from the test part before examination.

  • Sensitivity Level:

A descriptive term for the capability of a penetrant system to indicate surface-connected discontinuities.


Fluorescent Penetrant Testing is an indispensable tool for ensuring the quality and integrity of various components and materials.

Its non-destructive nature, versatility, and sensitivity levels make it a preferred choice in industries where precision and safety are paramount.

Understanding the nuances of Fluorescent Penetrant Testing including sensitivity levels and common terms, is key to its effective implementation in quality assurance processes.

For more details check out OnestopNDT.

Key Takeaways 

  • Fluorescent Penetrant Testing is a vital NDT method used in various industries to detect surface defects and discontinuities in materials and components.
  • Fluorescent Penetrant Testing involves applying a liquid penetrant, removing excess penetrant, and applying a developer.
  • It is effective in identifying surface cracks, weld defects, porosity, and other irregularities, preventing catastrophic failures.
  • It is a cost-effective NDT method suitable for a wide range of materials.
  • Does not harm the inspected part, making it suitable for production and in-service inspection.
  • Fluorescent Penetrant Testing can detect subsurface discontinuities when combined with other NDT Techniques.

  • Fluorescent Penetrant Testing is crucial for quality assurance because of its non-destructive nature, where precision and safety are paramount. 


1. What is the FPI NDT method?

A. The FPI NDT method, also known as Fluorescent Penetrant Inspection Non-destructive Testing, is a technique used to detect surface defects, cracks, and discontinuities in materials, especially metals and non-porous ceramics.

It involves applying a fluorescent liquid penetrant to the surface of the material being inspected.

After a specified dwell time, the excess penetrant is removed, and a developer is applied.

Any penetrant trapped in surface defects will fluoresce under ultraviolet (UV) or visible light, making the defects easily visible for inspection.

2. How to do fluorescent penetrant inspection?

Fluorescent Penetrant Inspection (FPI) typically involves the following steps:

a. Pre-cleaning:

The surface to be inspected is cleaned thoroughly to remove contaminants and ensure better penetration of the fluorescent penetrant.

b. Application of penetrant:

A fluorescent liquid penetrant is applied to the surface, either by spraying, brushing, or immersion.

c. Dwell time:

The penetrant is allowed to dwell on the surface for a specified period, during which it seeps into any surface defects.

d. Excess penetrant removal:

Excess penetrant is carefully wiped off or rinsed away.

e. Application of developer:

A white, absorbent developer is applied to the surface. This helps draw out the trapped penetrant from defects and forms a visible indication.

f. Inspection:

The surface is examined under UV or visible light. Any indications of defects will fluoresce, making them easily visible and allowing for evaluation.

g. Cleaning and reporting:

After inspection, the surface is cleaned, and a report is generated documenting the inspection results.

3. What is a fluorescent test?

A fluorescent test, in a broader sense, refers to any test or inspection method that utilises fluorescence to detect or analyse certain properties or substances.

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.

In various fields, fluorescent tests are used for purposes such as identifying specific molecules, monitoring biological processes, or detecting defects in materials, as in Fluorescent Penetrant Inspection (FPI).

4. What are the three types of fluorescence?

A. There are primarily three types of fluorescence:

a. Natural Fluorescence:

This occurs when a substance naturally exhibits fluorescence under certain conditions without the addition of external agents.

For example, some minerals and biological materials exhibit natural fluorescence when exposed to ultraviolet (UV) light.

b. Induced Fluorescence:

This type of fluorescence occurs when a substance that does not naturally fluoresce is made to do so by introducing external agents, such as fluorescent dyes or markers.

These agents are used in various scientific and medical applications.

c. Phosphorescence:

Phosphorescence is similar to fluorescence but involves a delayed emission of light after the excitation source (usually UV light) is removed.

Glow-in-the-dark materials exhibit phosphorescence.

Tree PNG back


Tree PNG back




Tree PNG back


Application Notes