The use of the telecentric beam path in ophthalmic optics to determine the corneal vertex distance

Purpose:

Nowadays, a video centering system is often used to determine centering data. Nevertheless, these are often additionally checked manually afterwards. An important parameter is the vertex distance (VD). This is often determined using a measuring ruler with a millimeter scale. Also during refraction a classical millimeter scale on the trial frame is used to determine the VD. Due to the difference between the location of the measuring scale used and the location of the real VD, there is a systematic measurement error.

Material and Methods:

At the Berlin University of Applied Sciences (BHT), a measurement tool was developed on the basis of an object-side telecentric beam path, which enables the determination of the real VD independent of distance. The validation of this measurement tool was performed in two different series of measurement. In the first series of tests, the measurements were performed on test subjects using a measuring tool, a video centering system (“visuReal” from Hoya) and a measuring ruler, and the results were compared. In the second series, VD measurements were taken using the measuring tool and millimeter scale on defined test sections for objectification purposes.

Results:

In the first series of measurements, the VD values obtained using the millimeter scale were lower than those determined with the other instruments (statistically significant at the 5% level with Friedman test). The difference between the VD values of the measuring tool and the video centering system, on the other hand, was small on average and not statistically significant. In the series of measurements on defined test sections, the measuring tool showed a significantly better approximation to the actual length of the defined test distance than the measuring ruler.

Conclusion:

The series of measurements show that the determination of the VD by the measuring tool and the video centering system are very similar on average and that the measured values of the measuring tool are closer to the original value than the measured values of the measuring ruler. Thus, the determination of the VD by means of a simple scale should be dispensed with in the future.