How to Choose 5 common Machine Vision Lights

How to Choose 5 common Machine Vision Lights

Oct 09 2020

Machine Vision Lighting technique describes how a light is physically positioned relative to the target. There are five common vision lighting techniques: ring lighting, backlighting, directional lighting (bright-field and dark-field), low-angle ring lighting, and coaxial lighting.

1. Ring Lighting

Ring lighting is the tried-and-true standard in vision lighting, used most often in general vision inspections. Standard ring lights mount directly to the camera or vision sensor and both parts can be mounted as one piece for convenience. Ring lights brightly illuminate the area directly in front of the camera and are especially useful for inspecting small parts.

2. Backlighting

With backlighting, the light source—normally a specialized backlight that provides even, high intensity light—is positioned behind the object and shines directly into the camera. Backlighting is the highest contrast lighting technique available, producing a binary image that is easy to process.
For opaque objects, backlighting creates an image that shows a dark shadow of the part (a silhouette). The silhouette can then be inspected for proper size and shape. The high contrast images created using a backlight are ideal for high accuracy applications like precision measurement, gaging small parts, detecting presence/absence of holes, and verifying part placement and orientation.
For translucent objects, backlighting is a common technique when looking for defects or detecting degree of translucency. Backlighting is not a suitable technique for detecting surface color or texture.  

3. Directional Lighting

Directional lighting creates shadows to detect changes in depth, illuminate specific surface angles, and avoid glare of reflective surfaces when directed at an angle away from lens. Directional lighting can be either bright-field or dark-field. 

In bright-field directional lighting, the light source is positioned so that it shines more or less perpendicular to the object, making the object appear bright. The features on the object’s surface are visible but not highly contrasted. However, bright-field lighting can produce glare and hotspots on highly reflective surfaces and may not be able to show surface texture.

In dark-field directional lighting, the light source is positioned at a low angle, so that its light reflects off the object, away from the camera. Dark-field lighting results in a darker overall image, but surface defects, raised features, and textures appear bright and stand out from the rest of the target. Dark-field lighting reduces glare, making it ideal for inspecting shiny objects, and this technique can also be used to measure height changes by creating shadows that can easily be measured.

Area lights, spotlights, and bar lights are common light sources that provide directional lighting. Area lights and spotlights provide even illumination in a concentrated area and can be mounted within your existing inspection, independent of the vision sensor’s optical axis. Users can change the direction or distance of the light for improved optical contrast without adding fixtures or realigning the inspected parts.

Meanwhile, bar lights, also known as linear array lights, offer higher intensity lighting to illuminate large areas, while cutting through dust, mist, and grime. Bar lights provide clean, bright, and even illumination to create the optical contrast necessary for reliable accurate inspections of larger objects at longer distances.

4. Low Angle Ring Lighting

A form of dark-field lighting, a low angle ring light has its LEDs mounted in a ring, pointing toward the part almost perpendicular to the camera’s direction of view. Low angle ring lights create shadows and bright spots to detect changes in depth and surface texture. Low-angle ring lighting is ideal for inspecting surface defects, etching, and embossed marks.

5. Coaxial Lighting

A form of bright-field lighting, a coaxial light uses a beam splitter to focus light along the camera’s optical axis, providing direct and diffuse light that renders reflective surfaces bright without glare. The camera looks down from the top, through the coaxial light, to the target part below.

This technique can be used to eliminate shadows, inspect shiny object, or detect surface texture differences. The coaxial light and the low angle light produce opposite images in terms of which features appear dark and bright.

Choosing the Right Wavelength of Light

In addition to choosing the right lighting technique, different wavelengths of light can also be used to create additional contrast, draw out features of interest, or reduce the visibility of insignificant features.

Colors of Light

Different colors of light can be used to create additional contrast to either obscure or draw out specific features. In general, when a light source is a similar color to the part, it will make the part appear brighter. For example, a red part illuminated with a red ring light will make the red part look bright on the image.

Conversely, when a light source color is different from the color of the part, the part will appear darker. An example is when the same red part is illuminated by a green light, the red part will appear dark.

Ultraviolet (UV) Light

Ultraviolet (UV) light can be used to draw out features of interest when there is very little contrast from the background, such as beads of clear adhesive on a part. Shining UV light on the adhesive may create a glowing effect (fluorescence) that makes the adhesive stand out clearly from the background. This makes it easier for the vision system to reliably identify when adhesive is missing or applied incorrectly.

Infrared (IR) Light

Infrared (IR) light can be used to hide insignificant features by reducing the contrast of certain objects. For example, some types of ink that appear dark in the visible spectrum reflect large amounts of IR light. In these cases, ink may disappear in a grayscale image—an effect that can be used to hide certain insignificant features. IR light can also be used to reduce glare and reflections when inspecting shiny objects.