# Advances in the Noninvasive Diagnosis of Dry Eye Disease
## 8 Interferometry  
The tear film lipid layer is chiefly composed of meibomian lipid, which is derived from the marginal reservoirs of the lids. The lipid layer plays a critical role in tear film stability for the maintenance of ocular surface health by preventing excessive evaporation of the aqueous layer. In obstructive or hyposecretory MGD, thinning of the lipid layer leads to excessive evaporation, causing evaporative-type DED [61]. When adequate light hits an oily layer, the result is the generation of an interferometric fringe. Furthermore, interference patterns are produced due to the phase difference between the light reflected from the lipid layer and the light that is reflected from the corneal epithelium [62; 63]. Interferences by thin films display different colors, depending on the thickness of the film, from a dark color, caused by a thinner film area, to a brighter color, caused by a thicker film area. Imaging-based ocular surface interferometry is a noninvasive technique that allows the measurement of lipid layer thickness (LLT) on a nanometer (nm) scale.  
In healthy eyes, LLT has been estimated and reported to be approximately 70 nm. Various methods utilizing interference patterns have been used to characterize this parameter. In 1968, [PERSON] used a gooseneck light [64], in 1980 [PERSON] et al. a bio differential microscope [65], Polaroid filters [66], monochromatic light [67], spectral discrimination [68],and a simple interferometer made of paper tool for lipid layer evaluation [69]. There are several interference imaging devices: the DR-1 tear interference camera (Kowa Co., Nagoya, Japan) [70], the LipiView II interferometer (TearScience Inc., Morrisville, NC, USA) [71], the LipiScanner 1.0 (Visual Optics, Chuncheon, Korea), an add-on to an existing slit lamp biomicroscope [72], the Oculus Keratograph 5M (Oculus, Arlington, WA, USA) [73], and the IDRA (Sbm Sistemi, Inc., Torino, Italy) [74; 75].  
The DR-1\(\alpha\) camera allows a qualitative analysis of the lipid layer. [PERSON] et al. proposed a grading system based on the first stable frames from the DR-1\(\alpha\) camera [76].  
[PERSON] et al. produced a computer-synthesized color chart of a human tear lipid interference image for the conversion from tear interference color information to tear lipid layer film thickness data [77]. By using the DR-1\(\alpha\) camera, it is possible to evaluate the kinetic spread and stability of the lipid layer. [PERSON] et al. evaluated the speed and pattern of lipid spread after eye opening and the stability of the lipid film after spread, and found that in patients with lipid layer deficiency, the lipid spread was slow and resulted in a vertically streaking and non-uniform pattern [78]. [PERSON] et al. demonstrated that the DR-1\(\alpha\) interferometer can measure the TMH as reliably as AS-OCT and that the interferometric TMH correlated with [PERSON]'s score. [79]. DR-1\(\alpha\) interferometry device can assist in the differential diagnosis of different subtypes of DED; indeed, [PERSON] et al., by comparing three interferometric patterns (pearl-like, Jupiter-like, or crystal-like), found a direct correlation with BUT, NIBUT and Schirmer test value [80].  
The LipiView II (LVII) interferometer can quantitatively measure the average LLT by analyzing the interferometric pattern of the tear film, a partial blink rate, and uses an infrared light source for imaging the MG. [PERSON] et al. showed that the average LLT value has a significantly positive correlation with age, OSDI, and Ocular Staining Score, and no correlation with the Schirmer test type I, BUT, or meiboscore [81]. By using LVII, [PERSON] demonstrated that younger patients with DED experienced more severe subjective symptoms, more incomplete blinks, and a thinner LLT [82].