Plexiglass used for flatening film

[daryl]

My 6x8 negs are not flat. So I lay them on the glass and place a piece of acrylic sheet over them. The acrylic is not laying right down on the neg. It is about 11x14, so it spans the whole surface of the scanner. The lid lays down nicely over it.
Anyway, my point is that it appears that I am still getting a good scan. Obviously I do not have the discerning eye that you people have and so I may be mistaken about the "good scan". What should I be looking for to determine if the acrylic is interfering with the scan? Do any of you use this method? Comments...

[The Crofter]

In terms of optical quality you would be better off using glass. Your local picture framer can supply you with 2mm float glass which will do the job. I have used this in the past and not noticed any newtonian effects but your mileage may vary.

[Costas L]

As long as the acrylic is between the light source and the negative, and not the negative and the capture CCD, its not likely to affect the quality of the scan. You just need to watch for dust and scratches on the acrylic.

Not sure how well acrylic behaves if you try and use ICE, will it absorb some of the red light source ? Pat's suggestion of 2mm float glass could be worth trying if there are problems in that area

[bez]

The only problem you may see (assuming the plastic is completely clean, clear & unscratched) is a circular pattern of light and dark bands in small patches, called Newton’s rings http://en.wikipedia.org/wiki/Newton%27s_rings but you’ll only get them if the plastic (or plain glass) is in contact with the film.
Anti-Newton glass has a very fine texture, usually on one side only, giving it a slightly diffuse appearance. You should be able to buy a small sheet from a specialist supplier, such as Better Scanning http://www.betterscanning.com/scanning/insert35.html ... to be placed on the lightsource side of the film as Costas says