How Do Borescopes Work?
There are three major types of borescopes: Rigid, Flexible and semi-rigid. All borescopes have an objective lens at the tip, much like a camera lens, to form an image of the object to be viewed. The objective is followed by an optical relay, to transmit the image down the tube. The eyepiece then magnifies the image to be viewed by the eye. The major difference in the three types of borescopes in the optical relay.
Rigid borescopes use lenses to relay the image down the tube. These can either be a series of achromatic doublets, Hopkins relay lenses (often referred to as rod lenses or Hopkins rod lenses), or a GRIN relay lens. These rigid borescopes give the best image quality of the three types. The resolution is typically diffraction limited at about f/5, meaning that the relay lenses have a resolution of resolve 5 microns. This means that a typical 4 mm diameter rigid borescope having a roughly 2 mm diameter optical channel, can resolve 400 lines(200 line pairs), or approximately 130,000 pixels. A typical 8 mm diameter rigid scope may have a 4 mm diameter image and therefore 800 lines (400 line pairs) or about 500,000 pixels. Rigid scopes have the best resolution by far. Prices range from $500 to $3000 depending on the features. The rule is always use a rigid if you can. Only use a flexible if you have to.
Rigid borescopes can typically be purchased with various directions of view for looking at different things. Zero degree (Straight ahead) and 90 degree are the most common. Ninety degree scopes are often used to inspect tubing, rifling, weld joints, cross, holes cylinder walls, turbine blades, etc. Seventy degree and 110 degree scopes are also commonly used for viewing valves in engines. Different fields of view are also available.
Flexible borescopes use coherent fiber optic imaging bundles to relay the image from the objective to the eyepiece. These bundles allow the scope to be flexible but at a high cost in resolution. Typically these bundles resolve 10,000 to 30,000 pixels … roughly a factor of 10 less than a rigid borescope. There are two types of fiber optic image relays: fused and leached. Fused fiber bundles tend to have smaller diameter fibers, giving higher resolution. However, they have cross talk between the fibers reducing contrast. Blacks aren't as black and whites aren't as white. They are also less flexible than leached bundles and are therefore more prone to break. Leached bundles have typically 10,000 to 20,000 pixels. The fibers are separated so that there is no cross talk, giving much higher contrast. It is hard to emphasize the importance of contrast in writing, but when you see the image, it is dramatic. High contrast is important, often more important than the number of fibers. Having more fiber doesn't do you a lot of good if the image is low contrast. You won't get the resolution you want anyway. Quality flexible borescopes are also much more expensive than rigid borescopes. Prices range from $1500 for non articulating to $15,000 for high-end articulating models. Once again, use a rigid borescope if you can, use a flexible scope if you have to.
Flexible borescopes can be either articulating, or non articulating. A 4 way means that the tip of the scope can be articulated up/down and left/right. A two-way scope would only be left/right. If you need to snake through a bent tube, a non-articulating borescope may be just fine, but if you want to go through a hole and look around inside a cavity, then you probably need articulation.
Semi Rigid Borescopes
Semi Rigid borescopes are built much like a rigid scope, but use a fiber optic image bundle to relay the image. These are typically less than 2 mm diameter and perform optically like a flexible, but mechanically like a rigid. They are made this way simply because it is difficult and expensive to make extremely small lenses that are required, often < 1mm diameter.
When using a borescope it is usually necessary to illuminate the object you are viewing. There are two basic types of borescope illumination: hot light and cold light. Hot light means the borescope contains a bulb at the tip. This is no longer common but some scopes still use this type of illumination. It has the downside of heating the object as well as the potential to break the bulb. Cold light means that there is an external light source and the illumination is transmitted through the scope using optical fiber. (Note: This is incoherent illumination fiber, not coherent imaging fiber as previously discussed.) Light sources vary from portable flash light types, which attach directly to the borescope, to metal halide arc lamps, or even 300 watt Xenon lamps. For the line powered light sources it is necessary to use a fiber optic light guide (or a liquid light guide) to connect the light source to the borescope. The amount of light you need depends on the size of the cavity that you need to illuminate, the amount of Illumination fiber in the borescope and the sensitivity of the detector, be it a camera or your eye. For illuminating most machined metal parts, metal tubes, rifle barrels, etc. a simple hand held portable light source can be used very effectively. When viewing larger cavities such as engine cylinder, large casting, inside of walls, fuel tank, oxygen tanks, etc, then you will need a higher-powered light source.