I should also mention that Stu is only describing CMYK backglasses. Those original glasses that were made of trillions of little "dots" - NOT solid colors. This is an area that no silkscreened backglass reproducers had trodden until we started experimenting with it starting 2-3 years ago.
It must be noted that a CMYK (photographic) backglasses cannot be redrawn. You cannot sit down and vectorize a CMYK backglass, doing black, blue, red, brown, orange, green, yellow, etc. They weren't made that way, and were usually based on watercolor or oil paintings as their master. So with no films available today, the only way to "remaster" new ones is literally lifting the image digitally off an original physical best-we-can-find NOS specimen. So the image used on the repro IS an actual scan of an NOS - RIP'd by a lab (Raster Image Processed) into new dots, and burned onto a new set of separations (films) by an imagesetter.
Those new dots *cannot* be pulled any finer than around 50% coarser than the "original" dot size. I cannot explain the science here, as it would be lengthy. Dots are not pixels ! So this has nothing to do with anything anybody would think of as something in Photoshop, working with pixels (resize, resample, dots per inch, etc). The labs work with lines-per-inch, halftones (dots), and angles. 100% totally different ballgame.
Stu does, when it applies, redraws certain layer elements on the CMYK backglass artwork - like the logos, sometimes text, score window frames, the lightblock backside, and of course the mirrored elements.
If you want to blow your mind with gobbley gook, check out these links:
http://www.csuchico.edu/~rpenne/screening/ScreeningBasics.html
http://graphicdesign.spokanefalls.edu/tutorials/tech/gray_tones/graytones.html
http://www.ekaprint.net/11screen.html
Now, all that said... not all backglasses we do are CMYK format.... SPOT COLOR backglasses are dead-on single-generation redraws, by art team members. Vectorized into brand new loss-less artwork, by the artist essentially tracing a giant comic book image (blackline + solid color fills). Spot Color glasses aren't made up of dots. They are a solid blackline, with 8-12 mixes of color inks individually printed to fill inside the blackline. Resulting in a solid image, where we have control over the tint of every single color. With CMYK, "purple" is a sandy array of magenta and cyan (and maybe some black) dots laying together in a certain array formation - determined by the lab - thus once we have the films, we cannot control the resulting "purple" color result on the glass. Working with Spot Color and CMYK are two totally different worlds. Thus, a Spot Color backglass is a much much truer rendition of an original, and sharp as a tack. Just look at our Solar Fire, Solar City, Evel Knievel, and Strikes & Spares glasses, for example.
I should also mention that color density of CMYK glasses appearing "darker" is resulting from the algorithm in the RIP when the films are made, due to pre-planning the "glow" of colors when the glasses are backlit. This is to prevent the appearance of our glasses being "washed out" when the lights come on. If it was a straight 1-1 translation from the NOS scan to the reproduction, the backglass (illuminated in the scanner with thousands of lumens of light) would be a bright "lit look" image on the final repro when you hold it in your hands... but then once backlit, you'd be introducing MORE light into the image, washing out the colors. So the algorithm involved takes into account the correct "glow" of the colors when light is introduced into the inks. So you gotta backlight the repro CMYK glasses. Get 'em in the backbox and light those suckers up! 
KEVIN
Classic Playfield Reproductions
http://www.classicplayfields.com
Post edited by KevinCPR : added material
West Point, UT
