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AboutHi,
Those still using MF3D viewers illuminated by six-inch F4T5 fluorescent tubes, might be interested in ordering a 5000K tube called the Toshiba FL4EX-N-H. It's a 4-Watt 6-Volt tube just like the many 3000K (Warm White), 4200K (Cool White) and 6500K (Daylight) tubes more readily available here in the States, but this tube is far more neutral.
This Toshiba 5000K tube is now my favorite for viewing MF3D transparencies, but Toshiba only sells it in Japan.
Here's the link to an eBay seller I've found who sells them, with 7-day shipping to the U.S.:
Super Bright F4T5 Fluorescent G5 Bi-Pin Base 4W 5000K Neutral White New!
Jon Hoggatt and I conducted some informal comparison tests using a single mounted view moving between two SaturnSlide viewers, to evaluate one pair of tubes at a time.
A 6500K (Daylight) tube took second place, exhibiting a noticeable but not annoying color cast that Jon describes as cyan, but which I would describe as closer to blue.
I had also brought a 5250K (Neutral) tube that's no longer manufactured. We agreed that it took 3rd place, with a slight green cast that doesn't "feel" as neutral as the 6500K tube, somehow.
A 4200K (Cool White) tube took 4th place, which we both found to have a fairly strong yellow-green cast, nowhere near as neutral as the 5250K tube.
A 3000K (Warm White, a.k.a. Soft White) tube was the worst of the bunch, with a strong red cast (designed to emulate incandescent lighting).
The 5000K Toshiba, when compared to our 2nd place 6500K tube, is startlingly better. Jon noticed how a faintly rust-colored patch in an otherwise grey wall of granite, suddenly became more distinctive with the 5000K tube.
All colors come to life with a more natural, uninhibited energy using 5000K illumination. Our eyes may quickly adapt to color casts, to some degree, but we cannot restore the dynamics of color available in the absence of a cast. Colors that directly contrast the prevailing color cast are attenuated the most, of course (magenta being muted most severely under a green color cast, for example), but even complimentary colors are affected adversely. Rapidly swapping back and forth between the 5000k tube and any other color temperature is very revealing.
For more info about the tubes we compared and some photos that illustrate the relative color casts, see the PDF at this link:
http://accessz.com/tools/Color_Temperature_Comparison_of_Five_F4T5_Fluorescent_Tubes.pdf
Mike Davis