Span/Level are settings that allow us to define a smaller group of temperature with-in the current Range that we are operating the imager in. There are a few ways to control the Span/Level. Manual (or locked span) or one of two Automatic options (continuous or on demand) depending on your imager. The Auto a feature that just about every imaging system has. Is that because the feature is that good or is it a selling point? “Come buy our camera, you just point and shoot no adjustments needed. The camera does all the work”. Manual span/level adjustments are now found on higher-end systems. Whereas back in the day (we have all heard this before) an auto function was found mainly on the higher end systems. Funny how that changes over time.
But as the title suggests, is the Auto function good or bad? To decide that issue, we need to understand just how the Auto function works in our imaging systems. The short simple version is that most of the cameras look at the entire FOV (Field of View) and finds the highest temperature (or detector output) and sets the upper span to that value. At the same time, the camera is also looking for the lowest value within the FOV and sets the lower span to that value. Giving us what the camera thinks is the best span/level for the FOV or image that we see on the screen. So, for a resolution of 320x240 that is 76,000 detector elements. The highest and lowest output from that grid is what is used. There are also some systems that have a user defined “area box” that the Auto function can be limited to, instead of the entire FOV.
Which presents a better image of the house? Which one has all of the colors of the pallet on the object of interest (which is the house) not the clouds and the trees? Auto does not always get you to where you need to be for the best image. But it can get you close, especially if you do not know where to begin in the first place.
If you are not careful, Auto can “hide” problems or make them very easy to overlook.
So the question is Auto a Friend or Foe? It can be both a very useful tool or a debilitating crutch. I would never recommend that running the camera in “Continuous Auto mode” is a good idea, but toggling between Auto and Manual modes (On-demand Auto if your system has it) is very useful. Auto to get close and Manual to fine-tune the image.
- Continuous Auto (auto-scale mode)
The camera continuously finds the hottest and coolest spots in the image and automatically adjusts the scale temperature limits to these values
- Manual (locked scale mode)
The scale temperature limits are locked by the user. Optionally the user can manually adjust the scale limits by adjusting the “level” and “span” values.
- On-demand Auto
In manual mode the user requests the camera make a one-time auto adjustment and then return to manual with the scale locked at these new values