Peter- adjusting the RGB LEDs to eliminate the mask was what I was thinking would be easier also. It's a bit iterative, but I think can be done. I've ordered some 10W RGB diodes and three PWM LED drivers (with knob adjusters). We'll see how it goes.
What are you using for your diffuser plates?
Wow, 10w led's would be bright! (A lot of people are using LEDs for lighting reef aquariums. My fixtures use 3watt cree's, and you can't look at them when they're on full.) Make sure to have them properly heat-sinked.
A few years ago I bought a 4x4ft sheet of white Plexiglas. I still have some left, and that's what I use.
Cemil Purut, who is making custom LED enlarger lamps for sale here advised me to use 3/16" #2447 white acrylic, there are a few different translucencies available and this is the most. For the sake of thoroughness, Peter has suggested using non-glare P99 acrylic to mount the neg to.
I just mocked up a light source using the LuminousFilm panel I linked to earlier in this thread. It's an array of 49 LEDs over a total 5 ¼" square area. Placing the 3/16" acrylic 2 ½" above it resulted in very even illumination. The difference between the center and outer edges measured -4 points in photoshop, a simple matter to correct in the final stitched scan. Though I don't hold much hope for it regarding optimal color spectrum I will test that aspect shortly. The manufacturer says the panel reaches a CRI of about 80 and is between 4,000K and 5,000K. Exposure was 1/100' @f/8 ISO 100. Incidentally, this thing barely gets warm.
That's a nice shutter speed. You might try using a second diffusion panel, ideally with some space between the two.
A second similar diffuser will help, I think it's 40% transmission so should come close to 1/30th sec. at that point, given some space and drop off. I made the world's worst spectrometer tonight and had a look at this light. It was encouraging not seeing sharp bands but rather smooth blends, similar to but not entirely like an incandescent spectrum. The blues were less smooth and seemed deficient. I plan to build another spectrometer (though really, they should have a different name, the way I build them : P) such that it can be attached to a camera, I'll put up the spectrum when I have it. The encouraging part was trying my splectromodor on other light sources at home and seeing in CFL's and other bulbs what I didn't want to see from my DSLR scanner lamp, narrow clearly defined bands surrounded by gaps. I'm very interested to see what my Epson V700 transarency lamp shows as well. Here's a link to the goofy job I plan to construct tomorrow.
In case it's of any interest, here's a sample of a 5"x5" area of the lamp as described at this stage.
In case it's not clear to any folks who haven't had the time to read everything posted previously, my present intent is to move the light source along with the negative to sidestep stitching artifacts due to less than perfect illumination or variations in illumination, repeated over individual 'scans' from a fixed light source as Peter's invaluable empirical evidence has shown can be an issue.
Struan, thanks so much for the spectrometer info!
Note to any splectromodor builders: two pieces of blue painter's tape iis very nearly .2mm, a preferred slit width for DIY jobs.
It seems to me that an x-ray viewing box may be good for this...Also, the standard size if 14x17 "
I finally had a few minutes to take some readings. I used an X-rite 811 densitometer in Status M mode, which is for color negatives. Taking a reading through the un-exposed but processed base of 35mm Fuji 100 color negative film, gave RGB numbers of R.22, G.64, and B.86. I then sandwiched Rosco gels with the negative strip and took readings to find the most equal R, G, B numbers. The winner was #70 Nile Blue. Sandwiched with the Fuji film, it gave R0.92, G1.01, B1.02.
Hopefully, I'll have some time to run some scan and see if this equalization of the RGB numbers helps.