Arrange that there's a bit of white paper somewhere in the frame - it can be outside the region that you're cropping so it doesn't show up in the final photo, but it ought to be lit about the same way as the ring, and if you can arrange it it would be good to have a shadow cast on it so you have a sample of dark-grey as well as of white - and then use the gimp/photoshop 'curves' tool to adjust the red/green/blue curves so that the white paper has red=green=blue at at least two points.
This doesn't work perfectly if your light is so dimly orange that there's no blue recorded at all, but it generally works quite well for me.
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Date: 2007-10-18 10:47 pm (UTC)This doesn't work perfectly if your light is so dimly orange that there's no blue recorded at all, but it generally works quite well for me.