A common optical system is relay lenses. Relay lenses are used when the user cannot be physically close to the object being observed (like an endoscope) or when images need to be inverted/rectified (like in a telescopic sight). Here we describe some steps that were taken to design an optical relay system for a client that needed a custom borescope.
What is a borescope
Borescopes are usually used when there is a need to inspect narrow or difficult access locations. They can be made with rigid or flexible components with an objective lens on one end and a camera or eyepiece on the other. The particular borescope shown in Figure 1 contains an objective lens and a set of relay lenses.
In order to design (or choose the correct) objective lens, there are different parameters that need to be considered such as working distance, field of view, resolution at the camera sensor, image size, etc. The relay lens translates this image from the objective lens to the imaging sensor (not shown).
Figure 1. Borescope concept. An objective lens creates an intermediate image that is picked up by a relay lens and is then translated to an image sensor (not shown)
The objective lens is the key part of the borescope system. The objective lens forms the initial image and provides the needed optical parameters and image quality. We designed this objective lens based on the client’s requirements of resolution, FOV, wavelength range (900-1700 nm in this case), and working distance. During the optimization stage, we carefully pick the best materials to reduce the amount of aberrations.
The borescope required an extension between 400-600 mm: instead of creating a single relay lens for the total length, we divided the length into 4-5 unit elements. That is, 4 or 5 relay lenses with an extension of 100-150mm each and concatenated one after the other to achieve the required borescope length. The relay lens translates image 1:1 while the aperture of the designed relay lens coincides with the objective lens and the relay lens allows image translation without significant degradation of image quality.
One advantage of this relay lens system is that, if the borescope needs to be extended further (or to be reduced), it will be a matter of removing relay lens sections without affecting the camera or objective lens.
The final optical design step that needs to be implemented is adding a camera (or eyepiece) to observe the image. Our client required a very specific image visualization, but in more general cases, the camera size and resolution should be taken in consideration when designing the objective lens. In our design, we looked at the aberrations and MTF curves at every step, that is, after the objective lens and after adding the relay system. Optimization is done using Zemax, and suggestions were made to the client about how to display the images and adjustments on the objective lens.
When the optical design is complete, the design is passed on to the opto-mechanical engineer who designs the lens housing and prepares the optics manufacturing drawings
Optical Layout of relay lens
When the optical and mechanical designs are complete, it’s time for prototyping. The finished product of camera, relay lens, and objective lens is seen below
The final product a thermal imaging borescope
