*3.3.4.2 Non- invasive tear break-up time (NIBUT)*

Tear film stability is believed to be affected by various factors such as temperature, fluorescein dye, humidity, air circulation so NIBUT is more reliable than the other tests.

Method –


A cut-off of < 10 seconds is used to diagnose DED. The sensitivity and specificity of the NIBUT is reported to vary according to the technique used, with values of 82–84% and 76–94% respectively [53, 62, 63].

#### *3.3.4.3 Thermography*

When the tear film is evaporated it leaves the ocular surface cool [64]. Infrared thermography is used to measure the absolute temperature and the spatial and temporal changes in temperature during the inter-blink period. It can be used as an index of tear film stability.

Purslow and Wolffsohn reported that the ocular surface temperature measured by infrared thermography is related to the tear film [65]. The literature has given many evidences that indicates the cooling rate of the ocular surface is faster in individuals with DED than in normal eyes, which is assumed to be as a result of a greater rate of tear film evaporation [64, 66–68].

#### *3.3.4.4 Osmolarity variability*

Osmolarity in the patients of DED varies which in turn affects tear film stability. The inter-eye variability of osmolarity in patients of DED is greater than normal people [69–70]. As the severity of DED increases, this inter-eye difference of osmolarity also increases [71].

#### *3.3.4.5 Tear evaporation rate*

It is used as an indicator of tear film stability [72]. Lipid layer is necessary to prevent tear film evaporation. An absent and non-confluent lipid layer of tear film is thought to have association with a 4-fold increase in evaporation rate in normal patients and in patients of keratoconjunctivitis sicca, tear evaporation rate is thought to be increased by 2-fold [73, 74].

Method: Different techniques are used to measure tear film evaporation such as vapor pressure gradient and the velocity of increase in relative humidity (resistance hygrometry) [74–77].

#### *3.3.5 To test the tear evaporation*

Goto, Shimazaki et al. in 2002 reported the importance of evaluation of tear evaporation in dry eye assessment [78].

It is a non-invasive procedure and aim at assessing tear dynamics, differentiates the subcategories of DED and evaluating the treatment [79–84]. There are three methods for the measurement:

#### *3.3.5.1 The evaporimeter system*

The two humidity sensors are placed at different heights from the ocular surface and they are used to evaluate tear evaporation rates [79].

#### *3.3.5.2 The closed-chamber system*

At a given ambient humidity in a closed chamber, the velocity of the humidity increases and it is used to estimate the tear evaporation rate [80–82].

#### *3.3.5.3 The ventilated chamber system*

The evaporimeter consists of an eyecup in the form of a ventilated chamber which tightly covers the eye [85].
