How to brighten the sides of an image?

On Wed, 19 Aug 2009 14:46:17 -0700 (PDT), iratecat wrote:

Hello,

I have a series of images, taken with a microscope / camera system, that I am trying to convert to binary. The problem is, the microscope lights unevenly, so the center of each image is brighter than the sides, and this is throwing off the threshold. Is there a way to selectively brighten the sides of the image, while leaving the center alone? Color fidelity is not an issue, obviously, but contrast in edges is important. Simply increasing contrast just merges the entire sides into big black blobs.

I have access to versions 6.0 and CS, and maybe others (haven’t looked).

The operation is called flat-fielding and it is commonly discussed in astronomical groups. Photoshop will not do this operation directly, since accurate flat-fielding requires arithmetically dividing channel values.

I suggest you try using the free Gimp utility for this task, since it does support the all-important "divide" layer mode.

In addition to your normal image, take a reference image of a clear area of slide, de-focusing if necessary to remove significant detail from the reference image. Stack this as a layer on top of the image you want to fix, and set its layer mode to "divide". This will have several effects on the image – the field will be pure white, with no color cast, and the lighting will be perfectly even, with no vignetting or doming.

For increasing contrast, you may be content with Levels for a while. I suggest that you consider using curves instead of levels, since curves provide more precise control over the contrast of individual areas. For sharpening, unsharp mask is probably the preferred method. If the image is reasonably sharp to begin with, keep the radius at about .5, and the amount at 100 percent or more. Both of these operations may be done in either Photoshop or Gimp.

I can only make an educated guess why this important function is missing: during the early years of Photoshop, divides were very expensive computationally. During the mid to late 80’s per-pixel divide was considered to be a show stopper, spoken of in whispers in the image processing community.
—
Mike Russell – http://www.curvemeister.com

iratecat wrote:

Hello,

I have a series of images, taken with a microscope / camera system, that I am trying to convert to binary. The problem is, the microscope lights unevenly, so the center of each image is brighter than the sides, and this is throwing off the threshold. Is there a way to selectively brighten the sides of the image, while leaving the center alone? Color fidelity is not an issue, obviously, but contrast in edges is important. Simply increasing contrast just merges the entire sides into big black blobs.

I have access to versions 6.0 and CS, and maybe others (haven’t looked).

Thank you!

Have you tried the DODGE tool?
OR
Use the circular selection tool…Feather around 25 -50 pixels Select the bright area…..SELECT > Inverse
IMAGE > Adjust > Levels
Brighten the darker area to suit your taste.
Bob Williams

Bob Williams wrote:

Have you tried the DODGE tool?
OR
Use the circular selection tool…Feather around 25 -50 pixels Select the bright area…..SELECT > Inverse
IMAGE > Adjust > Levels
Brighten the darker area to suit your taste.
Bob Williams

It is answers like this that make me wish there were a THINK TOOL. Bob, it is likely the darkened area is a graduated darkness.

If this is similar to lens vignetting, then photoshop has a lens correction filter that compensates for this. You adjust the amount of correction and the portion of the image affected.

A much more complicated alternative, but maybe giving you better results, is to use a gradient mask. One way to make the mask is to take a picture through the microscope of a plain white field, and then invert the image so that black becomes white, etc. Therefore the center will be black and it will taper off to gray near the edges. Then create an adjustment layer on top of the image you want to fix, using the levels control, brightening the whole image so that the edges look as bright as the center of the original image. Apply the mask to the adjustment layer, which will cause variable brightening from center to edge. You might have to adjust the transparency of the mask.

"iratecat" wrote in message

Hello,

I have a series of images, taken with a microscope / camera system, that I am trying to convert to binary. The problem is, the microscope lights unevenly, so the center of each image is brighter than the sides, and this is throwing off the threshold. Is there a way to selectively brighten the sides of the image, while leaving the center alone? Color fidelity is not an issue, obviously, but contrast in edges is important. Simply increasing contrast just merges the entire sides into big black blobs.

I have access to versions 6.0 and CS, and maybe others (haven’t looked).

Thank you!

iratecat wrote:

Hello,

I have a series of images, taken with a microscope / camera system, that I am trying to convert to binary. The problem is, the microscope lights unevenly, so the center of each image is brighter than the sides, and this is throwing off the threshold. Is there a way to selectively brighten the sides of the image, while leaving the center alone? Color fidelity is not an issue, obviously, but contrast in edges is important. Simply increasing contrast just merges the entire sides into big black blobs.

I have access to versions 6.0 and CS, and maybe others (haven’t looked).

Thank you!

Easy.

Go into Quick mask mode and install a gradient mask with a radial gradient. You will have to guess exactly where to start it.

Then exit quick mask mode and use curves or levels to adjust the outer part.

Doug McDonald

I’ve tired using radial masks for this type of thing, but the problem I’ve found is that the light fall-off (at least when it’s due to vignetting) is not linear, so no matter where I start the mask I can’t quite adjust it to match my image. This is easily visible if the darkening is in a uniform area like the sky. Still, it could be a good-enough approximation of this purpose.

"Doug McDonald" wrote in message

iratecat wrote:

Hello,

I have a series of images, taken with a microscope / camera system, that I am trying to convert to binary. The problem is, the microscope lights unevenly, so the center of each image is brighter than the sides, and this is throwing off the threshold. Is there a way to selectively brighten the sides of the image, while leaving the center alone? Color fidelity is not an issue, obviously, but contrast in edges is important. Simply increasing contrast just merges the entire sides into big black blobs.

I have access to versions 6.0 and CS, and maybe others (haven’t looked).

Thank you!

Easy.

Go into Quick mask mode and install a gradient mask with a radial gradient.
You will have to guess exactly where to start it.

Then exit quick mask mode and use curves or levels to adjust the outer part.

Doug McDonald