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Key words: lens testing, intraocular lenses
Many surgeons are turning to silicone IOLs since they are flexible and can be implanted through a smaller incision than conventional acrylic lenses. A smaller incision decreases scarring and distortion of the cornea. However, the index of refraction of silicone is lower than that of acrylic. To produce the same optical power in the eye, the silicone lens must have greater surface curvature. This presents no problem in the eye, but during testing these more highly curved surfaces create an effect known as spherical aberration. Images formed in air with silicone lenses are not as clear as those formed with acrylic IOLs. Thus, silicone IOLs are released with lower quality standards for resolving power in air. Unfortunately, the combination of spherical aberration and lower standards has the potential of allowing unwanted optical defects that may disturb vision after the IOL is placed in the eye.
OST scientists have been studying this process and have been evaluating a special lens that could be used during silicone lens testing to eliminate the effects of spherical aberration. This special lens, known as a "null lens," would undo the effects of spherical aberration while leaving the other optical properties under test the same. Thus, in principle, silicone lenses could be tested against the more demanding criteria applied to conventional acrylic lenses. The null lens was computerdesigned using lens-design software and consists of two optical elements working together. The second element cancels effects of the first lens except for spherical aberration. Just enough residual spherical aberration remains to cancel or "null" that produced by the silicone intraocular lens. For a perfectly made silicone IOL, the resulting resolution should be limited only by the effects of diffraction.
Bench testing of the first null lens design is currently underway. Removing spherical aberration sharpens the focus and this allows optical power to be determined with greater precision and overall accuracy and optical resolution to be improved. This allows the manufacturer to hold to tighter internal power tolerances. Resolution test results are closer to diffraction limited performance, thus permitting more demanding resolution tolerances to be used. The net result is that use of a null lens during manufacture might increase the surgeon's chances of achieving the intended visual result when a lens is actually implanted.
Null lens designs must be specifically tailored to that of the individual IOL. Since IOLs are manufactured in a range of powers, a set of null lenses would be required to cover that full range. Fortunately, the vast majority of implanted IOLs fall into a range of a few diopters. For this limited range, only one or two sets of null lenses may be quite sufficient for improved optical testing. [ProA, Stds, PostMS]
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Key words: intraocular illuminators, cataract surgery, retinal injuries
OST scientists initiated studies in FY 95 to evaluate the risks of retinal photic injury from intraocular illuminators used during vitreo-retinal surgery. This study was completed in FY 96. In this work, optical radiation emission data were obtained from intraocular illuminators being marketed in the United States. Measurements showed that the output from the measured devices varied by as much as a factor of 50 from one unit to the next. Also, these devices emit unnecessary UV radiation, which is potentially hazardous to the retina, since the light source impinges directly on the retina without the safety advantage of filtration by the cornea or lens of the eye.
When the spectral output of these devices was weighted with a biological effectiveness function, the results clearly demonstrated that the risk of retinal photic injury from these devices is similar to, and for some instruments higher than, that from operating microscopes. Analysis of the results indicates that the risk of retinal photic injury may be significantly reduced with the addition of filters to reduce ultraviolet radiation and blue light emissions from these devices. OST is continuing work on this project to ensure that the addition of safety filters will not interfere with the physician's visualization during surgery.
In addition, OST scientists assumed a leadership role in the development of ANSI and ISO performance standards for the quality of the optical radiation emissions from operating microscopes and intraocular illuminators. The ANSI Z80.7 and ISO TC172 committees completed work on developing specifications for optical radiation safety for operating microscopes and initiated work to develop safety performance specifications for intraocular illuminators. [PostMS, ProA, Stds]
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Key words: Sjogren's syndrome, noninvasive optical diagnosis
In a joint project with NIH, OST scientists are working on the diagnosis of Sjogren's syndrome (SS), an autoimmune disorder primarily affecting peri- or postmenopausal women. It is estimated there are more then 1 million persons with SS in the United States. Excisional biopsies of the minor salivary glands in the lower lip are considered to be the best criterion for the diagnosis of this disease. Due to postoperative discomfort (swelling, pain), frequently observed temporary loss of sensation of the lower lip, and its destructive nature, excisional biopsy is not generally applicable to serial studies of disease progression in a given patient. Given the superficial location of the minor salivary glands (0.5- 3 mm from the mucosal surface) and their optical accessibility, the noninvasive optical diagnosis of these glands appears a good clinical model to test a quantitative imaging scheme.
Simulation studies were performed for diagnosis based on 3-D imaging of fluorescent molecular markers (e.g., fluorescinated antibodies) that specifically bind to the diseased tissue. Rhodamine targets embedded in a Delrin disk were covered by Delrin slabs of various thickness. Surface images were obtained for different target depths and radial distances from laser excitation to target fluorophore. These images were analyzed and compared to calculations based on random walk theory in turbid media, using previously determined scattering and absorption coefficients of the Delrin. Good agreement between experiments and theory was obtained demonstrating the first step of feasibility of the proposed concept for a quantitative clinical diagnostic imaging technique. [ProA]
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Key words: laser products, laser measurements, night vision aids
OST continues to assist the Office of Compliance (OC) in evaluating illuminators used with night vision equipment imported from the former Soviet Union. The testing was performed in order to determine whether the source of radiation in the equipment was an infrared light-emitting diode (LED) or a laser diode. Equipment containing a laser diode as the illuminator is subject to the Federal performance standard for laser products. Measurements were performed with a spectroradiometer system to determine if each instrument incorporated a laser diode or a light-emitting diode. Previous testing has indicated that instruments incorporating laser diodes have a spectral bandwidth of between 5 nm and 6 nm. Instruments with light-emitting diodes have a spectral bandwidth greater than 30 nm.
OST evaluated five instruments in the laboratory. All five instruments were found to incorporate infrared LEDs as the source of illumination and are not therefore subject to the Federal performance standard for laser products.
In addition, OST continued to support OC through the laboratory evaluation of the radiation output levels of laser products. The testing was performed in order to confirm the manufacturer's classification of the product. Products evaluated include a laser pointer, a distance ranging instrument, and two laser gun sights.
A light-sound device was also evaluated to determine the flash frequencies of the light-emitting diodes incorporated in the eyepiece. The concern with this device is the potential for the flash frequency of the light-emitting diodes to induce seizures in susceptible persons, many of whom were not aware that they were susceptible. It was found that the light-emitting diodes flash at a variety of frequencies depending on the compact disk used with the device. Frequencies ranged from 4.0 Hz to 33.333 Hz. The results of testing were forwarded to OC for follow up. [Enf, PostMS]