We’re back from the Axon Accelerate Conference. What an incredible experience to meet so many law enforcement professionals who are enthusiastic about going from Capture to the Courtroom with reliable tools based in science and fact, not tools repurposed from the art world.
I’d like to share today the answer to a question posed to us at the Conference. The question was, “how do you quickly get rid of that annoying orange color cast that you find in images / videos taken in underground locations or grow houses.”
The answer is the Temperature Tint filter (found in the Adjust filter group). But, before we look at the filter and how it works, let’s talk about about Colour Temperature.
The chart above is from my old book, Forensic Photoshop. It’s helpful to look at colour temperature from the standpoint of the Sun as it rises – the horizon going from warm to cool. Another way to look at colour temperature is with the chart below that places temperature (the Planckian locus in Kelvin) as it relates to the CIE XYZ Color Space.
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish/orange via yellow and more or less white to blueish white; it does not make sense to speak of the color temperature of, e.g., a green or a purple light. Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature. [source]
In our use case today, we’re working with mercury / sodium vapor lights that radiate a light that is orange (about 3000k).
Our task force officer wants to correct the overall colour and light values in the above image. Let’s see how it can be done in Amped FIVE with the Temperature Tint filter.
The Temperature Tint filter manually selects the color temperature and tint of the light source illuminating the scene. This filter allows to the user to manually change the Color Temperature (orange-blue) and Tint (purple-green) of the light source illuminating the scene. The selected color is mapped to 6500K white. Increasing the Color Temperature will make the image less blue and more orange. The overall image can be made brighter or darker by adjusting the Exposure Correction parameter; if this parameter is 0, the average luminance (XYZ Y) of the image remains unchanged. The default settings are shown below.
Correcting this image, I’ve used the settings shown below. The file displaying the orange cast, I’ve set the Color Temperature to 3000 K, bumped the Exposure up a half stop in increased the Tint Correction by 1.
These settings produce the results shown below. The orange is gone from the walls. We can now further tune the colour problems.
Here’s another image in need of correction. This one is from a tunnel.
Using the Temperature Tint filter, we adjust the settings for mitigating the orange cast.
Here’s the before and after look at the file. The concrete is back to grey and the orange cast is gone.
Now that you’ve finished correcting the image, generate the report. Our reports are second to none. They give you the confidence to go and testify about your work. Everything you need to answer questions about the tools you’ve used is right there for you. This is part of what I meant in my old article, Democratizing Forensic Science. You’ve used a valid tool in a reliable, repeatable, and reproducible way (ASTM 2825-12). The report will illustrate that for you. You just have to read it when asked.
There you have it. The Temperature Tint filter. A powerful filter in your Global Color and Light Correction tool kit. It’s just one of over 110 filters and tools to be found in Amped FIVE.