It would be best to ask FLIR this question as the minimum temperature span is not a common specification on data sheets. The reason is that minimum temperature span is not a very meaningful figure.
1. A Microbolometer can be so noisy or unstable as to make any minimum span below a certain value pointless.
2. A Microbolometer operated without temperature stabilisation is unlikely to provide decent imaging stability at very narrow temperature spans of, say, 2 K. Modern Microbolometer based cameras and cores can come in stabilised or unstabilised types and the cheaper units tend to be unstabilised. The FLIR Ex series being a good example.
3. Industrial and Science grade cameras and cores normally offer the required FPA temperature stability to provide very narrow temperature spans but such performance comes at a cost.
4. The camera cores noise specifications, both NETD and the observation based specifications provide a user with an understanding of the minimum resolvable temperature difference in a scene. Minimum span does not. As you may be aware, the span just sets the ‘window’ size through which the user observes the collected ADU’s. This enables the user display bit depth to be applied to best effect. If there is only noise to be seen at lower temperature differential scenes, then greater bit depth does not help.
High gain and Low gain ...... yes these do help produce better imagery in low thermal contrast scenes but there are limits, as already detailed above. Let us use a Fire fighting camera as a working example......
When a fire fighting camera is used to search in smoke for casualties the room ambient temperature can be close to that of the human body, so producing a low thermal contrast scene. The camera operates in its highest gain mode that can resolve the smallest temperature differential within its hardware limitations. If the camera is temperature stabilised, this high gain mode can be tuned by the manufacturer to provide excellent low Delta T resolving power. Non temperature stabilised cameras, whilst capable of operation in a high gain mode, cannot resolve such low Delta T scenes as well due to the instability in the FPA. The difference in performance of modern fire fighting cameras, stabilised Vs non-stabilised, may not be that great however.
When the fire fighting camera is aimed at a wall on which a fire is present on the other side, the high gain mode soon reaches the limit of its upper temperature boundary. This is often around 120C. At this point the camera switches to a medium gain mode that can cope with a greater temperature range of say 20C to 600C. That range provides less Delta T resolving power in low thermal contrast scenes, but such is not important at the higher temperatures involved with a fire ! The wall may be at 260C and the camera easily copes with the range of temperatures from floor to ceiling as hot gasses collect higher in the room. If direct viewing of the fire or heated metals is required, the camera switches to low gain mode in order to provide adequate scene temperature coverage. The camera may then offer a temperature coverage of 20C to 1000C but at a much reduced Delta T resolving power. Again, this is not important in this usage case.
So from the above you will see that most generic thermal cameras operate in their most sensitive ‘high gain’ mode in daily use but when a higher temperature scene is involved, normally above 120C, a lower gain mode is used to provide the required maximum temperature increase, at the cost of Delta T resolving power.
Some cameras will be set up to provide coverage from -10C up to 260C with just one range. These cameras are a compromise solution and configured for best general operation whilst not excelling at any particular application.
I am not willing to detail how Microbolometers are ‘configured’ for optimum performance in various usage scenarios but that information may be available via internet sources. Fine detail of ‘tuning’ a Microbolometer for best performance is likely a bit sensitive for a public forum.
FLIR will likely be able to fully answer your questions and recommend a camera/core for your needs.
Hope this helps
Fraser
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