Improving Performance of monochrome cameras with Optical Filters

Advantages of combining Monochrome cameras with MidWest optical filters

This is the second editorial in a series of five which discusses MidWest Optical Filters for machine vision and how they can be used to improve results, simplify design or reduce costs. If you missed the first article, read about the technical benefits of bandpass filters with Monochrome Cameras.

Part 2 - Advantages of combining Monochrome cameras with MidWest optical filters

Part 3 - Boost Colour Camera Capability with Advanced Optical Filters

Part 4 - Boost Colour Camera Capability with Advanced Optical Filters

When it comes to machine vision applications colour imaging is not always more advanced than monochrome, in fact, monochrome imaging is often the best choice.

Monochrome Imaging can:

• offer higher contrast and better resolution
• provide better signal-to-noise ratio
• be sensitive to near-ultraviolet, visible and near-infrared spectrums
• a solution where others do not exist

When it comes to:

• optical character recognition and verification,
• barcode reading,
• scratch or crack detection,
• wavelength separation and more
  

monochrome cameras can be three times more efficient than colour cameras.

Need help selecting the right bandpass filter? Speak to the experts at Adept Turnkey.

Complementary metal oxide semiconductor (CMOS) sensors are made of silicon and exhibit sensitivity from 400nm (deep blue) up to 1000nm and sometimes 1100nm in the near-infrared (IR). By using a near-IR bandpass filter, a standard monochrome camera can be converted into an economical near-infrared imager.

Near-IR imaging can reveal contrast not visible under normal white-light illumination. The absorption and reflection characteristics of the object under inspection may produce significantly different images in the near infrared as compared to the visible spectrum. Using a bandpass of longpass filter that allows only wavelengths in the Near IR to pass, can highlight produce contrast in features otherwise not seen. Testing is necessary to determine the effectiveness of near-IR imaging for a given application. MidOpt IR bandpass filters are matched to infrared LED or laser diode illumination and are used with visible-blocking filters to maintain contrast under daylight or other broad-spectrum lighting conditions.

Pass IR & Block VIS Wavelengths to Increase Contrast
Before: Barcodes on the bottle labels interfere with accurate scanning of the shipping label barcode on the outer packaging. Illumination using only infrared LEDs does not provide enough contrast for reliable reading.	After: Using a BP850 near-IR bandpass filter on the camera isolates the outer shipping label from the bottle labels. In near-infrared light, the bottle labels are effectively invisible, providing maximum contrast for accurate barcode reading.

Fluorescence occurs when an object absorbs shorter-wavelength light, often ultraviolet, which excites a fluorophore and emits photons at a longer wavelength. In industrial applications, this emission is frequently blue. Bandpass filters centred on the emission wavelength block all other wavelengths including the brighter excitation light, ensuring the lower-energy fluorescence is detectable.

Filters are essential in fluorescence applications to isolate emission light from excitation sources. Common UV excitations include 365 nm, 395 nm, and 400 nm. By transmitting only the emitted fluorescence and blocking the excitation, filters allow features such as 2D codes, barcodes, or small printed markings to be reliably imaged in ambient lighting conditions. Polarising filters can also be applied to reduce reflections and enhance image contrast.

Fluorescence Imaging
Before: A postal barcode printed on the back of an envelope for high-speed sorting is illuminated with UV light. Without a lens filter, the orange characters are not detected because the camera sensor is overwhelmed by visible and UV light.	After: Mounting a MidOpt BP590 Orange Bandpass Filter on the lens enhances contrast by transmitting the orange fluorescence while blocking UV, near-IR, and other visible wavelengths.

Ultraviolet (UV) imaging captures reflected UV light from a surface rather than relying on fluorescence. The camera records the UV emission directly without wavelength conversion. True UV imaging is less common in machine vision but has expanded as UV-sensitive cameras and UV LEDs have become more accessible.

Monochromatic UV illumination produces sharper images compared with broad-spectrum sources. Shorter wavelengths improve detection of small features, making UV imaging ideal for applications such as detecting scratches, surface contamination, or photolithography in semiconductor manufacturing. Near-UV (290–400 nm) is commonly used, while deep-UV (below 290 nm) requires specialised optics such as fused silica or calcium fluoride lenses and often a controlled environment to prevent absorption by air. Filters are selected according to the UV range to optimise transmission and image contrast.

Ultraviolet Imaging
Before: Illumination from a UV light source without a lens filter fails to produce sufficient contrast to distinguish the plastic substrate from the background.	After: Combining the UV light source with a MidOpt BP324 UV Bandpass Filter on the lens increases contrast, clearly revealing imperfections in the plastic.

Protective filters shield camera optics from impact, dust, and harsh environments. Manufactured from glass or acrylic, they prevent damage to expensive lenses and lighting components while also blocking interfering ultraviolet wavelengths. Anti-reflection and anti-abrasion coatings enhance transmission and durability. Acrylic protective filters can be custom-cut to fit cameras, lenses, or enclosures providing flexible solutions for industrial imaging setups.