Microscope Illumination for Industrial Applications

DeRose, James; Doppler, Michael

March 20, 2023

Microscope Illumination for Industrial Applications

How to select the appropriate light source for microscope inspection

Inspection microscope image of a printed circuit board (PCB) taken with a ring light (RL) and near vertical illumination (NVI). PCB_taken_with_a_ring_light_and_near_vertical_illumination.jpg

This article gives users of inspection microscopes helpful advice when attempting to select optimal illumination or lighting systems for part or component observation. The illumination used for a microscope has a very important effect on the final image quality and greatly influences the details which are visualized. The following information should help users choose illumination that produce imaging results which are optimized for their inspection needs.

What is the illumination used for on an inspection microscope?

Inspection of components and parts for industrial manufacturing and production, process engineering, quality control and assurance (QC/QA), failure analysis (FA), or research and development (R&D) often is done with the aid of a microscope. The performance of the microscope used has a significant impact on inspection efficiency.

Choosing the illumination which helps users of inspection microscopes achieve the optimal image results depends upon the type of component or part and its details of interest that must be revealed [1-4].

Inspection microscope users can obtain helpful advice from this article when attempting to select optimal illumination for part or component observation. The following information should help users choose the appropriate illumination for microscope inspection.

What type of microscope light source is best for inspection?

Up until 10 to 20 years ago, halogen lights [5] were the most common type of illumination for inspection microscopes. However, since that time LED (light-emitting diode) lights [6,7] have been more and more utilized for microscope illumination due to the reasons given below.

Advantages of LED (Light-Emitting Diode) illumination

LED microscope illumination technology provides several advantages for microscopy imaging when compared to halogen lights. These advantages are:

Longer lifetimes (25,000 to 50,000 hours)
Lower power consumption
Natural color temperature
Constant color temperature even at low brightness levels
Less heat release (a “cold light” source useful for temperature-sensitive samples)
A more practical and compact design

Why is microscope illumination important during inspection?

There are several key factors to consider for high quality microscopic observation and imaging of a component or part when selecting the proper type of illumination:

Types of sample (components, parts, etc.) to be observed;
Sample characteristics (shiny or transparent areas, holes, scratches, surface structures, etc.) to be analyzed;
Difficulties noticed for specific applications (inspection, FA, R&D, etc.) with currently used types of illumination;
Need to access samples during microscope observation, e.g., handling with tweezers, soldering iron, or other tools, which requires sufficient working distance [8,9]between the sample and objective with illumination.

Users of inspection microscopes may have to try multiple types of illumination in order to find the optimal lighting [10,11].

Choose the right LED microscope illumination

The LED illumination solutions are described below. These include the LED3000 and LED5000 systems, mainly used with stereo [9] or digital microscopes [12], which are often exploited for inspection. Examples of other applications they can be used for are failure analysis (FA) and research and development (R&D). Some basic information about the LED3000 and LED5000 systems are shown in table 1.

Overview of LED3000 and LED5000 microscope illumination solutions

Ring light (RL) gives bright and uniform illumination; suitable for many types of parts and components. Additionally, diffusors and polarized light sets are available for both ring-light types. These accessories reduce the problems of glare and highlighting of spots.

Coaxial illumination (CXI), where the beam of light is guided through the optics and reflected from the part and component, works best for smooth and reflective components. It is especially useful if fine cracks or surface quality must be assessed.

Near vertical illumination (NVI) is achieved with LEDs positioned very close to the optical axis. It provides nearly shadow-free lighting and is practical for parts and components with recesses and deep holes or those that require long working distances.

Spotlight illumination (SLI) with flexible goosenecks offers high contrast lighting suitable for many types of parts and components.

Diffuse and highly diffuse illumination (DI and HDI) are designed for reflective, non-flat or curved parts and components. These can be difficult to image due to the amount of back-reflected light.

Multi-contrast illumination (MCI), utilizing repeatable contrast with lighting from 2 different directions and angles, is useful for parts and components with hard-to-find details.

Back light illumination (BLI) provides transmitted lighting for parts and components with transparent areas.

Results with Leica LED5000 and LED3000 illumination

Example images of various samples are shown below. The images were recorded with a Leica stereo microscope (M60 or M125) equipped with a Flexacam C3 digital microscope camera and a LED3000 or LED5000 illumination system. The types used were a ring light (RL) [with diffusor or polarizers], near vertical (NVI), coaxial (CXI), spotlight (SLI), multi-contrast (MCI), and diffuse (DI) or highly diffuse (HDI) illumination.

Reference Sample: Coin

Images of a metallic coin acquired with various LED illuminations are shown in figure 1. The coin images demonstrate clearly the differences in contrast which can be achieved.

Coin - Ring Light (RL), all segments — Figure 1a: Ring Light (RL), all segments

Coin - Ring Light (RL), right half segments — Figure 1b: Ring Light (RL), right half segments

Coin - Ring Light (RL), upper left quarter segments — Figure 1c: Ring Light (RL), upper left quarter segments

Coin - Near Vertical Illumination (NVI) — Figure 1d: Near Vertical Illumination (NVI)

Coin - Coaxial Illumination (CXI) — Figure 1e: Coaxial Illumination (CXI)

Coin - Highly Diffuse Illumination (HDI) — Figure 1 f: Highly Diffuse Illumination (HDI)

Coin - Multi-Contrast Illumination (MCI) — Figure 1g: Multi-Contrast Illumination (MCI)

Coin - Spotlight Illumination (SLI), both lights — Figure 1h: Spotlight Illumination (SLI), both lights

Printed Circuit Board (PCB)

Images of a PCB recorded with a RL, NVI, and SLI illumination are shown in figure 2.

Printed Circuit Board (PCB) - Near Vertical Illumination (NVI): Holes and recesses — Figure 2b: Near Vertical Illumination (NVI): Holes and recesses

Printed Circuit Board (PCB) - RL with crossed polarizers: Reflective areas — Figure 2c: Ring Light (RL) with crossed polarizers: Reflective areas

Printed Circuit Board (PCB) - Spotlight Illumination (SLI): Multiple sample characteristics — Figure 2d: Spotlight Illumination (SLI): Multiple sample characteristics

Processed wafer

Images of a processed wafer recorded with a RL, NVI, CXI, and SLI illumination are shown in figure 3.

Processed wafer - Coaxial Illumination (CXI): Surface texture — Figure 3b: Coaxial Illumination (CXI): Surface texture of processed wafer

Processed wafer - Near Vertical Illumination (NVI): Holes and recesses — Figure 3c: Near Vertical Illumination (NVI): Holes and recesses of processed wafer

Processed wafer - Spotlight Illumination (SLI): Multiple sample characteristics — Figure 3d: Spotlight Illumination (SLI): Multiple sample characteristics

Automotive component

Images of a sprocket recorded with a RL, NVI, and SLI illumination are shown in figure 4.

Sprocket - Ring Light (RL) with diffusor: Multiple sample characteristics — Figure 4a: Ring Light (RL) with diffusor: Multiple sample characteristics

Sprocket - Near Vertical Illumination (NVI): Holes and recesses — Figure 4b: Near Vertical Illumination (NVI): Holes and recesses

Sprocket - RL with crossed polarizers: Reflective areas — Figure 4c: Ring Light (RL) with crossed polarizers: Reflective areas

Sprocket - Spotlight Illumination (SLI): Multiple sample characteristics — Figure 4d: Spotlight Illumination (SLI): Multiple sample characteristics

Medical Devices

Images of a hip implant recorded with a RL, NVI, or SLI illumination are shown in figure 5.

Hip implant - Ring Light (RL) with diffusor: Multiple sample characteristics — Figure 5a: Ring Light (RL) with diffusor: Multiple sample characteristics

Hip implant - Near Vertical Illumination (NVI): Holes and recesses — Figure 5b: Near Vertical Illumination (NVI): Holes and recesses

Hip implant - Ring Light (RL) with crossed polarizers: Reflective areas — Figure 5c: Ring Light (RL) with crossed polarizers: Reflective areas

Hip implant - Spotlight Illumination (SLI): Multiple sample characteristics — Figure 5d: Spotlight Illumination (SLI): Multiple sample characteristics

Guide for choosing the appropriate LED illumination

Guide for choosing the appropriate LED illumination
The quick selection guide for the LED3000 and LED5000 series of illumination solutions is shown below in table 2. The LED3000 series is designed for more routine applications (e.g., inspection and QC) and the LED5000 series for more advanced applications (e.g., FA and R&D). This guide can be very helpful for microscope users attempting to find the most appropriate illumination for inspection of particular components or parts.

Figure 6: Quick selection guide for LED3000/LED5000

Other recommendations

In addition to the high-quality optics integrated into Leica microscopes, it is important to identify the component details to be analyzed and the field of view (object field) required for observation when selecting an illumination system. It is also worthwhile to consider the advantages of computer encoding of the microscope and the performance of the microscope’s optics, such as objective lenses in terms of transmission, chromatic correction, and planarity, i.e., planapochromatic, achromatic, etc.

Conclusion

Finding the right microscope illumination for inspection of components or parts can be challenging at times. However, the advice and recommendations mentioned here can aid users when investigating various illumination solutions in order to find the ones which give optimal results for image observation and recording.