What Is an Optical Filter?
Optical filters are used in the beam path of a camera or illumination and affect which parts of the light pass through. While certain wavelength ranges are allowed through, others can be blocked or attenuated. Which wavelengths are affected depends on the filter type. Bandpass filters limit light to a narrow spectral range, while longpass and shortpass filters allow wavelengths to pass above or below a defined limit. Polarization filters, on the other hand, do not affect the wavelength, but the direction of the light’s vibration.
In this way, the spectral range in particular reaches the camera that is relevant for the respective test task. Disturbing ambient light can thus be masked out and reflections reduced. At the same time, material properties such as color differences or surface structures can be highlighted that would be difficult or not visible at all without the right filter.
When Are Optical Filters Used?
Increase contrast
Emphasize relevant features more clearly
Minimize reflections
Improve repetition accuracy
Make material properties visible
Reduce disturbing ambient light
Important Characteristics of Optical Filters
Transmission
The transmission indicates how much light passes through the filter. The higher the transmission, the more usable light reaches the camera.
Exclusion Zone
The exclusion zone is the part of the spectrum that the filter blocks. A high blocking effect reduces unwanted light and thus the influence of external light on the image capture.
Half-Value Width (FWHM)
The half-value width describes the width of the spectral range that a filter allows to pass through. The smaller the half-value width, the narrower and more selective the wavelength range let through.
The Electromagnetic Spectrum
| Spectral range | Wavelength | Typical applications |
|---|---|---|
| UV | 200–400 nm | Fluorescence, material testing |
| Visible light | 400–700 nm | Standard image processing |
| Near infrared (IR) | 700–1,100 nm | Suppression of interfering light, material differentiation |
| SWIR | 1,000–2,500 nm | Moisture measurement, semiconductor |
| MWIR | 3–5 µm | Thermal imaging |
| LWIR | 8–14 µm | Thermal imaging technology |
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UV
|
|
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200–400 nm
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Fluorescence, material testing
|
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Visible light
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400–700 nm
|
Standard image processing
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Near infrared (IR)
|
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700–1,100 nm
|
Suppression of interfering light, material differentiation
|
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SWIR
|
|
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1,000–2,500 nm
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Moisture measurement, semiconductor
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MWIR
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3–5 µm
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Thermal imaging
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LWIR
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8–14 µm
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Thermal imaging technology
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What to Consider When Selecting Optical Filters?
Spectral Position
Filter Type
Degree of Transmission
Edge Steepness
Mechanical Compatibility
The right filter only works with the right illumination:
A filter alone does not produce an optimal image. Firstly, the illumination matched to the test object ensures that the desired features are highlighted. The appropriate filter reinforces this effect by masking out disturbing light components and allowing only the wavelengths relevant for image capture to reach the camera. Only then is a high-contrast, evaluable image created.
What Types of Optical Filter Are There?
Polarization Filter
Property: Polarization filters reduce disturbing reflections on shiny or transparent surfaces.
Typical application: Polarization filters increase contrast by filtering out disturbing reflections, glare and reflections on smooth, shiny or curved surfaces. This makes it easier to detect cracks, scratches or irregularities that could otherwise be overlaid by reflections. This makes them particularly suitable for glass inspection, inspection of plastic parts and foils, as well as liquid and surface inspection.
Bandpass Filter for Red Light (Color Filter)
Property: Red bandpass filters allow light from the red spectral range to pass and suppress other wavelengths.
Typical application: In combination with red LED lighting, red bandpass filters reduce disturbing light outside the red spectrum, ensuring stable contrasts. For example, print images, labels, barcodes or plain text can be reliably captured. Other applications include OCR applications as well as the recognition of color markings and packaging features.
IR Longpass Filter
Property: IR longpass filters block visible light and allow infrared light to pass through.
Typical application: In combination with active IR illumination, disturbing visible ambient light can be virtually completely hidden and the contrast increased. This enables reliable image evaluation even under challenging light conditions, for example in conveyor technology and robot vision as well as in presence control, material differentiation and code recognition.
Bandpass Filter for Visible Light
Property: Limits image capture to the visible spectral range of 400–700 nm and reduces disturbing UV and IR light.
Typical application: Bandpass filters for visible light reduce influences from surrounding or ambient light that can distort contrast and color reproduction. This makes it possible to achieve uniform values across all cameras in multi-camera systems. Color check, position control and quality control results also remain more comparable in changing lighting situations.
Bandpass Filter for Blue Light (Color Filter)
Property: Optimizes contrast in blue LED lighting and suppresses distracting ambient light.
Typical application: Blue bandpass filters use the lower diffraction of short-wave light to map fine structures with sharper edges and higher contrasts. At the same time, light outside the blue spectrum is masked out. This enables low-noise and high-contrast capture for electronics and PCB inspection, edge and contour detection, as well as surface inspection, structure detection and precision measurement.
UV Filter
Property: For UV illumination or UV fluorescence applications to highlight features that are not visible in visible light.
Typical application: In fluorescence testing, UV filters block the intense UV excitation radiation, while the weaker fluorescence signal emitted by fluorescent substances is used for image capture. This enables residues, defects or codes to be made visible. Typical applications include adhesive control, paint and coating testing, reading invisible security markings such as data matrix codes on glass ampoules, and material analysis.
