Concept – Structured Light

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This chapter describes the usage of structured light for surface inspection (deflectometry).

Concept of Structured Light

The basic concept behind structured light is to use a structured illumination, i.e. an illumination showing well known patterns. The way those patterns appear in the scene after hitting surfaces helps to further analyze or reconstruct the surfaces (see 3D Reconstruction / Structured Light).

Deflectometry is the procedure of analyzing the reflections of known patterns from specular or semi-specular surfaces. In such a setup, every pattern image must be shown by the display or monitor. It is then reflected by the specular surface under test, and a camera image of the reflection is acquired. Deformations of the pattern in the camera image are caused by the form of the specular surface which implies that defects on the specular surface can be detected.

In the following, the steps that are required to use structured light are described briefly.

• Create a structured light model: In the first step, a structured light model is created with

  • create_structured_light_modelCreateStructuredLightModel (ModelTypemodelTypemodel_type\(=\)'deflectometry'"deflectometry")

  or read with

  • read_structured_light_modelReadStructuredLightModel.

• Set the model parameters: The different structured light model parameters can then be set with

  • set_structured_light_model_paramSetStructuredLightModelParam

  or queried with

  • get_structured_light_model_paramGetStructuredLightModelParam.

  The pattern parameters 'pattern_width'"pattern_width", 'pattern_height'"pattern_height", 'pattern_orientation'"pattern_orientation", and 'pattern_type'"pattern_type" specify along with the stripe parameters 'min_stripe_width'"min_stripe_width" and 'single_stripe_width'"single_stripe_width" the specifications of the pattern images to be used to illuminate the surface. Finally, the 'persistence'"persistence" parameter can be enabled to debug intermediate results.

• Generate the pattern images: The pattern images are to be generated with gen_structured_light_patternGenStructuredLightPattern after setting all relevant parameters. Please ensure that the output images are as needed in the particular setup.

• Use the patterns to illuminate the surface and acquire the camera images: At this stage, the pattern images are shown on the display. The respective image of the illuminated surface is acquired by the camera for each pattern image.

• Decode the acquired images: The acquired CameraImagescameraImagescamera_images can be decoded with decode_structured_light_patternDecodeStructuredLightPattern. Upon calling this operator, the correspondence images are created and stored in the model StructuredLightModelstructuredLightModelstructured_light_model.

• Get the results: The decoded 'correspondence_image'"correspondence_image", as well as other results can be queried with get_structured_light_objectGetStructuredLightObject. For more details of the different objects that can be queried, please refer to the operator's documentation.

  The 'defect_image'"defect_image" can also be generated and queried with get_structured_light_objectGetStructuredLightObject.

Further operators

The structured light model offers various other operators that help access and update the various parameters of the model.

The operator write_structured_light_modelWriteStructuredLightModel enables writing the structured light model to a file. Please note that previously generated pattern images are not written in this file. A structured light model file can be read using read_structured_light_modelReadStructuredLightModel.

Furthermore, it is possible to serialize and deserialize the structured light model using serialize_structured_light_modelSerializeStructuredLightModel and deserialize_structured_light_modelDeserializeStructuredLightModel.

Further Information

See also the “Solution Guide Basics” for further details.