DxO ClearView Plus technology in DxO PhotoLab
What is haze?
Haze is caused by the dispersion of light by particles in the air. Even on a clear day, the haze created by diffracted light is very visible on objects that are more than 2 miles away. This is called “dry haze.”
When haze is created by the diffusion of water particles, we refer to it as “mist” or, if visibility is reduced to less than half a mile, as “fog”.
Causes of haze
A portion of the light reflected by a subject into the lens (in this case, the light reflected by the building and represented by the yellow arrows in Figure A below) is lost because it is absorbed or scattered by particles in the air. In other words, the light is redirected elsewhere, as represented by the arrows pointing in different directions. As a result, the subject appears fainter.
Some of the light coming from the light source (in this case, the sun) is redirected towards the camera due to light being reflected by particles in the air, creating even more parasitic light in the photo (see the blue arrows in Figure A). This phenomenon makes dark areas appear lighter than they actually are.
How do you remove haze?
In theory, to remove haze, you would need to know the distance between the camera and each subject, the amount of diffracted light, and the properties of the light source.
However, it’s impossible to get all of this information from a single image! As a result, choices have to be made, starting with the assumption that if a photo does not have haze, it’s because it includes local dark areas (and, more specifically, because at least one of the RGB color channels is low).
The haze removal process must bring the R, G, and B values back to zero (to compensate for light diffusion) and amplify the signal in order to compensate for the loss of light from the subject.