Posts Tagged ‘thermal images

Now I’ll be getting into a bit more detail on what kind of images a thermal camera produces and explain it to you how and why it is all a matter of interpreting that data and deciding how to show it visually…

How Does a Thermal Image Look Like
As I have already explained in my previous post the sensor of a thermal camera does not operate in the spectrum of visible light, but instead in a part of the infrared light and more specifically the Long-wavelength infrared region (LWI). This is the reason that these cameras do not record in color, though color is still used to make for a better and easier to read representation of the thermal data. Thermal cameras do record only temperature information for each pixel of the image they create instead of color information and then use software to turn that data in something visual and easy to understand and analyze. A special color palette is used with a different color mapped to a different temperature value and as a result you get a strangely looking image where different colors represent a different temperature. One of the most common such color palettes is called Iron and you can see a sample thermal image using this palette in the photo above, though this is not the only one. I’ll get more into the different representations of thermal images in a moment, but first I want to talk about something else as well…

Since thermal cameras do not record visual information, but only thermal one, other the resulting image may not be very detailed and it can even be hard to recognize multiple objects on the image if they have closer surface temperatures. This is especially true if you are using a lower resolution thermal imager like in the case of most affordable consumer oriented thermal cameras. So as a means to make things look better and the lower resolution thermal image to look more like a higher resolution one there is an interesting trick being used to help. Thermal camera manufacturers add a second camera that works in the visual light spectrum, just like the camera on your smartphone, that has a higher resolution than the thermal one and they combine the two images to produce a better looking thermal image as if it was recorded with a higher resolution.

On the image above you can see a monochrome representation on how a thermal image may look like, and next to it how a visual camera of the same thing (my hand in this example) can look like. These two images are recorded by the two different sensors on the same FLIR thermal camera with the visual image actually being double the resolution of the thermal sensor. Do note that even the monochrome representation of the thermal data on the left is just that, a representation of the thermal data in a visual way with false colors (black to white – colder to hotter), the same as when using different colors for different temperatures with other color palettes such as the Iron one mentioned already.

So what happens when we combine the thermal image with the visual one, normally a bit of a mess, though if we take only the contours of objects from the visual image and superimpose them on top of the thermal image we get more of what we need. We are essentially improving the visual detail of the thermal image making it easier to distinguish details and objects on the image without diluting too much the visual representation of the thermal information. Of course there are some other ways to help us combine the data of the two camera sensors in order to make things more useful for the person that will be analyzing the thermal information.

You can also get thermal fusion with opacity or even a picture in picture mode for combining the two different sets of information – visual and thermal. In the first image we are fusing the two images with the thermal one being with some opacity to show the visual image in the background with enough details. The second image just puts the two images in a picture in picture mode with the higher resolution visual image as a background and the lower resolution thermal one on top in the center. It is up to the user to decide which one of these different methods works best in each case and will be more useful for analyzing the thermal information when needed. If you have the original thermal image you can always generate any kind of the available visual representations, however that may not always be the case as you might just be looking at a report with some resulting photos and without the actual thermal data and tools available for you.

False Color Representations of Thermal Images
There is a number of commonly used color palettes that have been named specifically being used for representing the thermal information from a thermal camera in a visual way. You have already seen examples of the most commonly used color palettes – the Iron and the Grayscale ones, but there are some more you can also see being used less often. For example in the movie Predator that I have used as an example in my previous post a variation of the Rainbow color palette has been used to represent different temperatures in the thermal image. Aside from it you may see the Arctic or Lava color palettes used as well and even some custom combinations as it is not that hard to create your own color palette to use.

Here is an example that I have made with a custom color palette that essentially goes from black for colder areas to brighter green for hotter ones… and there you have it a green strange looking hand on an image that represents thermal data. The key thing that you need in being able to analyze the visual information that you see, regardless of the color palette used, is to have a color-temperature chart like the one on the right. Without such a color chart to tie the temperature to a certain color value it is really hard to do a proper analysis of the thermal information. Just imagine using the Grayscale color palette, but reversing it and using black for hotter elements and white for colder instead… if you don’t know that and assume it is a regular Grayscale representation you will be getting confusing results.


Iron color palette


Grayscale color palette


Grayscale REVERSE color palette


Arctic color palette


Lava color palette


Rainbow color palette


Rainbow High Contrast color palette

thermal-image-iron-palette

We have already talked about what are and how thermal cameras work, but now it is time to do a quick look at what do thermal images look like usually. Thermal cameras do not actually detect color as the infrared range they operate in is way beyond the range of the visible light, instead they record thermal information and that thermal information is then displayed in a way that we can visually interpret it in the form of an image. To make it easier for people to easily analyze a thermal image visually it is represented using a false colors representing the difference in temperature and the most commonly used color palette for that is the so called Iron one (shown above) where black is for the coldest areas, then blue and purple for slightly hotter areas, the mid-range of temperatures is usually red, orange and yellow and then going to white for the hottest parts. These false color visualizations usually do come with a small scale next to the image that show the colors used and what temperature range they cover as otherwise the person seeing a thermal image may get the wrong idea about the actual object temperature. It all depends on the temperature range that has been recorded, so black (the coldest part) on a thermal image can represent 0 degrees Celsius, 23.5 degrees C or another value and the same goes for the hottest and whitest part it could be 62.3 degrees Celsius or 200 degrees C. In thermal images using false colors to represent the difference in temperature there is no specific temperature representing specific color from the color palette used, the colors are just there to make it easy to distinguish the coldest from the hottest parts.

thermal-image-other-color-palettes

Another very common way is to represent a thermal image is in the form of a grayscale image, where you get only black to white colors passing through various levels of gray to represent the difference in temperature. This way of representing thermal images is often used in thermal security cameras or night vision thermal devices, but you will probably see it rarely used in other areas when thermal cameras are needed. The reason for that is, because it is harder for a normal person to distinguish the difference in temperature when only a single color is used and only the level of intensity is varied. It is much easier when you use a color palette with multiple colors. Aside from the most common Iron color palette that we’ve shown to you above here you can see some of the other often used false color representations used for thermal images. These are the Grayscale palette that we’ve already discussed as well as Arctic, Lava and Rainbow, and you may also find a versions of these with higher contrasting color palettes to make differences even more apparent. There of course could be thermal images using different color palettes as well, but as long as you have the smaller scale with the used colors and what temperatures they represent you should be able to quickly get an idea on what you are seeing in terms of temperature. Another interesting way of focusing the viewer’s attention to a specific area of the thermal image is to use grayscale thermal image with color only on specific areas that are either below or above certain temperature or if they fit in a specified thermal range.


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