Lighting

An algorithm can only evaluate features that are visible in the image. Lighting therefore often decides the success of an application more than additional camera megapixels. With the right light geometry, a barely visible scratch can appear with strong contrast.

The choice depends mainly on: surface material object geometry the feature you want to see camera direction working distance speed the wavelength required There is no form of lighting that is optimal for every application.

In a front lighting arrangement, camera and lighting are on the same side of the object. Depending on the angle of incidence, bright field, dark field or diffuse lighting effects arise.

A ring light sits in a ring around the camera optics and illuminates the object from several directions. This can reduce shadows. The actual effect on the image depends heavily on the angle of the LEDs: a shallow ring light can act as dark field, a steeper one as direct front lighting.

In dark field, light hits the surface at a shallow angle. Smooth areas reflect much of the light past the camera, while edges, scratches, engravings or raised features can scatter light towards the camera. Surface defects therefore often appear bright on a dark background.

Typical applications: scratches engravings embossing cracks surface structures DPM codes edges Whether it actually works depends on the material and surface structure.

In coaxial lighting, light is directed onto the object along the camera axis via a beam splitter. This makes it particularly suitable for flat, specular or directionally reflective surfaces.

Examples: wafers polished metals printed circuit boards flat glass surfaces print markings on specular surfaces On strongly curved surfaces, a diffuse dome geometry is often the better choice.

Dome lighting produces light from many directions around the object. This reduces harsh reflections and shadows. It is particularly suited to glossy, curved or irregular surfaces.

With backlighting the object sits between camera and light. The object is rendered as a silhouette. Typical applications: outer contour diameter gap bore presence geometry

With collimated light the rays run more parallel than with a normal diffuse backlight. This can render edges more sharply and reproducibly for certain measurement tasks.

Telecentric or strongly collimated measuring backlights produce highly directional rays and are used with suitable optics for particularly accurate contour measurements. For high-precision metrology, a telecentric lens and telecentric or collimated lighting can be combined.

Line lighting concentrates light onto a narrow area. It is used particularly often with line scan cameras, because only the object line currently being captured has to be brightly lit.

There is no universally best light color. The optimum wavelength depends on which materials or colors have to be separated from one another. The spectral reflectance of the object and the sensitivity of the sensor are decisive.

nm means nanometre. 1 nm = 10⁻⁹ m Visible light spans roughly violet/blue through green to red; machine vision also uses UV, NIR and SWIR wavelengths, among others.

Red light is used very often because: many monochrome sensors are well sensitive to it powerful LEDs are available matching bandpass filters are available Whether red delivers the best contrast depends on the surface and color of the part being inspected.

Blue light has a shorter wavelength than red. This can offer optical advantages with very small surface structures and produces different contrast ratios on differently colored materials than red light.

Green can make sense where it improves the spectral contrast of the workpiece or where camera and optics work particularly well in the green range. Monochromatic green lighting is common in measurement applications in particular.

White light contains a broader visible spectrum. It is necessary, or sensible, where genuine color information is to be evaluated with a color camera.

NIR can render visible color or print patterns differently from visible light. Applications can include: certain packaging inspections material differentiation suppressing visible printed information inspecting certain biological materials The specific effect has to be established by testing.

SWIR stands for short-wave infrared. SWIR cameras capture wavelengths above the classic visible/NIR range and can reveal material properties that a normal silicon camera cannot see, or can barely see. Typical applications include moisture differences, material separation, semiconductors and inspection through certain packaging materials.

UV is often used to excite fluorescence. Suitable substances then emit light at a different wavelength. Typical applications: adhesive inspection coatings security markings contamination certain printing

A polarizing filter preferentially passes light of a particular polarization direction. In machine vision, a polarizer on the lighting is often combined with a second one in front of the lens. Cross-polarization can strongly reduce certain disturbing specular reflections.

No. How effective it is depends on the material, angle, surface structure and type of reflection. Metallic reflections and strongly curved surfaces can be more difficult, for example.

A bandpass filter passes only a limited range of wavelengths. If a red LED is combined with a matching red bandpass filter, for example, ambient light outside that range can be reduced considerably.

A longpass filter passes wavelengths above a defined cut-off wavelength and blocks shorter ones.

A shortpass filter passes wavelengths below a cut-off wavelength and blocks longer ones.

An IR cut filter suppresses infrared light. This is useful in color applications in particular, so that IR components do not falsify the color rendition.

With continuous lighting the light is permanently on. That is easy to set up, but it produces heat continuously and often reaches a lower peak light output than permissible strobe operation.

Here the lighting is only switched on for a short window. The lighting is synchronized with the camera and, on products designed for it, can be driven at a higher current for short pulses.

Advantages: high instantaneous light output short exposure times less motion blur lower average thermal load than the same peak power in continuous operation The permissible pulses depend entirely on the lighting and controller, however.

Overdrive means driving an LED above its normal continuous rating for a very short period. This is only permissible where lighting and controller are expressly designed for it. Manufacturers such as CCS and Falcon specify product-specific limits for this.

The duty cycle describes the proportion of time during which a signal is active. For a periodic flash: duty cycle = flash duration / cycle duration × 100% Example: 100 µs flash 10 ms cycle 0.1 ms / 10 ms × 100 = 1%

That is product-specific. There is no universally valid maximum duty cycle for machine vision lighting. Check the datasheets of the light and the controller.

That is product-specific too. The permissible peak current, pulse duration, frequency and duty cycle together form the thermal limit.

PWM stands for pulse width modulation. Brightness is controlled through the ratio of on-time to off-time while the electrical pulse amplitude can remain largely constant. With cameras, make sure that PWM and exposure time do not produce unwanted brightness variations.

Depending on the model, a controller can: regulate current dim the lighting receive triggers generate flash pulses set delays provide overdrive synchronise several channels

There is no general answer. For flat specular metal, coaxial light or diffuse lighting are interesting, for example. For scratches and engravings, shallow dark field often works very well.

A very shallow angle of incidence is often effective. The smooth surface stays dark while the scratch scatters light towards the camera and appears bright.

Possible candidates are: dome diffuse coaxial lighting polarisation dark field Which solution is optimal depends on whether scratches, print, contour or a DPM code have to be made visible.

Here too the feature you want to see is decisive. For glossy plastic surfaces, diffuse lighting or polarization often work. For surface defects, dark field methods can be suitable.

Possible approaches: backlighting for contours dark field for edges and defects directional light for surfaces polarisation for certain stress or reflection effects Glass is one of the applications where practical lighting tests matter most.

Depending on the inspection feature, the candidates are: diffuse front lighting dome coaxial light low-angle / dark field Falcon names low-angle diffuse lighting for PCB and solder joint inspection, for example.

Dome or large-area diffuse lighting is often a very good starting point. Additional polarization can reduce further directional reflections.

Rather than just reducing the exposure, the lighting geometry usually needs to change. Possible solutions: dark field dome coaxial light more diffuse light polarisation change the camera or light angle

Possible reasons: wrong distance light too small wrong angle insufficient diffusion object geometry shadowing Optimise the lighting mechanically first; flat-field correction can then compensate for the remaining differences.

Possible causes: PWM mains frequency unsynchronised controller incorrect trigger timing exposure time hitting different PWM phases Camera and lighting should be synchronized in time.

Often the light pulse is not correctly synchronized with the exposure. Check: trigger polarity strobe delay pulse duration exposure time controller luminous flux cable

LEDs do not convert all the electrical power into light. The resulting power loss has to be dissipated as heat. With powerful industrial lighting, adequate thermal coupling is therefore essential for light stability and service life.

Then optical contrast is usually missing. Instead of an even higher-resolution camera, try first: changing the light angle changing the wavelength dark field coaxial light dome polarisation The aim is to make the defect clearly visible in the raw image already.

duty cycle [%] = pulse duration / cycle duration × 100

Do not start with a manufacturer but with the optical task: contour → backlight scratches → dark field flat specular metal → coaxial light curved glossy part → dome general front lighting → ring or bar light Then choose the matching size, color, output and controller.

We need: object size the feature you want to see surface working distance camera angle light color continuous or strobe operation required ingress protection Determine the lighting geometry first and then select within the appropriate product family.