I'm sure this question has been asked here a dozen times, but here we go again.....How do I prevent or remove these pesky lines? I'm trying to paint light rays onto a dark background, but no mater what I try I still end up with this effect. I believe it has something to do with resolution? How can I fix this - or better still avoid it in the first place? Thanks.

Banding often happens like that if you are working in INDEX mode or setting your resolution to 72DPI with a limited color palette. Are you working in RGB? Are you using WEB SAFE colors? What is the resolution and the image size? Save the brick! We'll help you! :) retrocity

The image is 2000 by 2500 pixels. The Resolution is 200 dpi. It is in RGB mode. I can't understand why this is happening.....reaches for the brick

PS - if it makes any difference I'm using colors from the Pantone Coated Pallette....

Yikes! That's very odd (or my brain isn't working...) What method are you using to create the "rays"?

It looks like you've got a gradient in this image. It's not uncommon to get banding in a large gradient. Use the noise filter on it. If you set it right, you can get rid of the banding without introducing any visible noise. - Ken Heronheart -

Thanks Heron - I'll give it a try. But no there is no gradient. Retrocity - I was creating the light ray effect using the airbrush on a new layer. I've since tried doing it on a solid color layer with much better results. However if I adjust the levels even slightly the bands will show, such as above. But even though I was creating them on a new layer, surely Photoshop is a powerful enough tool to be able to handle the transperancy. Could low computer memory be at fault?

Here's a larger view so you can see the extent of the interference. It was intended to be backlighting for a forest scene. First I used the fill tool in very dark green, and used the airbrush to brush long diagonal strokes over the top, in various shades, to give the impression of greenish light rays. Then motion blur was applied to help blend together. But the end results are pretty appauling. The motion blur only adds to the problem. But I still don't think this is normal; I also have a similar problem when using the lighting filter. Also, adjusting the levels feature adds further agravation. I tried using the noise filter as suggested above, but because of the severity of the pattern, I would have to use a lot of noise which looked worse than what we have here.

Try adding about 15%-20% Gaussian Monochromatic noise, and then a Gaussian Blur with a pixel radius equal to about 5%-10% of the shorter image dimension.