The geometry of a surface in combination with a light source has a significant effect on the resulting camera image.
The simple rule 'angle of incidence = angle of reflection' applies to reflected light, especially when using directional, point light sources.
This is particularly true for collimated incident light ( telecentric / condenser) as the emitted light is extremely parallel and no further diffuse light is emitted. However, diffuse lighting techniques can also produce directional light: diffuse coaxial incident lighting emits largely directional light.
Illumination from the top
If the object is illuminated from above, there will be a strong reflection towards the camera and the flat surface of the component will be bright.
Light is refracted sideways by angled edges, curvatures and phases, which appear dark in the camera image because this light does not hit the camera sensor.
Flat and sloping surfaces with
incidence of light from top
Illumination from the side
If light falls on the side of the flat surface of the target, it will be reflected to the side and not onto the camera sensor.
The surface appears dark. In our example, the ring light has been positioned so that the oblique phase is exactly at the 'viewing angle'. The light is reflected towards the camera, which appears bright.
Flat and sloping surfaces with incidence of light from side
Direct illumination of spherical and cylindrical surfaces
The ideal test object for the influence of surface geometry on the direction of light is a polished steel sphere (or, for simplicity's sake, a smooth metal cylinder). This is a good way to see what homogeneous light can illuminate.
Directed light from above creates a strong light reflection on the smooth surface. Light is mainly reflected. With a spotlight, the angle of incidence is equal to the angle of reflection. With direct incident light, such as an LED ring light, the solid angle that the light can illuminate is very small. The beam angle of LEDs is typically 15 degrees, but the brightness distribution over this angle is not constant, so the emitted light distribution actually appears to be much more directional.
Directional light on a spherical shape
Diffuse incident lighting on a curved surface
The situation improves the more diffuse the lighting and the larger the illuminated area. If diffuse lighting illuminates a spatial angle of 90 degrees without gaps, a homogeneously illuminated image area of 45 degrees can be expected.
Caution: Light emitted over a large area will not have a large angle of illumination if only simple diffuser panels diffuse the light from point light sources.
Diffuse incident light on spherical shape
Dome-like illumination on sperical and cylindrical shapes
Truly homogeneous light can only be produced with spherical dome illumination. The solid angle that is illuminated on the sphere without immersing the specimen in the dome is ideally 90 degrees with an illumination of 180 degrees.
Therefore, when illuminating the edge of a spherical object, there will be a large loss of light and significant shadowing, as the light incidence without diving into the dome can only be a maximum of 180 degrees.
Dome light on spherical shape
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