Main lighting methods of image measuring instruments

Lighting is a critical link in the measurement process of image measuring instruments. A good lighting solution plays a vital role in accurately measuring various values ​​of the measured object.

In image measuring instruments, the three main lighting methods are: surface light lighting, contour light lighting and coaxial light lighting. Surface light and contour light lighting methods are relatively common, while coaxial light lighting plays a vital role in the measurement of certain special workpieces. In addition to surface light and contour light, the automatic image measuring instrument developed by Tianjue Company (www.tztek.com) has added coaxial light lighting, which can be used specifically to measure the hole depth and blind hole depth of workpieces, and has a very unique effect on measuring reflective and mirror workpieces, and has a strong auxiliary effect on the accuracy of grabbing primitives.

Surface light lighting: Surface light lighting generally uses ring or point lighting, which is on the same side as the camera. Ring lights are a commonly used surface light lighting method, which is easy to install on the lens and can provide sufficient lighting for diffuse reflective surfaces. Surface light lighting uses surface light sources. Surface light sources provide general illumination of the workpiece surface. The current application trend is to use annular surface light sources to provide surface illumination in multiple directions and incident angles. Tianjue's image measuring instrument provides 6-ring 8-zone surface light illumination, with a total of 48 independent controllable partitions. Each partition can independently control the switch and brightness, with a brightness level of up to 200 levels. And the incident angle of each ring is different, ensuring that different lighting methods can be provided for different workpieces.

Contour lighting: Contour lighting is to place the light source on the back of the object. This lighting method is very different from other lighting methods because the image analysis is not based on reflected light but incident light. Contour lighting produces a strong contrast. When contour lighting technology is applied, the surface features of the object will be lost, but the contour information of the object can be clearly obtained. The light source for contour lighting is called a contour light source. Contour light sources distinguish between transparent and opaque parts, with transparent areas appearing white and opaque areas appearing black, thus obtaining a black and white contrast image. An important point about contour lighting is that it is necessary to ensure that the emitted light is parallel light, so that different intensities of light will not cause changes in the contour of the workpiece, thereby affecting the accuracy of the measurement.

Coaxial lighting: Coaxial lighting is when light from the same direction as the axis of the camera is irradiated onto the surface of the object. Coaxial lighting uses a special semi-reflective mirror to reflect light in the direction of the camera's lens axis. The semi-reflective mirror only allows light reflected from the surface of the object and perpendicular to the lens to pass through. This lighting technique provided by the coaxial light source is useful for achieving uniform illumination of surfaces with mirror features on flat objects. In addition, this technique can also highlight the surface angle variation, because the light reflected from the surface that is not perpendicular to the camera lens will not enter the lens, resulting in a darker surface.