Infrared photography with 8800?

Started Jan 8, 2005 | Discussions thread
Zayla New Member • Posts: 16
Re: Another 8400 Ir image

JensR wrote:

May I ask where you got the glass from?

Jens,

I obtained that RG715 filter (and a similar piece of RG780 glass which I have not yet used) about 30 years ago when conducting spectrographic work in a laboratory using rather slow 10 by 25cm glass plates (Kodak 1N?). So I cannot help with its current availability.

I posted the last infrared image because it showed that disturbing off centre green patch. There was some very bright cloud above the image area which might have been responsible. The filter also has some very slight surface etching which I cannot remove by polishing, so that might contribute too.

It is interesting to compare the histograms of the RGB components of the infrared images taken with auto white balance and white balance preset on foliage. The latter often needs an exposure of 2 seconds at about f/4.5, and its RGB histograms are very similar in shape and levels. With auto white balance (or set on daylight), at typical exposures of 1/2 second at f/4.5 to prevent saturation of the Red channel, the Blue and Green histogram curves are much lower in level and maximum luminosity, as would be expected for the small amount of visible light transmitted by the filter. But they are also of quite different shape from the Red component, showing that they respond not only to a little of the infrared radiation, but also to some of the visible. There is a gain both in sensitivity (of about 2 stops) and in discrimination of foliage in using the daylight white balance. I am experimenting with making new colour images from the red channel of the infrared image with the green and red channels of the normal colour image shifted to blue and green, so that foliage discrimination shows clearly as false colours. I do not have anything worth showing yet, but do see intriguing colour fringes on the moving clouds!

Zayla

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