Although most camera modules are designed to be used with the object at a larger distance (infinite conjugate), conventional testing techniques usually place a test target with various patterns – typically a test chart – at a shorter distance (finite conjugate) from the module.
Today, testing with test charts in finite distance is standard, done by image analysis software in real time.
Figure 1 shows a schematic representation of a test chart system in finite configuration. The device under test (DUT) is placed in front of an illuminated chart with different markers and patterns on it. An image is then taken with the DUT, and software algorithms analyze the position and shape of the markers, and extract the performance parameters of the camera module.
A modified version of this setup incorporates a relay lens in between sample and chart to create a virtual image of the chart as seen from infinite distance. This approach usually requires a custom, high-quality relay lens, especially for larger-field-of-view applications and broadband (e.g., daylight) illumination, which can make such a setup complex and commercially unviable. This solution should be used only if infinity conjugate measurement and a more complex chart evaluation are required.
This test chart technology is relatively easy to deploy and combines a high density of measurement points with the ability to use different types of markers and patterns on different field positions of a single chart. It is typically used for sensor resolutions of up to 13 MP and testing in finite or close-to-infinite distances. However, because of recent advances in manufacturing technology, this technology reaches its limits as the industry moves to sensor resolutions of 13 MP and higher, which tightens the requirements on optics and the alignment of the individual components of the module.
Common precision and repeatability bottlenecks include the need for high homogeneity of the illumination over a large area, edge contrast and precision of the patterns on the test charts, and the required reproducible spectral distribution lighting, which is usually done with LEDs.
The size of the test instrument is another important factor in a production environment: For the wide-angle, large-field-of-view optics typically found in consumer camera modules, a large test chart is required. The larger the angular field of view, the larger the test chart must be for a given finite object distance, which cannot be set too small for infinity-set fixed-focus objectives. The situation is similar for modules that have a focusing capability – in this case, shorter object distances can be used, but the setup does not properly reflect the final application. For large test charts, the corresponding large instrument requires more precious factory floor space.