I understand your question is about thermal images, but I will use some information of depth images to provide the explanation, so please bear with me.
There is a very good tutorial from StereoLabs that explains how depth information is visualized:
If you use the Image plugin, since the
depth data are published on topics of
type sensor_msgs/Image it differs from
a “normal” image in that the data is
encoded in 32-bit (floating point) not
8-bit.
The parameters are the same as for
Image, with three additions:
Normalize range: Since a floating
point image is not directly rendered,
it is converted to an 8-bit grayscale
image. Enabling this field means the
normalization range is automatically
calculated
Min value: If Normalize
range is unchecked, you can manually
set the minimum depth range in meters
Max value: If Normalize range is
unchecked, you can manually set the
maximum depth range in meters Manually
setting the normalization range is
useful if you know the maximum value
measured for the depth and you want to
keep the image scale static
Further in REP 118:
https://www.ros.org/reps/rep-0118.html
Depth images are published as sensor_msgs/Image encoded as 32-bit float. Each pixel is a depth (along the camera Z axis) in meters.
Moreover:
Alternatively, a device driver may publish depth images encoded as 16-bit unsigned integer, where each pixel is depth in millimeters. This differs from the standard units recommended in REP 103.
I believe the same is happening with your thermal images as they also subscribing to Image plugin, take a look at this example of thermal sensor: http://wiki.ros.org/optris_drivers
thermal_image_view (sensor_msgs/Image)
color encoded image to be displayed with e.g. image_view
visible_image_view (sensor_msgs/Image)
RGB image from visible channel (bi-spectral technology necessary, only PI200/PI230)
And corresponding parameters:
palette (int, default: 6)
coloring palette in the range of 1..12
paletteScaling (int, default: 2)
scaling method for color conversion (determination of temperature bounds for high contrast coloring, 1=manual, 2=min/max, 3=1sigma, 4=3sigma)
temperatureMin (int, default: 20)
minimum value of temperature range (if manual scaling method chosen)
temperatureMax (int, default: 40)
maximum value of temperature range (if manual scaling method chosen)
