Optical Design and Engineering Service 



Pictured here is a camera system designed for NASA at Glenn Research Center. Labeled the High Bit-depth Multi-spectral camera (HiBMs) it was designed to be used aboard the International Space Station in the Destiny Laboratory Module as part of the Fluids and Combustion Facility. The camera was launched and placed into service in 2007.

The design was required to be telecentric in both object and image space, incorporated a Liquid Crystal Tunable Filter (LCTF), and has a motorized focus using a dovetail prism. The camera is of modular design allowing replacement of parts in case of component failure or upgrade. The modular design also allows for different fields of view to be incorporated by swapping out what is called the focusing module. The optical design is such that only the lenses in the focusing module need be changed to change the field of view. Two different fields of view were initially designed, a 90 mm field of view and a 50 mm field of view.

The camera was designed to be used as a visual diagnostics tool to gather data in combustion science experiments. The primary operation of the camera was to perform data gathering in soot volume fraction and non-contact temperature measurements. Soot volume fraction is the measurement of the amount of soot or airborne residue present within a given volume of airspace round a combustion event. When combined with a collimated illumination source for back-lighting the combustion event, the variable telecentric stop in the camera system will reduce the illumination from the combustion making it so only the soot is visible, allowing for detailed measurements of soot volume both inside and outside of the flame. The temperature measurements are made using the fast switching Liquid Crystal Tunable Filter. The LCTF is an electronically controlled optical filter that allows only a single wavelength to propagate through. When two images are taken quickly in sequence, each with a different wavelength set in the LCTF, the calibrated response in the resulting digital images can be compared to the Planck distribution curve to determine temperature at a particular point in the image.

This is design is for a digital uretoscope. A uretoscope is a form of endoscope that goes into the kidneys via the ureters that attach the kidneys to the bladder. The use of a uretoscope allows diagnosis and in some cases treatment of kidney stones, cancers and other pathologies of the kidney. Because of the small size of the ureters, a uretoscope must be under about 3 mm in diameter, posing special challenges for optical and electro-optical design.

The lenses shown here are approximately 1-1.2 mm in diameter. The small size of the optics requires particular care in the design phase. For instance, it is far easier to create plano-concave, and plano-convex lenses in this size format. This may be true with all lenses, but in the 1-2 mm diameter range, the cost difference between a plano-convex and a bi-convex or meniscus is significant. This is because in this size range, it is very difficult for standard lens fabricators to grind powered surfaces on both sides of a lens, while maintaining accurate center thickness on the finished product. There are similar problems in creating doublets at this size. The end result is that the lenses often must end up much simpler in form that what is common in a larger format. One advantage to designing for such small lenses is that, because of the small size, a designer can often choose among a much larger array of glass types, since volume of glass used in any particular lens fabrication is much smaller with this type of system.

The graph below the lens profile is known as a Modulation Transfer Function (MTF) which is a common method of displaying the resolving performance of an optical system. The MTF curve graphs image contrast as a function of spatial frequency. Each curve on the chart corresponds to a field point and each field point has two orientations, saggital and tangential. Contrast has no units and is charted on the vertical axis and is a measure of how different shades compare to one another. Spatial frequency is in units of cycles per millimeter, is on the horizontal axis and is a measure of the different response between the different shades. The higher the spatial frequency at a given contrast, the higher the resolution. A perfect system would have a contrast of “1.0” across all spatial frequencies. However, all optical systems have limits to the resolution they are capable of, based on several factors. Most well corrected optical systems will follow a curve similar to an exponential decay shown here.

The image here is a cross section and close up of a tapered light guide launching into fiber bundle used to collect and concentrate illumination from white light LED’s. This model uses non-sequential mode of Zemax to simulate the light source, tapered cone light guide and fiber bundle.

The light source, at the top of the cone, is a simulation of a commercial off the shelf, high intensity, white light LED. The white light LED has four 3 mm squares which emit light in a Lambertian (uniform) pattern. White light is simulated by setting up multiple source wavelengths and weighting each appropriately based on the manufactures specifications sheet.

The tapered light pipe is a cylinder of BK-7 glass that is tapered such that it is large enough to cover the size of the four light sources and tapers to a size small enough to launch into the fiber bundle. The fiber bundle is a rectangular array of 1849 fibers that simulates the entrance face of a flexible fiber bundle light pipe. The fiber bundle is modeled by modeling a single fiber as a core and cladding pair that are then placed into an array in Zemax. The closeup of the output of the fiber bundle (inset) shows that each ray is “captured” inside its respective fiber until it reaches the exit face.

The non-sequential mode of Zemax is typically used to model illumination, display, beam shaping and other non-imaging types of optical design. The non-sequential mode is more mechanically intuitive, but because of the large amounts of rays that must be traced for an accurate estimate of performance, it is also much more computationally intensive.

This is the objective lens array on a very low distortion laparoscope. A laparoscope is a rigid surgical scope allowing surgeons to view inside the body with very small incisions. Combined with other instruments, these devices allow doctors to perform minimally invasive surgery, reducing scaring, post-operative pain, and chance of infection. Laparoscopes are typically 10-40 cm in length, 5-10 mm in diameter and are mostly used in abdominal surgery. The optical system of this type of scope consists of an objective, shown here, a relay lens system that passes the imagery through the length of the scope, and an ocular that conditions the imagery for direct viewing with the human eye. Although laparoscopes typically are suited for direct viewing with the eye, they are typically coupled to cameras that allow the images to be displayed on monitors within the surgical arena.The design here was conceived to model the performance of a high quality scope and the ultimate image quality of an integrated system when coupled with a low cost digital camera. The design of this objective is for a very low distortion scope. Distortion, as defined in optics, is a change in magnification as a function of field height or distance from the center of the field of view. The result of high distortion in an optical system will result in the edge of an image appearing stretched or compressed, depending on the type of distortion present in the system.

The plots below the lens layout are typical for the analysis of this type of system. The modulation transfer function (MTF) is at the top left showing the MTF curves of all of the fields plotted in the lens layout. On the top right is a diagnostic called the spot diagram. This shows the classical optics ray tracing results of many rays emanating from a geometric point in object space to where they land in image space. Each spot corresponds to a field height shown in the lens layout. The lower left shows plots of field curvature and distortion. The plots indicate approximately 0.010 mm of field curvature and a maximum of 1% distortion. The plot in the lower right is a graphical representation of the field curvature at the image plane.

This is a design for a document scanning system that exhibits elements of both sequential and non-sequential ray tracing. The system incorporates a laser line generator, several power monitors and associated beamsplitters, an imaging optical system, chromatic filters and beam steering mirrors.

Light is generated from an commercially available laser diode. The laser is shaped in to a line using custom designed line generating optics. The laser line is focused on to the document being scanned and the reflected light is then focused on to a digital sensor with imaging optics. There are two beamsplitters which direct a portion of the light to monitoring sensors. These sensors are placed at critical locations so real time monitoring of laser power is possible.

The imaging optics in this system were designed using the standard sequential mode of Zemax. The system consists of two off the shelf lenses and one custom designed lens. The laser line generator, beamsplitters, beam steering mirrors, filters and windows were designed using the non-sequential mode. For the final system performance analysis, the imaging optics from sequential mode were imported into the non-sequential design. This system is an example of how sequential and non-sequential design techniques can be combined in a mechanically complex system.

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