Improving Performance of Colour Cameras with Optical Filters

Boost Colour Camera Capability with Advanced Optical Filters

This is the third 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.

Part 1 - Five Technical Benefits of Midwest Optical Systems Bandpass filters with Monochrome Cameras

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

Part 4 - Boost Colour Camera Capability with Advanced Optical Filters

Colour cameras are used to achieve natural colour rendering, equivalent to what the human eye can see. However, the sensitivity of the sensor and spectra of the light source being used can significantly affect the colour or image quality. Using a filter is the easiest, most cost-effective way to:

• Achieve accurate colour rendition
• Reduce specular glare
• Reduce light intensity
• Regulate exposure

Colour cameras are a popular choice for photography, license plate recognition, security, surveillance and agricultural inspection and can be helpful in any application where accurate colour inspection is required.

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

Natural Colour Rendition

Colour cameras differ from the human eye in that they are sensitive to near-infrared (IR) wavelengths. When near-IR light is present, images can take on an unnatural appearance, even when using white light from natural sources or broad-spectrum illumination. To achieve realistic and consistent colour output, it is necessary to block near-IR wavelengths during imaging.

Near-IR blocking filters are designed to prevent all, or selected portions, of the near-IR spectrum from reaching the sensor. In some cases, these filters also reduce specific red wavelengths. Their use is important in colour imaging applications where accurate colour rendition and stable image quality are required. Near-IR block filters may also be used with monochrome cameras to manage excessively bright lighting conditions, such as those encountered in molten metal inspection.

Glare Reduction

Specular glare occurs when light reflects off a non-metallic surface and becomes partially polarised. Surfaces that are smooth, or coated with grease, oil, or liquid, are common sources of glare. This reflection can interfere with image quality and obscure details

The most effective way to reduce glare is to use a polarising filter on the camera lens combined with a polarising film over the light source. MidOpt offers three types of polarisers to suit different applications:

PR/PL Series Linear Polarisers - the most common type, effective in reducing glare over the 400-700 nm wavelength range.
PC Series Circular Polarisers - also effective over 400-700 nm, particularly useful when cameras have light metering or auto-focus functions, making them popular in photography.

Pi Series Infrared Polarisers - advanced wire-grid linear polarisers that reduce glare over the 700-1100 nm wavelength range.
Optimal glare reduction is achieved by aligning a linear polariser to pass only light polarised perpendicular to the reflected glare. Beyond glare reduction, polarisers can improve image contrast, increase colour saturation, allow stress evaluation in transparent plastics and glass, and even be paired to create a variable neutral density filter.

Light Balancing

White light is composed of the three primary colours: red, green, and blue. The perceived tint of white light depends on its colour temperature. Light with a high colour temperature contains more blue wavelengths, while light with a low colour temperature contains more red wavelengths.

Light Balancing Filters are used to correct the effects of artificial lighting so that colours appear more natural. For example, cool white LED lamps emit a higher proportion of blue light; a light balancing filter can block much of this excess blue, resulting in a more natural appearance of the object being inspected.

Using optical filters to balance light is generally preferable to relying on software corrections. Software-based adjustments can degrade image resolution, whereas optical light balancing preserves image quality and ensures more accurate colour representation.