How to Diagnose and Eliminate Ghosting in Optical Prisms?

In the realm of optical systems, ghosting represents a prevalent issue with optical prisms during practical applications. The issue manifests as overlapping or ghosting of the image passing through the prism, or splitting of the laser beam after passing through the prism. This has a significant impact on the imaging quality and accuracy of the optical system.

The ghosting of optical prisms is the result of a number of issues, including prism processing defects, installation errors, usage wear and tear, and environmental interference. This article, in combination with the working principle of optical prisms, systematically analyses the root cause of ghosting, providing technical support for precisely solving the problem.

1. Analyse the characteristics of ghosting imaging to identify the appropriate troubleshooting approach

The imaging performance of ghosting is directly related to the root cause of the fault.

• It is essential to pay close attention to the number and superposition modes of ghosting. If it is a double image and the distance between the two images is fixed, this is usually related to the processing accuracy or installation positioning of the prism. In cases where there are multiple shadows and the images are blurry and divergent, the cause may be surface contamination or internal defects of the prism. If the double image changes when the prism is shaken, this is likely to indicate that the installation is loose or the positioning reference is offset.
• It is important to observe the brightness difference between the double image and the original image.If the brightness of the double image is significantly lower than that of the original image, this may be due to coating defects on the prism surface or contamination of the reflective surface, causing abnormal reflection of some light. If the brightness of the double image is similar to that of the original image, it is necessary to consider that the parallelism of the optical surface of the prism exceeds the standard or that there is a birefringence phenomenon.
• It is also important to check the stability of the double image: If the double image morphology continues to change under temperature variations or vibration conditions, this may be due to poor thermal stability of the prism material or insufficient rigidity of the installation structure. If the ghosting is consistently stable and fixed in position, the processing defect of the prism itself is the primary concern.

2. Focus on the self-inspection of the prism and trace the source of the core fault

Inspection of the optical surface quality is paramount. The accuracy of the incident, exit and reflection surfaces of the prism directly determines the direction of the optical path.

• The optical surface is inspected using the strong light side illumination method, which involves the use of parallel strong light shining from the side of the prism. In the event of a reflecting surface exhibiting defects, such as scratches, pockmarks, or chipped edges, it will cause some light to undergo diffuse reflection, resulting in the formation of stray light and double images.
• When inspecting glued prisms, the focus should be on the glued surface. If there are bubbles, impurities or delamination on the surface, the light will undergo partial reflection and refraction, resulting in obvious double images. This can be observed by the transmission method. If uneven bright spots appear on the glued surface, this indicates the presence of defects.
• Some optical prisms utilize crystal materials, such as calcite and quartz. If the crystal cutting direction is deviated, a birefringence phenomenon will be observed. Place the polarizing filter at the incident end of the prism, then rotate the polariser to observe the changes in the double images of the emitted light. If the double images alternate clearly as the polariser rotates, this indicates the presence of a birefringence problem.

3. Check the installation and system compatibility, and eliminate the deviation of optical path regulation

• It is also necessary to check the installation positioning reference of the prism.Prisms are typically fixed by positioning pins, pressure blocks or bonding. If the positioning pins are loose or the pressure blocks are unevenly subjected to force, it will cause the prism to tilt or shift slightly. This will result in a deviation of the optical path and the appearance of double images.

Coaxiality verification of the prism relative to upstream and downstream optical components. In the configuration of an optical system, it is essential that a prism is positioned in alignment with components such as lenses, lenses, and diaphragms. The laser collimation method is a highly effective detection tool. It involves the emission of parallel laser beams that pass through each optical component of the system in sequence. It is important to observe the laser spot on the receiving screen. If the spot splits or shifts after passing through the prism, this indicates a problem with the coaxiality. The installation position of the prism will need to be adjusted.