When choosing a new instrument, we tend to end up looking at a specification sheet. But what do all of these numbers mean? Bigger numbers are better surely?
Whilst there are lots of specifications that we can compare, this is often meaningless without context. A prime example of this is when comparing lasers, which lie at the center of many of our instruments.
For example, the Zetasizer Advance series instruments are available with different laser powers – is more better? What does this actually mean?
How do I describe a laser?
There is much more to a laser than just how much energy it emits.
|Power||Typically measured in mW. How much optical power does the laser output. Often we will also be interested in the stability of the laser power.|
|Wavelength||Typically measured in nm. This effectively denotes the color of the laser light, but also relates to the frequency and energy of the electromagnetic wave. Wavelength is often quoted as a peak value, with a width.|
|Coherence||This describes how the propagation of the laser beam remains in phase. This dictates how well the laser light will interfere or mix.|
|Polarization||The orientation of the energy of the laser beam relative to its propagation. This could be very linear or over a range of angles. This will dictate how light is scattered or reflected by a material.|
|Beam divergence||How the laser beam propagates over time. This may vary with orientation.|
|Beam shape||Typically a “good” laser beam is a circular spot with a Gaussian profile but this may be elliptical, have a halo or other features.|
For different applications, these parameters will be of varying significance. For example, in illumination, coherence and polarization may not be relevant. Coherence is critical when laser light has interfered (mixed) for signal analysis. Looking at the list above, it is hopefully clear that more laser power is not necessarily better.
Often some of these parameters may be balanced or compete with each other. Some lasers include a feedback loop to stabilize their power output, but power may be stabilized by allowing the coherence to change with time. If our beam shape isn’t particularly great, then optical components may help us fix that, but in doing so we lose some laser power.
What does this mean in practice?
Lets look at the Zetasizer as an example of an instrument that uses a laser and see what these specifications mean.
The Zetasizer performs particle size measurements via a technique called dynamic light scattering (DLS). Here we can measure the size of very small particles measuring how laser light scattered by our sample changes over time, due to the motion of the particles in a liquid. A sensitive device called an Avalanche Photo Diode (APD) is used to detect the scattered light.
Lets look at this technology description and relate it back to laser specifications:
“very small particles” – we may benefit from a high laser power if our sample doesn’t scatter much light.
“scattered” – the polarization and beam shape need to be well defined.
“changes over time“ – we need the laser output to be very stable.
“due to the motion of the particles” – this means there will be some signal analysis, so we need a very coherent laser.
“in a liquid” – oh. This will scatter light too, as will the container. More light will also mean more background noise.
‘sensitive … APD“ – This device has an optimal intensity that it can detect so we might need to control how much light is on the sample.
Here we can see that too much laser power may actually be an issue. For most DLS measurements, the laser beam is actually attenuated.
If our laser has poor coherence and polarization, then we may improve our signal to noise by increasing laser power, but we also increase the background noise of the measurement.
Choosing the right balance
Going back to the Zetasizer Advance instruments, the higher laser power option will give some increased sensitivity, but only for very low concentration samples. Both use a helium-neon (HeNe) laser which is highly coherent and stable over time. More powerful diode lasers are available but not necessarily with other optical qualities.
If we compare data quality with a higher power laser with poorer coherence, we might actually see lower data quality. A convenient way to monitor this is the correlation function intercept. This is a measure of the signal to noise – the higher the intercept the higher the signal and lower the noise.
We can see in the example below that the correlation intercept for the Zetasizer Ultra Red is comparable between measurement angles and only drops slightly for very low concentrations of low scattering material. When using a much more powerful alternative laser the intercept and the data quality vary significantly between measurement angles and also drops off for low concentrations.
Be specification smart
If you do find yourself comparing technical specifications, please remember to ask yourself which specifications are appropriate to compare. This doesn’t just apply to scientific instruments but applies to all purchases. Numbers are only part of the picture and may not mean what they imply. If a vendor boasts about how much higher one of their specifications is, we might need to ask what they are compensating for.