Introducing the Luminosity Blend Mode

Whether you produce your images for web or for print, at some point you’ll want to sharpen the images for output. Sharpening, as we’ve discussed in earlier tips, is a process in which the edges of detail are enhanced by making the dark side of the edge slightly darker, and the bright side slightly brighter – in effect, increasing contrast along the edge. However, in many cases, sharpening can introduce unwanted hue shifts as a result of the way the darkening and lightening affects the image. We can use Photoshop’s Luminosity blend mode to remove the color artifacts, leaving us with a cleaner result.

For example, take a look at this document, which has some noticeable color shifts along the edges, as a result of sharpening. We can use Luminosity blend mode to prevent this type of result:

 Before we start with the sharpening workflow, we’ll need a basic understanding of the Luminosity blend mode. Luminosity is one of the four tonal blend modes located near the bottom of the blend mode list. In its simplest terms, when you combine a layer in Luminosity blend mode with another layer, the resulting color keeps the original layer’s hue and saturation, but the luminosity value ONLY comes from the new blending layer. Let’s demonstrate with a quick example.

 We’ll start with a one-layer document, and add a gradient as shown below. The gradient transitions from a red value (RGB: 128, 0, 0) to a blue value (RGB: 0, 0, 128).

 If we examine the foreground and background colors with the color picker, notice the HSB values located directly above the RGB values. In both cases, these colors have a “B” (Brightness) value of 50%. This indicates that while the “H” (Hue) and “S” (Saturation) values change across the range of the gradient, the Brightness, or Luminosity, of the colors is constant – 50% throughout the image.

 Before proceeding, we want to change our foreground colors again – change the foreground to an RGB value of (180, 255, 82). Notice that the luminosity of this color, as evidenced by the “B” Brightness value, is 100%:

 Again before proceeding, let’s change the background color to an RGB value of (54, 77, 24). Notice that the luminosity of this color (the “B” Brightness value) is 30%. Also notice that the Hue and Saturation values are identical to the foreground color:

 Now add a new layer to the document, and fill it with the foreground color by pressing Alt-Backspace (Option-Delete on a Mac). With this layer still selected, we select the command Filter > Render > Fibers and accept the default values in the dialog box by clicking OK. We have this result:

The fibers are rendered in colors ranging from the foreground to the background colors, which differ ONLY in their Brightness, or luminosity value. Let’s bring blend modes into play. With the fibers layer selected, change the layer blend mode to “Luminosity:”

We can see that, although the layer with the fibers has a range of garish green colors, the Hue and Saturation of the greens are completely ignored, but the Luminosity, or Brightness, remains. The Hue and Saturation of the original bottom layer shows through, but its Brightness values are over-ridden with the Brightness of the fibers layer.

The key point to take away from this – it doesn’t matter at all what Hue or Saturation the fiber layer is – ONLY the Luminosity is retained when using this mode.

We’ll use this principle to great advantage next week as part of a sharpening workflow. Stick around!