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It has been discovered that ocular tissues have the absorption bands almost equal to the absorption bands of water, opaque to far IR.20,21 The emissivity of water is 1. On IR spectral above 3 µm, it behaves similar to a black body. For ocular tissues, the radiated spectrum falls between 1 µm and 30 µm, with a maximum emission of 9 µm under normal circumstances (32◦C). Hence, the spectrum emission of any ocular tissues is wholly absorbed by the tissues anterior to it.

The transmission between the cornea and tear film occurs differently. On the spectrum above 3 µm, water generally behaves like a black body. However, when the thickness of a water layer falls around 10 µm, the transmittance on spectrum 8 µm to 13 µm is determined to be around 30%; that value approximates to zero if the thickness value is 40 µm or above.22,23 Tear film is determined to have a thickness around 40 µm.24 The spectrum emitted by cornea is fully absorbed by tear film. Therefore, the OST measured in thermography is in fact the temperature of tear film.23,25,26

The above finding is verified by a recent study.27 In this study, tear film stability was determined to be correlated with OST. Parameters such as corneal curvature, depth of anterior chamber, or central corneal thickness have no significant effect on OST.27

In clinical thermography, if one views a curved anatomical surface at an angle beyond 90◦, the measured temperature will be 4◦C less or more, since emissivity varies with angle of viewing. The amount of thermal radiation of other sources reflected from an anatomical surface also increases when the angle of viewing is greater, resulting in greater error in measured temperature. The resultant error is negligible when the angle of view falls within π/4. In practice, the error incurred due to the above factors is negligible for the recording of OST, since the angle of viewing on an eye with respect to thermography is assumed within π/4. As a result, the recorded OST can be seen as the temperature of tear film.

8.3. The Acquisition of OST

In general, an ocular thermogram is an image stored either in gray-scale or RGB format. RGB thermogram usually provides better visual representation of temperatures for manual inspection, and thermogram of gray-scale is useful for quantitative analysis.

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The cornea-sclera boundary is indiscernible in ocular thermogram. Often in literature, the acquired OST refers to the temperature of a specific site or some defined region on anterior surface.14,28–32 The approaches adopted by various researchers to study OST can be classified into three categories: manual measures, semi-automated methods, and fully automated methods.

8.3.1. Manual Measures

Efron et al. measured surface temperature in the way as illustrated in Fig. 8.2(a).33 In total, 11 points are placed across the anterior surface, and the sixth point coincides with the estimated geometric center of the cornea. Horizontal temperature profile of OST was determined, expressed in terms of regression polynomial

T = ax2 + bx + c,

where a = 0.01◦C/mm2, b = 0.003◦C/mm, c = 0.01◦C. x is the distance between the point of interest and the geometric centre of cornea.

Recently Tan et al.34 developed a similar measure based on the method by Efron et al.33 They determined the horizontal temperature profile and the vertical profile. Twenty points were used, as depicted in Fig. 8.2(b).34 Purslow et al. devised a method that seems like an extended crosshair.35 As illustrated in Fig. 8.3(a), they put up a structure of 23 points over the anterior ocular surface and recorded the corresponding thermal data.

The above-mentioned methods measured OST by the number of disjointed points. There were two similar methods, however recorded five thermal data points of various sites along the horizontal meridian.

Fig. 8.2. Methodology in the acquisition of OST by (a) Efron et al.33 and (b) Tan et al.34

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Fig. 8.3. Methodology in the acquisition of OST by (a) Purslow et al.35 and (b) Galassi et al.36,37

Fig. 8.4. Methodology by Morgan et al.25,38,39

Nevertheless, these schemes were slightly dissimilar to one another. Besides, instead of using points, research groups such as Morgan et al.25,38,39 adopted squared boxes to acquire OST (as shown in Fig. 8.4).

8.3.2. Semi-Automated and Fully Automated

Acharya et al. developed a semi-automatic method to capture OST using a standard procedure.40 This algorithm located the circular cornea, but the user had to crop a rectangle on the region of interest manually. The temperature deviation along cornea (TDC) (illustrated in Fig. 8.5) was calculated in the work as:

TDC = f(x, y) − f(x, y + 1).

y

Snake algorithm, coupled with target-tracing function, was employed to achieve fully-automatic acquisition of OST,41,42 as shown in Fig. 8.6. It was developed by Tan et al., to enable localization of eye and cornea41 without

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Fig. 8.5. Horizontal temperature profile across cornea.

Fig. 8.6. The acquisition of OST by Tan et al.41,42

any manual intervention. The automated method derived the corneal radius and center based on the final snake points. The corneal center was equaled to the snake centroid. Corneal radius was obtained by a formula deduced on the basis of work in the past.43

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