What this means is that your eye may be drawn to changes in achromatic signals in the colour palette and mistakenly consider them to be an incorrect temperature change. The intensity rises and falls along the gradient itself. The palette that comes closest to a greyscale, slightly monotonic decreasing palette is FLIR’s Yellow palette. Sadly, my own favourite palette is shows a slightly curvilinear function with some intensities rising before falling again. The other palettes however are not all the same as the greyscale. Clearly the greyscale gradient shows a monotonic decline in intensity, as you would expect for an achromatic response. See how the different palettes show varying degrees of linearity and non-linearity. different colours), the achromatic signals are what I am trying to highlight above. All palettes are compared in the lower right-hand graph to show how these different palettes translate “intensity” differently.Īlthough the human eye can certainly detect chromatic signals (i.e. All I did was import the images into ImageJ and drew a line down the gradient to obtain the “grey intensity” as a function of the colour palette. Inset line graphs depict the “intensity” of the gradient scale used in common units. The picture above shows the yellow crowned night heron depicted using 5 different palettes. I have purposely rescaled the image to range from 5 to 32C, which actually masks some of the potential colour differences but it also shows how the different palettes may obscure potential temperature differences. So, why all the fuss about palette choice? Let’s consider the same image as I’ve shown you above, but this time using 5 different palettes. This maximizes the potential contrast of temperatures within the visible colour palette without saturating or eliminating any potential temperature. Note how I have set the temperature range to go from 22 to 32C, where 22 is the lowest colour on the gradient and 32 is the highest. My own favourite palette is the Iron palette, as demonstrated in the following image of Yellow Crowned Night Heron from Galapagos: Although greyscale might not offer a large degree of contrast between similar temperatures, it is a simple, natural scale of intensity where black is the lowest intensity, white is the highest intensity and greys provide a gradient of intensities in between.įor presentation purposes and for publication purposes, however, a visually appealing colour might be appropriate (instead of greyscale). People generally do not believe me until i demonstrate it, but I often prefer to use the greyscale palette when imaging animals in the field for the simple reason that it is easier to focus on edges. Some palettes were designed specifically to create a large contrast in colour intensity across a narrow range of temperatures. Indeed, for some purposes this is entirely appropriate. Each palette ‘translates’ temperature into a specific colour, with cold colours at the bottom (typically darker colours) and warm colours at the top (typically lighter colours).Īt first glance it might be tempting to choose the funkiest colours, since it looks ‘pretty’ (this is a common reaction I get from people when I show them thermal images). There are a number of palettes that you might find available on your thermal imaging camera:įrom left to right, these palettes are described as: Greyscale, Iron, Rain, Glowbow and Yellow (terminology comes from FLIR™). This spectrum of colours is referred to as a palette in most graphical software. When presented online, then, you have a colourised rendition of temperature that is essence a filter of the real data mapped onto a pre-existing spectrum. However, images captured are eventually converted into a digital format that is easily visualized by others, typically a jpg, png, tiff or bmp file. Almost all thermal imaging devices actually provide a proper temperature estimate for each pixel on the screen and as the images are typically stored in proprietary format in 12 or 14 bit files, this is not a concern from a quantitative perspective. My reasons will be illuminated below, but suffice it to say it relates ultimately to contrast and detectability of different temperatures through the use of colour itself.įrom a computational perspective these concerns are not important. Why be concerned about which colour to use? Isn’t it just for presentation? It happens to be one of my pet peeves with thermal imaging and how people new to the field do not appreciate how a colour choice might enhance small differences in temperature and overemphasize something rather minor. This might sound like a strange thing to write about, the choosing of a colour palette for a thermal image.
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