There are four filters in Photoshop that are designed to reduce noise (and one filter to add noise). One of the most useful of these filters is the Median filter. The Median filter looks at pixels, within a specified radius, and blends the luminance of the pixels. The filter does this by calculating the median value (a type of average) of the pixels within the radius and assigning that value to the center pixel. In a sense, it averages out the noise.
When working on an image to reduce noise, it is always best to duplicate the Background layer and perform the noise reduction work on the duplicated layer. This allows more control and leaves the Background layer untouched. Thus, the first step is to duplicate the Background layer by dragging the Background layer to the Create a new layer icon on the Layers palette (see Figure 3). The new layer is renamed the Median layer (see Figure 4).
With the Median layer selected, the Median filter is launched (choose Filter/Noise/Median). The Median dialog box appears (see Figure 5).
The Median dialogue box has only one setting: the radius. The radius must be set to a point where the noise is reduced, but the detail is maintained as much as possible. A radius setting of 4 works well for this image. Figure 6 shows a crop of the image after the Median filter was applied.
To further improve the image, it is necessary to apply the noise reduction that was just performed to the smooth areas (where the noise is the most noticeable) but not to the edges (where the noise reduction reduces detail). This can be done be creating a mask that protects the edges from the noise reduction.
The next step is to duplicate the Median layer by dragging the layer to the Create a new layer icon on the Layers palette. The new layer is renamed the Edge layer (see Figure 11).
The Edge layer is now sharpened with Unsharp Mask (choose Filter/Sharpen/Unsharp Mask). The purpose of this step is to sharpen the Edge layer so that it will be easier to detect the edges in the next step.
During this sharpening step, it is important to pay attention to the Threshold setting. The goal is to sharpen the detail without sharpening the noise. The Threshold setting is the key to this. The threshold should be set to a level that will prevent the minor tonal differences, which are caused by the noise, from being sharpened while allowing the more significant tonal differences, which are caused by the detail, to be sharpened. This will require some experimentation. For this image, the experimentation showed that a Threshold of 2 worked best.
Keep in mind that the purpose of this layer is simply to create a mask. This layer will not be part of the final image. Rather, before the procedure is done, this layer will be thrown away. So, don't spend too much time trying to create the perfect sharpening. The settings used for this image are shown in Figure 12.
With the Edge layer still selected, the Find Edges filter is run by choosing Filter/Stylize/Find Edges. The Find Edges filter will do just what its name implies. It will find the edges in the image. The image now appears as show in Figure 13.
Unfortunately, this is a challenging image. The problem is that some of the minor detail doesn't have much more tonal variation than the noise. This is shown in Figure 14.
Figure 19 show the Channel Mixer dialogue box settings used for this step. It can be seen that the Channel Mixer is bringing in most of the detail from the green channel, a little bit from the red channel, and very little from the blue channel (make sure that the Monochrome box is checked at the bottom of the dialogue box).
Figure 20 shows the results of the Channel Mixer. The Edge layer now has much less noise, but it still has the detail.
It is best to save this selection by choosing Select/Save Selection. The Save Selection dialogue box appears as seen in Figure 26.
Once the selection has been saved, the Edge layer is no longer needed. So, the layer is dragged down to the Delete layer icon at the bottom of the Layers palette.
The next steps are designed to add some of the detail back into the image. This is done by duplicating the Background layer by dragging the Background layer to the Create a new layer icon on the Layers palette. The Background Copy layer is renamed the Detail layer. The Detail layer is then moved above the Median layer. The Layers palette now looks as shown in Figure 28.
The Detail layer is a copy of the original image. The Median filter has not been applied to this layer. Thus, the detail (i.e., the edges) in this layer has not been affected by the Median filter. It is necessary to add the edges from this layer to the image without adding any information from the smooth areas. This is accomplished by using the selection that was just created to generate a mask for the layer (if the selection is no longer active, choose Select/Load Selection). With the Detail layer selected, choose Layer/Layer Mask/Reveal Selection.
The image is now ready for further editing, sharpening, and printing.
This technique is a great way to remove luminance noise. However, it does not handle color noise very well. For that, one more step needs to be added.