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P. King–Smith, B.A. Fink, R.M. Hill, J.J. Nichols, K.K. Nichols; Human Tear Film Breakup Studied by a New Imaging Interferometer: Preliminary Observations . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4400.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: A new imaging interferometer is described which provides novel information about the structure of tear film breakup and the mechanisms which cause it. Methods: The optical system is modified from Doane's interferometer (1989, Opt. Vis. Sci. 66, 383). Broad– and narrow–spectral band, infrared, video images of the pre–corneal tear film are recorded simultaneously. The broad–band image shows the lipid layer of the tear film. Full–thickness interference fringes (as in Newton's rings) are derived by dividing the narrow–band image by the broad–band image (King–Smith et al., 2004, The Ocular Surface 3, S80). The images also show any distortion of the tear film caused by tear film breakup. Video resolution is 656 x 494 and the area imaged is about 7.5 x 6 mm, with a frame rate of about 20/sec. Results: Two main types of 'breakup' (pronounced change in the images) are observed. First, the reflection may develop a grainy appearance whose mean intensity is similar to that of the smooth–surfaced tear film. This grainy appearance indicates exposure of the rough epithelial surface; the lipid layer of the tear film may become 'draped' over the glycocalyx on top to the microplicae of the epithelial surface. The contrast of the grain is weak initially, but becomes stronger, perhaps because the lipid layer drapes over epithelial cells at different depths. This type of breakup may be in the form of small spots, linear streaks, or extensive areas. The breakup can be correlated with observations of the lipid layer and full–thickness fringes. For example, spot and streak breakup can be correlated with features of the lipid layer before breakup. Further, an extensive area of inferior breakup can be seen to be a consequence of changes in full–thickness fringes, which may be related to tear flow driven by a surface tension gradient. A second type of breakup consists of irregular streaks, which are brighter than the first type and do not have their grainy appearance. These may correspond to mucus strands (e.g., Tabery, 2003, Eye, 17, 488). Conclusions: This interferometer can provide information about tear film breakup which has not previously been available. Breakup can be related to both the distribution of the lipid layer and to changes in the full–thickness fringes of the tear film. These studies should also help to elucidate the origin of the bimodal distribution of tear film thinning rates, determined using spectral oscillations (2004, Nichols et al., Ocular Surface, 3, S96). To the extent that tear film breakup is an important causative factor of dry eye conditions, the interferometer should provide information of clinical relevance.
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