Requirements Specification for a Vision System: What Requirements Must Be Defined Before Requesting Quotes?

Structured layout of a qualified image-processing requirements specification

Vision projects often lead to unsatisfactory results because requirements were not fully and concretely defined at the outset. A precise requirements specification is therefore one of the most important foundations for a successful project.

1. Detailed specification of inspection features

Define exact limit values for each individual feature:

• Defect definition: Which deviations count as rejects (e.g. scratches, inclusions, pores, discolouration, deformation)?
• Geometric limits: From which exact dimension (e.g. length ≥ 0.2 mm, width ≥ 0.05 mm) must a defect be detected?
• Zone definition: Are there non-critical areas (B-surface) and highly critical functional surfaces (A-surface)?

2. Part specifications and variant overview

• Dimensions: Maximum and minimum part geometry (length, width, height in mm) to determine the field of view (FOV).
• Material properties: Material (e.g. POM, PA6, anodised aluminium), reflectivity (high-gloss, matte, transparent) and colour spectrum.
• Positional tolerances: How imprecisely is the part mechanically positioned in front of the camera (X/Y offset in mm, rotation in degrees, height variation in Z)?

3. Process dynamics and environmental conditions

Two technical core parameters are particularly often underestimated in practice and belong in the requirements specification: the requirements for the PLC connection (trigger, fieldbus, cycle time) and the camera resolution calculated from field of view and smallest feature, covered in our article on flash detection.

• Time framework: Minimum cycle time per part as well as the maximum permissible software response time (processing time) for PLC evaluation.
• Motion state: Does the part remain completely stationary during image capture (stop-and-go), or does inspection occur in motion (continuous transport at speed v in m/s)?
• Disturbance factors: Occurrence of machine vibration, dust ingress, cutting fluids, and installation space constraints for camera and lighting.

4. Defining acceptance criteria and test procedures in advance

The type of later acceptance testing should be determined before ordering the vision system. For the design of the inspection system, it is relevant whether only the reliable detection of defined good and bad parts must be demonstrated, or whether, for example, a measurement system analysis (MSA) with defined requirements for repeatability and measurement deviation is required.

For measuring inspections, acceptance criteria directly influence the required accuracy of the overall system. Camera resolution, optics, lighting, calibration and mechanical positioning must be designed so that the required measurement uncertainty can be achieved under real production conditions.