One of the issues with automated projection perimeters is the fact that it is not currently possible to make the mechanical projector movement completely silent. That is why when this type of perimetry was introduced, False Positive errors became an important measurement of patient cooperation because if they were simply following the sound of the moving projector, they would incur a False Positive fixation error. Compared to the original projection units, the sound has been improved greatly, but can still be a factor in patients trying to outwit the field test.
We decided to compare the sound of the projectors using two of the newest units on the market, a Zeiss HFA Model 860 (the current top-of-the-line HFA) and the Optopol PTS 2000 projection perimeter. Here are the findings using a standard decibel meter with accuracy of within +/- 1.5 dB.
What is significant in terms of decibels? Experts such as ABD Engineering tell us that the human ear can barely perceive a decibel level change of 3dB. At 5dB, most listeners report a noticeable change. At 10dB, the average listener reports hearing “double” the sound.
So, when you look at the comparison above, even though the HFA measured the lower dB level at its quietest point, the range was approximately 11 dB whereas the range for the Optopol is a fraction of that, approximately 3 dB. This is significant because it’s all about what the patient may notice and what will take their attention off of the functional visual test and on to the sound. In other words, when one is in an environment in which similar sounds are presented, having one that is twice as loud may cause the patient to take notice.
To learn more about OPTOPOL perimeters, schedule a quick virtual demo below: