I have discussed the issue of scanner re-reflected light when scanning reflective media and am interested in how much of this occurs in different manufacturer's scanners. Should anyone test their scanners with the included image, could they please report model and results?
Briefly, re-reflected light is from the illumination of nearby areas that reflects back onto nearby structures in the scanner and back onto the media being scanned. When the surrounding colors are dark, little light is reflected back but when the surrounding colors are highly luminous the media is illuminated by the sum of both the original illuminant and the re-reflected light from nearby areas.
This causes, in extremis, a large variation in the scanned colors. For instance a scan may indicate a L* value of 88 on color that is surrounded by white but a value of 82 when that same color is surrounded by black. And this can't be corrected by a profile as it's a response baked into the scanner design.
In order to provide a consistent technique to quantifying the re-reflection error magnitude with different scanner I have made a image with a pair of nine circles, decreasing in size from 2" diameter to .125" diameter. Each decrease reduces the area in the circle by half and so the diamter of each circle is reduced approx. 29% from the prior circle.
One set of circles is surrounded by "white" while the other set of circles is surrounded by black. Each circle is printed with a slightly gray white using RGB values of 220. The 2" diameter circles are marked with a "1"
This was printed and scanned with my two scanners. One, an Epson V850 and the other is an HP M477fwd scanner/printer combo. The latter scans flat or with a sheet feeder but isn't as consistent or repeatable as the Epson. However, they both exhibit re-reflection errors.
To use the chart, print it at 100% on 8.5x11 US letter size on photo paper. Doesn't matter if glossy, semigloss or matte. You will get similar results on any paper. You can eve print it with a laser printer on regular white paper as the effect is pretty large. Print it using Perc. or let the printer manage color. Again, doesn't make much difference. But don't use Abs. You want the "white" to be unprinted and Abs. will not do that.
Then scan it using the scanner's internal default profile or a custom one if you wish. That also don't matter much other than shifting the luminance a bit. Scan at 200DPI or higher.
Photoshop CC technique to read the L* values:
Load the scanned image in Photoshop. Set the info display to 32 bits and the displayed colorspace to Lab. This will show fractional values of L*. Scans are noisy, so the next thing is to set the eyedropper to average 31x31 pixels. This will give good averages and make fractional measurements of L* useful.
Now measure the center L* of the first 7 circles in both the white surround and black surround. For the last two circles change the eyedropper averaging range to 5x5 and make sure you take a measurement in the centers.
Here's the result on my two scanners showing the variation in L* with circle diamter in the black and white surrounds. The 2" circles are the left side of each chart line.
The blue lines are after re-reflected light has been removed.The top yellow lines are from the HP office scanner. The orange lines are the V850 using the built-in ICC which scans to Adobe RGB. The blue lines are from a custom profile after running the scanned tif file through scannerreflfix.exe which models then subtracts, the re-reflected light. It is attached to an Argyll profile set to Abs. Col.
The actual, spectrophotometer measured L* of all the light gray circles is 86.4 which is what my printer printed RGB 240 at. The important thing is the up ramp change on the white surround circles and the down ramp on the black surround circles.
The most significant metric is the dL* between the 9'th patch with the black surround and white surround.
The V850 has more re-reflected light than the HP office scanner coming in with a dL* of 6.6 while the office scanner dL* is 4.5. However, the re-reflected fixup program (blue lines) does a decent job of correcting this. The most current version will process 8 bit and 16 bit scan tifs with arbitrary DPI settings.
However, note how lumpy the office (yellow lines) L* values are. The office scanner has more illumination variation across it's surface than the Epson. Also, the office scanner has narrower illumination channels. This means there is less re-reflected light with the office scanner. It also illuminates only from one side while the Epson illuminates from both sides.
Attached is a zip file containing the scanner circle re-reflectance test chart as well as my command line program
and dlls for removing reflected light. However, it's only applicable to the V850 and possibly the V800 which I understand has the same reflective scanning structure.
Program updated to retain embedded profiles and add a new option, the most significant one is the -A option which corrected reflected light in an image from Epson Scan that has been converted to Adobe RGB (1998). It's not quite as accurate as scans converting RAW (That is unmodified, no color management Epson Scan tiff files) since the profile conversion is done prior to reflection correction but it is reasonably good.