This knowledgebase contains excerpts from National Instruments' white paper "A Practical Guide To Machine Vision".
Vision Illumination Sources and Spectral Content
The lighting sources now commonly used in machine vision are fluorescent, quartz halogen, LED, metal halide (mercury), and xenon. Fluorescent, quartz halogen, and LED are the most widely used lighting types in machine vision, particularly for small- to medium-scale inspection stations. Metal halide, xenon, and high-pressure sodium are more typically used in large-scale applications or in areas requiring a very bright source. Metal halide, also known as mercury, is often used in microscopy because it has many discrete wavelength peaks, which complements the use of filters for fluorescence studies. A xenon source is useful for applications requiring a very bright strobe light. Figure 2 shows the advantages and disadvantages of fluorescent, quartz halogen, and LED lighting types and relevant selection criteria, as applied to machine vision. For example, whereas LED lighting has a longer life expectancy, quartz halogen lighting may be the choice for a particular inspection because it offers greater intensity.
Figure 2.Comparison of common vision lighting sources.
Historically, fluorescent and quartz halogen lighting sources have been used most commonly. In recent years, LED technology has improved in stability, intensity, and cost-effectiveness; however, it is still not as cost-effective for large area lighting, particularly compared with fluorescent sources. However, if application flexibility, output stability, and longevity are important parameters, then LED lighting might be more appropriate. Depending on the exact lighting requirements, oftentimes you can use more than one source type for a specific implementation, and most vision experts agree that one source type cannot adequately solve all lighting issues. Consider not only a source’s brightness but also its spectral content (Figure 3). Microscopy applications, for example, often use a full-spectrum quartz halogen, xenon, or mercury source, particularly when imaging in color; however, a monochrome LED source is also useful for a black and white CCD camera, and also now for color applications, with the advent of “all color—RGB” and white LED light heads. In those applications requiring high light intensity, such as high-speed inspections, it may be useful to match the source’s spectral output with the spectral sensitivity of your particular vision camera (Figure 4). For example, CMOS sensor-based cameras are more IR sensitive than their charge-coupled device (CCD) counterparts, imparting a significant sensitivity advantage in light-starved inspection settings when using IR LED or IR-rich Tungsten sources.
Figure 3.Light Source Relative Intensity Versus Spectral Content (The bar at the bottom denotes the approximate human visible wavelength range.)
Figure 4.Camera Sensor Absolute Quantum Efficiency Versus Wavelength (The bar at the bottom denotes approximate human visible wavelength range.)
Additionally, Figures 3 and 4 illustrate several other relevant points to consider when selecting a camera and light source:
- Attempt to match your sensor’s peak sensitivity with your lighting source’s peak wavelength to take the fullest advantage of its output.
- Narrow wavelength sources, such as monochrome LEDs, or mercury are beneficial for passing strategic wavelengths when matched with narrow pass filters. For example, red 660 nm band pass filter, when matched to red LED light, is effective at blocking ambient light on the plant floor from overhead fluorescent or mercury sources.
- Sunlight has the raw intensity and broadband spectral content to call into question any vision inspection results–use an opaque housing.
- Even though your mind is good at interpreting what your eyes see, the human visual system is woefully inadequate in terms of ultimate sensitivity and spectral dynamic range–let your eyes view the image as acquired with the vision camera.