Summary of Test Prints using Xerox 7750 and iGens (July 2006)
The Xerox 7750 is capable of giving almost the same quality prints as the iGen. However, it is not colour-calibrated. I have tested four 7750s and the colour cast ranges from too green to too red. The iGen does not have this problem. I suspect the 7750 can give accurate colour rendition but it would need to be calibrated regularly. The colour casts I have seen are actually a good indication that the 7750 can give accurate colour – the variations says that the colours can be adjusted. It would be an indication of a poor machine if the colour was the same each time -- and wrong.
iGen Results
The half-tone frequency and angles for the C, M, Y and K toners, change for each resolution setting (150, 175, 200). The half-tone dots for each toner are laid out in a grid on two axes at right angles, and along those axes the resolution is not the same, even for the same overal resolution setting. eg for the black toner at 175 linescreen, the horizontal resolution is 175, but the vertical resolution is 150.
Frequencies and Angles
(Screen for the iGen is half-tone dots; for the 7750 it is lines)
Colour.....iGen 175 resolution........iGen 150 Resolution...........Xerox 7750
Cyan..........150 @ 40º, 175 @ 130º.......125 @ 73º, 135 @ 163º............190 lines @ 0º
Magneta....175 @ 58º, 150 @ 148º.......125 @ 13º, 135 @ 103º ............135 lines @ 135º
Black.........150 @ 90º, 175 @ 0º............135 @ 47º, 135 @ 137º ............135 lines @ 45º
Yellow.......Screen is not discernable - appears as blobs for the iGen and 7750
Screen Problems at 150 resolution for colour
The screening became visible at 150 resolution because of an interaction between the C, M, Y and K screens. It is a similar problem to Moire patterns. What happens is that the screen frequencies that the iGen uses at 150 resolution interact to form visible lines at about 60 to the inch. Small regularly-spaced colour cells are formed by the C, M, Y and K screens. The effect was only obvious in the darker areas and was not visible when the image was magnified. This could be fixed if the iGen allowed user-selected line-screen frequencies; but the effect may then reappear in different areas of the image. This problem may not be correctable.
DO NOT use 150 linescreen for colour images.
150 vs 175 resolution for Grayscale Images
Grayscale images showed a slight improvement in gradation in the highlight areas at 150 screen versus 175 screen, with no other discernable image quality loss. I could only detect the improvement in highlight areas where, for the 150 screen image, the gradation was smoother with less contouring. The improvement is very small, and difficult to detect, but it was definitely there.
DO USE 150 linescreen for grayscale images, in preference to 175 linescreen.
Text Colour
For the iGen, 100% cyan text printed as about 90% cyan. For the 7750, coloured text printed as pure colour.
Low-Density Grays
The iGen appears not to be able to print densities less than about 3%, but it varied between the machines I tested. The clearest indication of this was the yellow gradient. The grayscale version of the yellow gradient cut out 80 mm from the 100% end on a 111 mm gradient. This equates to 28% yellow density. (A different iGen gave 13%.) A yellow level of 28% is equivalent to 3% grayscale (measured using the eyedropper on the original image), i.e. a level of 247. That means I should limit the range of levels when printing a B&W image in CMYK to no more than level 247.
Similarly, I checked the K-only gradient. It cut out 92 mm along a 96.5 mm gradient i.e. 4% density. Thus, to avoid the problems of highlights appearing, I should probably limit the white level to 5% density; ie level 242, say 245.
Low-Density Cyans
In the Blue Bird Cafe colour image (in CMYK, but actually a B&W image) there are highlights visible in the awning. These occur at low densities where yellow toner is primarily being used to achieve the low density required. (To retain the overall gray appearance, the yellow is balanced by magenta and cyan as you move away from the extreme highlight.) The highlights occur because cyan is introduced at higher densities and forms a boundary that becomes visible. Like the height contours on a map, if the cyan is introduced at a certain density (say 5%) it will form a contour where it first appears. And this contour is visible. This effect is only noticeable at low densities and occurs between where there is no cyan, and when the cyan is first introduced.
The highlight was not noticeable in the same area of the CMYK image converted to grayscale, which was covered by small half-tone dots.
This problem in the colour version of the image is not due to the low density (say less than 3%). It is caused by the CMYK mapping trying to achieve gentle gradation by introducing cyan. This causes a noticeable level jump – a contour – because cyan is significantly darker than yellow.
Converting to Grayscale using the iGen
Do not use the iGen to convert colour images to grayscale because it converts all gray densities above 90% to 100%. ie dark shadows end up as 100% solid black shadows with the accompanying sheen. I measured this figure from the K-only gradient. It appeared solid black for the first 9mm of a 96 mm black-to-white gradient. All densities between 100% and 91% ended up as 100% for the three iGens tested.
