Luminance Corrected LRGB
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Introduction:   When combining a luminance layer with RGB in Photoshop, the color is oftentimes subdued and desaturated.  Rob Gendler describes several approaches to minimize this problem, and I describe another approach here.   The inspiration for developing this technique comes from Paul Kanevsky's website, which describes a related approach, although I've modified it in important ways as described below.

Rationale and Method:  In the standard LRGB technique, desaturation occurs because the luminance layer is too bright, relative to the RGB signal.  Simply decreasing the opacity setting of the luminance layer in Photoshop to achieve a luminance "blend" results in greater color saturation, but at the expense of luminance detail (since the luminance contribution that we work so hard to obtain is diluted out by this method).   One solution to this problem is to maintain the luminance image at 100%, but modify the luminance image by 1) decreasing the overall intensity of luminance signal (which will improve color saturation), and by 2) selectively sparing luminance signal in areas of high color content (which will preserve luminance detail).  This new method adaptively changes the luminance image to better match the RGB layer, thereby improving color saturation.  This is why I refer to it as "Luminance Corrected LRGB"The technique is surprisingly simple and is described as follows:

The following directions apply to users of Photoshop:

1.  Obtain your best RGB image in the usual manner.  Although not necessary, it is preferable to use an HaR blend for the red channel when processing emission nebulae.  In some cases, it may also be helpful to apply a mild Guassian blur to the RGB image, prior to proceeding with the next steps (this depends upon how much noise is present in the RGB image).

2.  Duplicate the RGB image into another layer.  Desaturate this layer only (go to Image, Adjustment, Desaturate).  Name this layer "Lum correction".

3.  Now construct the color image as follows:  Luminance image (e.g., Ha or clear filter Luminance) in the BOTTOM layer; "Lum correction" image in the MIDDLE layer; and RGB image in the TOP layer.

4.  Assign "normal" blend mode to the luminance image layer (BOTTOM); assign "multiply" blend mode to the Lum correction layer (MIDDLE); assign "color" blend mode to the RGB layer (TOP).

5.  Adjust the opacity of the "Lum correction" layer until you achieve the proper balance between color saturation and image detail.   An opacity of about 30-40% is a good starting point, but you should experiment with this. 

6. Adjust the RGB layer
(TOP) using the Selective Color and/or Hue/Saturation tools (under the Layers menu in Photoshop) as needed to enrich color.

NOTE:  The technique as described above will permit the "multiply" layer to affect all parts of the underlying luminance layer equally.  This may be fine for some images, but not others.  For instance, an opacity setting of 40% may be optimal for one part of the image, and yet an opacity of 20% might be best for another part of the image.  To achieve greater control over the opacity level, try the following:

1.    Choose a reasonably high opacity setting for the multiply layer (like 30-50% for instance).
Create a layer mask for the multiply layer as follows:  Click on multiply layer so that it is active, then go to "Layer, Add Layer Mask, Reveal All".
4.    Choose the Brush tool, Hardness level of about 25-30%, and assign the color to black.
5.    Set the brush tool opacity (i.e., setting on the top menu bar) to about 50% to start with.  This will be changed depending upon your needs as noted below.
6.    Click on the newly formed layer mask (i.e., the white box created in step 2) to make it active. 
7.    Go to the image and use the paint brush to mask out areas as needed, in order to reduce the multiply effect in those locations.  Experiment with adjusting brush size, brush hardness, and brush opacity to reveal the best detail, depending upon the region.
8.    Repeat in other areas of the image as needed.

You will find that this technique permits a great deal of flexibility in constructing the final LRGB image.

Please try this "Luminance Corrected LRGB" method and e-mail me with feedback. 


All Images and Content  Copyright Steve Cannistra unless otherwise stated.

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