Imagine trying to watch a movie while a third of the theater is chatting away. Would you sit and enjoy the movie thinking, “Well at least most of the audience is quiet”? Of course not, it’s clearly noisy! But that’s essentially what one does when using RMS noise instead of 3-sigma to choose a laser measurement sensor. Wait, weren’t we talking about movies? Yes. No. Let me explain.
Noise is an intrinsically random phenomenon. It follows, therefore, that we cannot state an absolute noise threshold. Instead we get a Gaussian or bell-shaped distribution, which shows the probability of different values of noise. We could just state the average noise (μ), but for a wide bell, this could be far off from what actually happens much of the time. The root mean square (RMS) method is a standard which says that the noise will be within the range of ±σ 68% of the time, as can be seen in the graph below. This is equivalent to saying that the noise will be outisde that range 32% of the time.
Many laser measurement companies use RMS as the standard to set the stated minimum measureable power. It is true that at a power equal to the RMS noise, you will be able to measure something. You will be able to know that there is a signal. However, since the signal is the same level as the noise, you will not be able to measure in any meaningful way how much it is.
At Ophir, however, we don’t just give the customer enough information so that if he has an eye for detail and knows what to look for he’ll understand the product’s capabilities. We know that when someone is looking for an Ophir sensor, they expect the highest standards of quality and precision. As such, we set the minimum measurable power to 20 times the noise level. Furthermore, in calculating this, we use the 3 σ value for the noise where only 0.2% of the noise is outside that level. Thus the Ophir standard for minimum measurable power is 60 times as stringent as some other companies!
So, would you go to a movie where 32% of the people are talking? Probably not. What about 0.2%? Well it’s not perfect, but there’s always going to be that one guy that won’t shut up. If the world were perfect, there would be 100% accuracy without any noise. Until then, Ophir will continue to strive to give you specifications that you can rely on without reading the fine print.
You might also like to read: How to determine the lowest power your sensor can measure?