72 PART IV Digital Documentation
Signal-to-Noise Ratio (SNR or S/N)
To acquire images with detectable content requires the signal from the sample
to be distinguished from the background noise. If you need a measure for the
detection quality or low light sensitivity of your camera, you can use the signalto
noise
ratio as a quality parameter. If your signal-to-noise ratio is larger than 1,
you will start getting a detectable signal, but if it is smaller or equal to 1, noise is
dominant and it will obstruct your signal. The higher the SNR, the better your
image will appear.
SNR becomes especially crucial when you work with low light samples. In this
case you cannot optimize your signal so you have to minimize the noise. Cameras
with minimized noise are usually more sophisticated and specialized and
therefore more expensive.
The maximum SNR of a camera is a measure of the so-called available dynamic
range. This describes the relation between the smallest detectable signal and the
brigthest portion in a single image that can be recorded by a digital camera system.
For example, if you look at a printed circuit board with a macroscope in a
quality analysis workflow, you want to check the quality of the solder spots in the
dark shadows next to an electronic component as well as the quality of the reflective
solder spot surface.
But how can you influence and reduce the noise?
✔✔Use purpose-built microscope cameras with good quality sensors.
✔✔Allow free airflow to your camera and do not heat it further by hot
surroundings.
Increasing the gain setting doesn’t improve the SNR. When you increase
the gain, both the signal and the noise will increase. As a result,
the SNR does not change. Gain only changes the contrast of
your image.
Frame Rate
Frame rate is the inverse of the time the camera needs to acquire and read out an
image. This depends on a number of factors:
✔✔ the readout and data transmission speed of the sensor and interface
technology