Introduction to light source selection terminology

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Forward Light

Forward light refers to light that is directed onto the surface of an object from above, with the light source positioned above the object being measured. Based on the angle of illumination from the light source, forward light can be categorized into high-angle light, low-angle light, and shadowless light, which encompasses both high and low angles of illumination.

High-angle Light

Light Path Description: Light rays with an angle greater than 45° relative to the horizontal plane are referred to as high-angle light.

Effect Analysis: With high-angle illumination, light rays reflect off the smooth surface of the object under inspection and enter the lens, resulting in higher grayscale values in the image. However, light rays cannot penetrate the lens if the surface is uneven, leading to lower grayscale values in the image.

Main Applications: Positioning, character detection, contour detection, scratch detection, dimension measurement.

Common Light Sources: High-angle ring light, linear light, area light, coaxial light, point light, etc.

Coaxial Light

Light Path Description: When the reflected light rays are parallel to the lens, it is referred to as coaxial light.

Effect Analysis: After the light rays undergo planar reflection, they enter the lens parallel to the optical axis. At this point, the object under inspection acts like a mirror, reflecting the information of the light source. Any irregularities or deformations on the object will be particularly pronounced in the image.

Main Applications: Scratch detection, imprint inspection, detection of surface irregularities, contour detection.

Common Light Sources: Coaxial light, parallel coaxial light, area light, linear light.

Backlighting

Backlighting refers to the placement of the light source below the object under inspection. When there is no obstruction, the light directly enters the lens, resulting in high grayscale values in the image. However, if the object obstructs the light, the grayscale values in the image will be low. This low grayscale value reveals the contour information of the object.

Low-angle Light

Light Path Description: Light rays with an angle less than 45° relative to the horizontal plane are referred to as low-angle light.

Effect Analysis: With low-angle illumination, reflected light from the smooth surface of the object under inspection cannot enter the lens, resulting in lower grayscale values in the image. However, light reflected from uneven surfaces can enter the lens, resulting in higher grayscale values in the image.

Main Applications: Positioning, character detection, contour detection, scratch detection, dimension measurement.

Common Light Sources: Low-angle ring light, linear light, line light, etc.

Diffuse Backlighting

Light Path Description: The light source passes through a diffuser to scatter the light.

Effect Analysis: When extracting contours, for products with thickness and curvature, the edges in the image may appear blurred and less sharp. However, for flat products, the image effect is stable, offering a cost-effective solution.

Main Applications: Positioning, dimension measurement, presence/absence detection, defect detection.

Common Light Sources: Area light.

Shadowless Light

Light Path Description: The light path is altered through structural elements or diffuser plates, resulting in a light emission angle that includes both high-angle and low-angle components.

Effect Analysis: Combining the effects of high-angle and low-angle light, shadowless light provides multi-angle illumination for the object under inspection. Surface textures and wrinkles are minimized, resulting in a uniform appearance in the image.

Main Applications: Positioning, dimension measurement, surface inspection of curved products.

Common Light Sources: Dome light, ring shadowless light, square shadowless light, lightbox, etc.

Parallel Backlighting

Light Path Description: Light sources emit parallel light through parallel structures.

Effect Analysis: Parallel light enables precise outlining of irregular objects, resulting in clear and sharp imaging of the edges of products such as cylindrical objects or those with chamfers and rounded corners. Typically used with telecentric lenses, it offers high precision.

Main Applications: Dimension measurement.

Common Light Sources: Parallel area light, parallel coaxial light.

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