Flat Field / Flat Frame Correction

Often, an image will be illuminated unevenly, or it may be brighter in the center than the edges due to image vignetting. Vignetting is a natural consequence of lens geometry. Before an image can be quantitated, it is necessary to ensure that all parts of the image are illuminated evenly. This can be done with a flat frame (sometimes known as a ``neutral field correction''), which is obtained from an image of a featureless white or gray object taken under exactly the same conditions as your image. The program calculates a correction factor for each pixel, then applies it to your image. This guarantees that measurements taken from all parts of the image are comparable to each other. The array of correction factors can be stored on disk and reused.

Flat-frame correction differs from dark current subtraction in that the pixels are multiplied by some factor instead of subtracting a value.

The drawback of a flatframe correction is that it can reduce the pixel depth of your image. For example, if pixels at the edge of the image are only half as bright as those in the center, your effective pixel depth is reduced by one bit. Therefore, it is recommended always to use 16 bits/pixel or higher when quantitating images.

To make a flat frame correction, do the following steps:

1. Click on ``Flatframe filename'' and enter the name of the flat frame file. This is an array of numbers that you created previously that defines the correction factor for each pixel. If you haven't created this file yet, go to step 3.
2. Click Accept. The flatframe image is now in memory and can be used for any number of corrections.
3. Alternatively, if you do not have a flatframe correction file, load the flatframe image as a regular image and specify its image number in the clickbox labeled ``Image for flatframe''. Select ``Create from image'' and click Accept. Note that the flatframe image must be grayscale and must be the same size and pixel depth as the image to be flattened. Use Image...Crop, Color...Change Image Depth, and Color...Convert to Grayscale if necessary.
4. If you want to save your flatframe array for future use, click ``Flatframe file name'' and enter the desired filename. Then click ``Save to file'' and Accept.
5. To apply a flatframe correction to an image, click ``Image to change'' and specify the image number. Then click ``Apply to image'' and Accept. Note that the flatframe correction file must be in memory before this will work.