Improving Performance of monochrome cameras with Optical Filters

Five Technical Benefits of Midwest Optical Systems Bandpass filters with Monochrome Cameras

Many assume that colour imaging is more advanced than monochrome imaging but that’s not always true when it comes to machine vision applications.

Monochrome cameras combined with high-quality optical filters:

• can offer higher contrast and better resolution
• provide better signal-to-noise ratio
• can be sensitive to near-ultraviolet, visible and near-infrared spectrums
• provide 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. This is the first editorial in a series where we look at the advantages of using Midwest optical filters in monochrome Machine Vision imaging.

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

In monochrome imaging the value of each pixel is composed exclusively of shades of grey, varying from black to white. Maximum contrast in imaging is achieved when black pixels (the weakest signal) are measured against white pixels (the strongest signal). Maximum contrast can be achieved by passing only relevant wavelengths while suppressing others or by reversing this process to highlight background features. When contrast is maximised, errors in interpreting results will be minimised. This can greatly improve a system’s accuracy and reliability.

While colour cameras may be the first thought when separating items by colour, monochrome cameras paired with colour bandpass filters can provide a more effective and efficient solution. In the example above, to brighten or highlight an item that’s predominantly red, transmission in the red portion of the spectrum is maximised and all other portions of the spectrum blocked.

The image above demonstrates how Midwest Optical Systems bandpass filters are matched to LED illumination sources, enabling straightforward evaluation of monochromatic imaging performance

Midwest filters have the ability to control the quality and quantity of light, block all unwanted ambient light, pass only the output of light necessary for inspection and significantly increase contrast and resolution. This reduces the effect of unwanted ambient light, improves image clarity, increases resolution, and reduces reliance on physical enclosures or shrouds.

Before: Lack of contrast between the cookie dough and chocolate chips, in addition to cellophane packaging glare caused by ambient light, challenges the accurate counting of chocolate chips in cookies.	After: A BP660 Dark Red Bandpass Filter mounted on the lens lightens reflections and blocks interfering ambient light, resulting in improved contrast.

Chromatic aberration occurs because lenses cannot focus all wavelengths at the same distance and onto the image sensor. In fact all lenses exhibit chromatic aberration to some degree. Apochromat lenses are designed to minimise chromatic aberration but are expensive and hard to find these days.

Typically the chromatic aberration from a lens in the visible spectrum is acceptable but the use of an optical filter with monochrome cameras is still useful. This however is not the case with broad spectrum imaging, where ultraviolet (UV) and infrared (IR) wavelengths would otherwise contribute to reduced clarity.

A bandpass filter reduces the light to a small band of wavelengths to allow sharper focus and so improve resolution. High-pass and Low-pass filters can be used in the same way for the same reason.

Light reflected from a non-metallic surface becomes polarised, resulting in specular glare. When a smooth surface is covered with oil, grease, or liquid or the surface is inherently “shiny”, the application will often produce specular glare in the image. This can obscure detail and lower image quality. Polarising filters provide an effective method of controlling glare when applied to both the lens and the light source.

Midwest Optical Systems offers three types of polarisers. Linear polarisers (PR/PL Series) are widely used across the visible range. Circular polarisers (PC Series) are recommended for applications requiring compatibility with auto-focus or metering systems. Infrared polarisers (Pi Series) are designed for wavelengths between 700–1,100 nm. Proper alignment of linear polarisers achieves optimal suppression of reflections.

Optimal glare reduction is achieved when a linear polariser is orientated to pass only light polarised in the direction perpendicular to the reflected light (the glare). Aside from reducing reflections, polarizers help improve contrast, increase colour saturation, allow for evaluation of stress in transparent plastic and glass and can be used in pairs to form a variable neutral density filter.

Before: Reflections off a sealed cap illuminated by ambient light and a white LED light source result in specular glare that renders the cap unreadable.	After: A PR032 Polarizing Filter installed on the lens and PS030 Polarizing Film over the light source greatly reduces specular reflections.

Neutral Density (ND) Filters are primarily used for exposure control, limiting an overwhelming amount of light that might otherwise inundate a subject. They are also used to retain image resolution and reduce depth of field.

ND filters are used to manage exposure by reducing transmission across a defined spectral range. This ensures balanced imaging, preserves resolution, and supports depth of field control with the aperture.

ND filters transmit light uniformly without altering colour or contrast. Midwest provides absorptive ND filters (ND Series) and reflective types (Ni Series), each defined by optical density (OD). Filters may be combined to achieve higher densities, although stacking reflective filters is not recommended due to potential interference.

Traditional ND filters may not block ultraviolet or infrared radiation effectively, leading to reduced performance in broad spectrum imaging. Midwest Ni filters provide spectrally flat suppression into the near-IR region, ensuring consistent results. These filters are suitable for applications such as welding inspection, analysis of bright light sources, or situations where precise control of exposure is required.

Before: The camera lens iris is set to f/16, the smallest aperture available. Still, over-saturation caused by the light that is under inspection results.	After: With the iris set to f/16 and a ND200 (1% transmission) Neutral Density Filter mounted over the lens, overall transmission is uniformly decreased to manageable levels. Individual lighting elements can be distinguished.