All a question of light
The correct lighting is immensely important in reliably solving an inspection task using a camera sensor. This is because a camera needs light to be able to see. However, not all light is the same – the arrangement of camera, test object and lighting is key. A distinction is generally made between incident light illumination and transmitted light illumination. With incident light, the illumination is located above the object plane. With transmitted light, however, the light source is implemented as backlighting and is located below the object plane.
When should which type of lighting be used? – A brief overview
The illumination methods for incident light are described below: Classic ring lighting is used for inspecting smooth, reflective or shiny surfaces, e.g. for checking electronic components to ensure that the contacts are complete. So-called dome lighting, which enables extremely uniform illumination of a test object, is suitable for more complex, reflective objects, e.g. films, crown caps on bottles or data media (Blu-ray, CDs, DVDs). Dark field lighting is usually positioned very close to the test object and consists of e.g. flat ring lights, line lights or spot lights. This type of lighting is frequently used for detecting flaws on object surfaces (e.g. scratches or scoring) or when inspecting engravings.
With transmitted light illumination, a test object is lit from below or behind so that the beams from the light source are directed toward the camera lens. This generates a kind of silhouette of the test object, whereby the object background is visible as a bright area in the image detected by the camera sensor. This illumination method is mostly used if e.g. the presence of features (such as punch holes or drill holes) on objects is to be checked in cases where dimensional information is not of primary importance for the test result.
This brief overview serves only as an initial introduction to the extremely complex topic of illumination. If you would like to learn more about this topic, we recommend our white paper "Camera sensor technology part 3 – All a question of light", which is available for free on the ipf electronic website under the heading "Know How".