GRIN lens simplifies precision borescope design
Douglas Kindred
Rigid endoscopes, or borescopes, offer better image quality than flexible designs and are suitable for a variety of medical and industrial applications. Traditional rigid scopes incorporate relay systems composed of several conventional optical elements. The most common configuration is the Hopkins? rod system, which uses spherical lenses cemented to homogeneous glass rods (see Fig. 1). Relaying an image one time requires a pair of rod lenses, each of which is composed of a singlet, a solid glass rod, and a doublet, all cemented together. Each relay is made u¥of eight distinct optical elements.
The aspect ratio for each relay module is about 25 to 1; in other words, to make the scope long enough to be useful, most rigid endoscopes require three to seven modules of the image. This adds u¥to from 24 to 56 individual optical elements ranging from 1 to 4 mm in diameter. Each component must be separately ground, polished, and centered; it is easy to see why rigid endoscopes are typically priced at $2500 to $5000.
Color-correction with GRIN glass
Radial-gradient-index (GRIN) lenses provide an alternate approach to rigid endoscope relays. A single rod with flat end faces can easily relay an image three to seven times, eliminating multiple element fabrication and alignment costs. A typical Hopkins relay costs from $200 to $400, while GRIN relays cost $50 to $100.
Despite the cost advantages and simplicity of the GRIN approach, the technology has not replaced the Hopkins configuration. The primary drawback of conventional GRIN relays is that they suffer from excessive dispersion of the same sign as conventional lenses; that is, the system focal length for the blue end of the visible spectrum is shorter than for the red end of the spectrum, making it impossible to produce a color-corrected system. Using a proprietary fabrication technique, engineers at Gradient Lens Corp. (GLC; Rochester, NY) have developed a gradient-index glass and produced the endoGRINS relay, a component that introduces dispersion of the opposite sign to cancel that of conventional lenses, allowing designers to make an achromatic borescope.
Blue light traveling through GRIN glass still experiences a larger refractive index than red light, but the change in refractive index, Dn, is larger for red than for blue, making the relay lens stronger in red. This allows the lens designer to easily balance the axial chromatic aberration in the relay with that of the objective and eyepiece, thus producing a color corrected endoscope (see Fig. 1, inset).
EndoGRINS relays are manufactured by ion exchange in GLC?s proprietary lithium-containing glass. The glass is first drawn into rod form, then lowered into a molten salt bath containing sodium ions. The exchange of sodium ions in the salt solution for lithium ions in the glass forms a nominally parabolic refractive-index profile in the glass with a Dn of ?0.003, producing an f/5 relay with an aspect ratio of approximately 25:1 for a single pitch length, or relay of the image; this is roughly the industry standard.
The rods have excellent image quality, resolving more than 200 line pairs per millimeter. The dispersion of the relays is only ?0.25%; a rod 2 mm in diameter and 1.5 pitch lengths (150 mm) long would induce ?0.375 mm of axial color in the image, with red wavelengths coming to a shorter focus than blue wavelengths. This condition is easily balanced by the chromatic effects of the eyepiece and objective lenses, in which blue light comes to a shorter focus than red light.
Hawkeye precision borescope
The endoGRINS relay lens allows optical designers to produce simple, economical, high-quality endoscopes and borescopes such as the GLC Hawkeye Precision Borescope (see Fig. 2). The optical design contains only three optical elements, yet the instrument performance is competitive with conventional Hopkins designs. The scope uses a single-element, 2.7-mm-diameter GRIN objective lens manufactured by NSG Corp. (Osaka, Japan). The GLC EG-27-3P/2, a 2.7-mm-diameter by 195-mm-long relay, transmits the image down the scope to a single-element BK-7 eyepiece with an effective focal length of 15 mm. The borescope offers good performance over a 40! field of view.
Because the endoGRINS relay is a symmetric 1:1 relay lens, the relay itself introduces no coma, distortion, or lateral color. The system must only be corrected for spherical aberration, field curvature, and axial color.
The objective introduces a negligible amount of spherical aberration. A small amount of overcorrected spherical aberration in the endoGRINS relay balances the undercorrected spherical aberration of the BK-7 singlet eyepiece. The slight backward curving field of the relay corrects for the inward curving field of the objective and eyepiece.
The most important contribution of the relay is its ability to correct for axial and lateral chromatic aberration introduced by the other optical elements of the system: the axial color of the objective and eyepieces are balanced by the negative dispersion of the endoGRINS relay, and the lateral chromatic aberration in the objective is balanced by the eyepiece. The result is a well-corrected borescope with high resolution, high contrast, and a retail list price of $350.
Rigid endoscopes have a host of industrial applications. Gunsmiths and target shooters use borescopes to check the quality and cleanliness of the barrel, while locksmiths and vault technicians view lock mechanisms. Aircraft, automotive, and diesel mechanics use the instruments to inspect cylinder walls, pistons, manifolds, and fuel-injector ports. In the injection-molding industry, the borescope allows technicians to examine the surface quality of finished molds. Quality-control departments also use the Hawkeye to inspect welds and solder joints and to search for cracks in inaccessible areas. The endoGRINS technology provides the low cost, simplicity, and image quality to make it all possible.
FIGURE 1. Conventional Hopkins-type relay (top) consists of a singlet, a rod lens, and a doublet cemented together; two of these assemblies are required for a single relay. Gradient-index (GRIN) relay lens (bottom) performs the same task with a single element; the lithium-doped material
corrects chromatic aberration by introducing negative dispersion (inset).
FIGURE 2. Hawkeye borescope uses a 2.7-mm-diameter 195-mm-long EndoGRINS lens to relay an image with only three components.