6.        Tear Film Lipimetry: OSDx® brings several firsts in lipid layer analysis of the pre-corneal tear film like analysis of the central corneal LLT distribution accurate to 10 nm, in the entire range of 20 to 200 nm, live at 10 frames per second with a histogram of thickness distribution and a colour coded map of the region analysed.

Thickness analysis of the tear film lipid layer is based on white light interferometry. Although it is called interferometry there is actually no interference pattern. Instead, the relative path difference of the component wavelengths of white light set up a colour pattern due to relative changes in velocity of the component colours as it enters from the air into the first layer of the tear film. This phenomenon is much like the colour patterns you see in soap bubbles. Tear film lipid layer usually ranges from 0 to 200 nm and in this range the colours do not repeat themselves so enabling a reliable thickness measure by colour comparison. Although it is technically possible to have a continuous colour comparison it is of little clinical value to measure down to the accuracy of a nanometer. Instead, OSDx® measures in bands of 10 nm in the range of 20 to 200 nm. OSDx® uses a patented technology to cast a uniform white light onto the central 5 – 6 mm of the cornea being examined (the exact size of the region of the cornea measured depends on the radius of curvature of the cornea i.e. K value). It is important to protect the projection cone of the device and prevent it from being damaged or blemished. Each projection cone is calibrated for uniformity and if you have any doubts you should send it back for re-calibration.

1.        Method: Clip on the magnetic projection cone on to the OSDx® device taking care to maintain a reasonably safe distance from the patient. Do not touch the inside of the cone; the outside has a metal casing for manipulating the cone. If there are blemishes on the inner surface of the cone you can try using a damp cloth to clean it out. Do not try to scrape or otherwise remove any blemishes and marks on the cone.
2.        Switch to Lipimetry mode and click on the start camera button to start the test. The device will switch on white light. Room lights do not interfere with the test and need not be switched on. However, care must be taken to ensure that there are no other Purkinje 1 reflections on the cornea except that created by the device.
3.        Keep the white illuminated circle on the centre of the cornea and centred in the Live Window, move forward and backward until the edge of the circular patch of light is sharp and well-focused. The live view window in lipimetry mode is divided by dotted lines into 9 zones – the Purkinje image should be kept inside the middle rectangle for data acquisition to start. Ask the patient to look into the central unilluminated (black) part of the lipimetry cone. If the patient blinks you will be able to see a shifting pattern of lipids. Even if you do not see it do not worry. By default, the area of analysis is auto-detected – when it is, a yellow ellipse appears indicating that data is ready to be acquired. Sometimes auto-detection of the Purkinje image fails, especially in deep-set eyes and corneas with scars. In such cases select the check box of Manual RoI and use the slider to adjust the yellow rectangle to a proper size to cover the Purkinje image (in the live video window). Do remember, however, that Manual RoI selection makes the lipid layer analysis more prone to errors (as explained in section 6.1.5).

4.        The device acquires data approximately every 50 ms (20 fps). One second worth of data is processed for quality and if acceptable figures will start appearing in the right (results) pane. The figures are: mean and SD (global) and Upper: Lower ratio. Also, a graph will be visible (histogram) showing the frequency distribution of the various thicknesses. OSDx® automatically stops acquiring during blinks and eye movements. The patient should be advised to look into the centre of the light being projected on his/her cornea (this area is not illuminated as it contains the camera window and makes it more comfortable for the patient). OSDx® automatically excludes lashes from analysis so patient does not need to open his/her eyes wide. In fact, opening the eyes wide will cause the lipid layer to be stretched thin. When the image and data stabilise check the Auto capture check box to capture data and end the test.

5.        Sometimes the analysis area is not detected automatically and you may need to define the analysis area manually. Check the “Manual RoI” check box. Remember that this method stops auto-exclusion of artefacts (like lashes) in the RoI (region of interest) although other quality checks are intact. This makes it more prone to error. User has to take the responsibility that patient’s lid or lashes do not enter the RoI and that the eye is centred. You can vary the dimensions of the RoI by dragging the track bar below the Manual RoI check box. Optimal values will be between 200 and 300 pixels.

6.        OSDx® requires at least 90% of the pixels in the area under the analysis (whether auto or manually detected) to be fit for analysis (i.e. within the colour range). If this is not achieved you will get a message below the live window ("Error acquiring data ... consider increasing gain"). If you are unable to get rid of this message by moving the RoI around on your screen you can try increasing the gain by dragging right the track bar labelled Gain. However, please note that this is strongly deprecated as it may interfere with results. The gain value is visible to the right of the gain track bar – the default value is 30.
7.        Once everything is set up and you are getting a consistent analysis area (yellow ellipse) – you can turn on auto-capture by checking the Auto capture check box. The next valid data set will be frozen and the camera will be stopped. To get audio cues from OSDx® make sure your sound is not disabled/muted.
8.        The data is now ready to be saved. Click the save button and select the other eye to start analysing the other eye. You cannot move to the other eye without saving the test. To delete the results of the current eye (for a re-measurement) click the Purge button and select “Clear unsaved data in this test”. You will lose all unsaved data from the current test for the current eye.