The Shape and Size of Glare Around Bright Lights
It's unpolished. But I've got a polisher, and it will be interesting to see how well it works without polishing. I was quite surprised what a good image you get reflected from a matt metal surface, so maybe a window might work somewhat with not polishing.
Here's a metal surface reflecting my hand in thermal IR.
But, it's matt, with near-zero specular reflectivity.
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When looking at jet engines in IR, another factor besides glare comes into play.
The elongated hot exhaust plume itself is a source of IR radiation as well. So the black (or white) blob that you see when looking directly at the back of the jet will also be caused by the exhaust plume, which is a rather thick cloud of hot gas from that line of sight (looking directly into the end of the exhaust plume). The IR image from that hot gas will of course not rotate in the gimbal case.
The images showing rotating glare in IR and visible light can be a bit misleading in that sense, since they only show glare.
The images demonstrating glare from jet engines do show the typical spikes radiating out, which indeed do rotate with the optics, but the center of the blob does not since it is a thermal image of something physical (a hot exhaust cloud), not glare.
The elongated hot exhaust plume itself is a source of IR radiation as well. So the black (or white) blob that you see when looking directly at the back of the jet will also be caused by the exhaust plume, which is a rather thick cloud of hot gas from that line of sight (looking directly into the end of the exhaust plume). The IR image from that hot gas will of course not rotate in the gimbal case.
The images showing rotating glare in IR and visible light can be a bit misleading in that sense, since they only show glare.
The images demonstrating glare from jet engines do show the typical spikes radiating out, which indeed do rotate with the optics, but the center of the blob does not since it is a thermal image of something physical (a hot exhaust cloud), not glare.
Can you show an example of what you mean?
There were some questions related to exposure and contrast in this thread so I thought I'd look into what the ATFLIR in particular might be capable of in that regard.
ATFLIR related patents refer to the "time of each exposure, typically measured in milliseconds", but there's no mention of exposure in this ATFLIR technical manual, only gain and level settings. The manual says:
Both exposure and gain/level could increase the brightness of the image but for example this document about selecting IR cameras says that there are pros and cons, that "the shorter the exposure time, the less likely there will be blurring for high-speed events" and that "some cameras have better thermal resolution and are therefore more sensitive. We can deduce that it requires less exposure time for the higher-sensitivity cameras to get the same image", where thermal resolution is defined as the "smallest temperature change the IR camera can detect above its noise floor" and for example this presentation mentions that increasing the gain also amplifies the noise. We don't know what the thermal resolution is for the ATFLIR. This article says that cameras "with a cryocooler ... can result in significantly improved thermal sensitivity". This paper says "Cooling air is provided to the ATFLIR by the F/A-18", and the manual also mentions the "aircraft cooling air for the components of the ATFLIR pod". We can see a comparison of cooled and uncooled cameras here. These points provide a potential reason for why they might've chosen to use a low fixed exposure time, to keep motion blur low and perhaps a high enough sensitivity might've allowing them to only vary the gain, but it's very inconclusive.
Among others here, some VFX artists analyzing Gimbal also made the point that explosions in IR show changes in the effective dynamic range of the image. Many such explosions imaged with ATFLIR can be found by looking up VFA cruise videos. The following are just a few examples of what it looks like for the first frame or two of the explosion, and what it looks like after the camera immediately changes exposure/gain after that, with the background appearing to become much colder.
From here:
From here:
From here:
It appears that some information about the background from before the explosion is lost with a huge glare suddenly covering it, and then as the camera changes settings the size of the glare is drastically reduced and some of that background information becomes visible once again. But as the camera internally uses 14 bit values containing more information than just the 8 bit output we see, and if gain is an analog effect that could create some contrast where a purely digital effect couldn't, could these effects be due to just gain/level changes, or are exposure changes required for them ?
People have noticed that the clouds in Gimbal appear to have some contrast with some hotter regions which seem unusual given how cold the scene is supposed to be at that altitude, at night, with the clouds below blocking any radiation from the ground, and that this could be indicative of the thermal range being narrow in the image, perhaps because for example the gain (contrast) setting is higher than usual ?
ATFLIR related patents refer to the "time of each exposure, typically measured in milliseconds", but there's no mention of exposure in this ATFLIR technical manual, only gain and level settings. The manual says:
According to an article about the "Basics of Digital Camera Settings"The digital non-uniformity correction (DNUC) channels pixel-to-pixel non-uniformity and converts the analog signal to digital. The low-noise 14-bit DNUC receives the ATFLIR detector video output and applies level and gain corrections to each pixel. A fine level calibration takes less than one second.
...
Level is adjusted from 0 (least brightness) to 9 (highest brightness)
...
Gain is adjusted from 0 (least contrast) to 9 (highest contrast).
...
ALG pushbutton switch: ... When boxed, selects automatic level and gain controlled by the FLIR pod
The ATFLIR manual doesn't say which type of gain the pod uses, but as the gain is applied by the unit that converts analog to digital, and as it is intended to increase the contrast, it is most likely real gain. Still there's a question of whether the exposure time in ATFLIR is fixed and there's only the "automatic level and gain" algorithm that the manual mentions or if there's some undocumented auto exposure algorithm in addition to that.Gain can be before or after the analog-to-digital converter (ADC). However, it is important to note that gain after the ADC is not true gain, but rather digital gain. Digital gain uses a look up table to map the digital values to other values, losing some information in the process. ...
In general, gain should be used only after optimizing the exposure setting, and then only after exposure time is set to its maximum for a given frame rate.
Both exposure and gain/level could increase the brightness of the image but for example this document about selecting IR cameras says that there are pros and cons, that "the shorter the exposure time, the less likely there will be blurring for high-speed events" and that "some cameras have better thermal resolution and are therefore more sensitive. We can deduce that it requires less exposure time for the higher-sensitivity cameras to get the same image", where thermal resolution is defined as the "smallest temperature change the IR camera can detect above its noise floor" and for example this presentation mentions that increasing the gain also amplifies the noise. We don't know what the thermal resolution is for the ATFLIR. This article says that cameras "with a cryocooler ... can result in significantly improved thermal sensitivity". This paper says "Cooling air is provided to the ATFLIR by the F/A-18", and the manual also mentions the "aircraft cooling air for the components of the ATFLIR pod". We can see a comparison of cooled and uncooled cameras here. These points provide a potential reason for why they might've chosen to use a low fixed exposure time, to keep motion blur low and perhaps a high enough sensitivity might've allowing them to only vary the gain, but it's very inconclusive.
Among others here, some VFX artists analyzing Gimbal also made the point that explosions in IR show changes in the effective dynamic range of the image. Many such explosions imaged with ATFLIR can be found by looking up VFA cruise videos. The following are just a few examples of what it looks like for the first frame or two of the explosion, and what it looks like after the camera immediately changes exposure/gain after that, with the background appearing to become much colder.
From here:
From here:
From here:
It appears that some information about the background from before the explosion is lost with a huge glare suddenly covering it, and then as the camera changes settings the size of the glare is drastically reduced and some of that background information becomes visible once again. But as the camera internally uses 14 bit values containing more information than just the 8 bit output we see, and if gain is an analog effect that could create some contrast where a purely digital effect couldn't, could these effects be due to just gain/level changes, or are exposure changes required for them ?
People have noticed that the clouds in Gimbal appear to have some contrast with some hotter regions which seem unusual given how cold the scene is supposed to be at that altitude, at night, with the clouds below blocking any radiation from the ground, and that this could be indicative of the thermal range being narrow in the image, perhaps because for example the gain (contrast) setting is higher than usual ?
Last edited:
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