What to do with developed slide film cutoffs

and about

Digital Ice scratch repair

Uploaded 3/4/05

 Here is a little photographic demonstration on an interesting property of Ektachrome film. We all have those cut off ends of developed film laying around, left over from developing those hard earned astro photos. Films such as Elite chrome, or Provia are developed E6, and the unexposed frames are always jet black, letting little light through. But here's a nifty fact you may not have been aware of: These pieces of black cut off ends make very good deep infrared filters, that can be used to photograph daytime scenes an reveal the hidden beauty of a deep IR shot. They are actually black till about 800nm, then they become optically transparent. With the right web cam, or digital camera, you can take dramatic infrared images.

This article is broken up into two parts:

1. The infrared process and sample daytime images

2. How this technology is used in scanning your astro films in slide scanners that use Digital ICE for scratch removal.

The Infrared Process

LEFT: Using a Phillips TU Cam, this shows that the infrared remote from my television which transmits at 910nm can be easily picked up as a bluish white bright source. This is the difinitive test to see if your web cam or digital camera will work with IR imaging.

RIGHT: A jet black piece of Elite chrome 100 slide film that has been developed but unexposed allows nearly 100 percent of the 910nm light through, but no visible.

LEFT: When we take the camera outside, a typical scene toward the snow covered Colorado Plateau is seen with pines in the foreground seen here in normal visible wavelengths.

RIGHT: Through the film chip, the auto gain of the camera boosts up and yields this bizarre scene. The trees and plants are pure white as chlorophyll has a very high IR reflectivity. The clouds become more dramatic in contrast.

LEFT: Normal scene from my balcony looking over the pine trees toward the town of Payson.

RIGHT: Through the film chip, the scene looks like a snowy winter shot in far IR.

LEFT: Blades of green grass (I need to mow my lawn!) with a tree in the background.

RIGHT: In deep IR the grass is pure white and shows contrasts and details unseen in white light.

LEFT: Final image, a prickly pear cactus in my back yard sitting in the sun, normal spectrum.

RIGHT: Even the common cactus takes on a very strange and eerie appearance in deep IR.

How Digital ICE works

 About half the scanners today have whats called "Digital ICE" for removing scratches from negatives and slides scanned. When you put that astro photo into the scanner, every scratch will show up in the final image because of the high contrasts we stretch our images to when we process. Using ICE can optically remove the scratches WITHOUT degrading the fine detail of an image. But HOW it does this has always been a mystery to most people.

How it works.

A normal slide scan consists of three passes on the image, one for Red, one for Green and one for Blue. When ICE is activated, a fourth scan is done in infrared light. You'll see below why this works.

The LEFT image is a normal visible light shot with my Philips camera of a schmidt camera slide taken back when the sky was clear long ago. Behind it is a star chart printed out with my ink jet printer using Megastar. There are a few penciled in areas of interest on the chart as well.

RIGHT: In infrared using the film chip as a filter, something very strange happens to both the slide and star chart print ! The slide is now totally transparent and you would be able to easily see any scratches on the film without interference from the actual image. Digital ICE corrects ONLY those areas by an algorithm that lightens them to match the background of the image. But look though the side and you can see on my chart one of the penciled in regions showing where the Fox Fur nebula is located. The word "Cone" is at bottom too.

Where are the stars on the chart? They are invisible in infrared! Only the graphite pencil line notes and sketches I made are seen.

I hope you've enjoyed this write up, write me if you found it interesting.



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