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This post discusses some aspects of fading of dye based inks, and uses information posted in (http://www.nifty-stuff.com/forum/viewtopic.php?id=498).
It has been noted that when black inks fade that they often take on a red color cast. The attached image shows the spectra of several colors swatches that may help to explain why this happens.
The first set of data compares Formulabs dye and pigment black inks, showing several points of interest:
1. The pigment ink is darker across the entire spectrum, giving a darker color. The pigment ink is very effective at all wavelengths.
2. The dye ink loses effectiveness at the longer wavelengths (far reds), but this is at the far limits of our vision and may not even be seen.
3. The dye ink is also slightly less effective at the shorter wavelengths (violet/cyan/blue), and this gives a blue cast to the dye black.
The second set of data shows cyan and PC. For now, just look at the far right of the cyan spectrum - it is far better at blocking the reds than the black. For a test, a mixture of 2/3 black&1/3 cyan was painted and measured. The third set of data shows that the ability of the blend to block the reds is better than the black alone, but at the expense of reducing the blocking ability at the shorter wavelengths. I am not suggesting that cyan should be added to black ink, but there may be other dyes that could extend black's blocking ability further into the reds without reducing its effectiveness at the short wavelengths. The reason that this is important is because of what happens when the black fades. It is unknown whether an ink's blocking ability is uniformly reduced at all wavelengths as it fades or not, but (since we have no real information on this) let's assume that a faded ink acts like a diluted ink so that we can guess what might happen when the black fades.
The bottom set of data shows the spectra for black when the ink is at full strength and diluted to 50% and 25%. Just like we saw with the diluted inks in the previous post, the effect of dilution (and therefore probably fading) is that the transition points are shifted sideways so that (in this case) shorter wavelengths aren't affected and only light at the long wavelengths (reds) will be increased. Obviously, more red light reflecting from the paper will give a red color cast to the image.
Another color that is often mentioned as having fading problems is cyan. It was previously shown that PC has about the same color as a 25% mixture of cyan ink and 75% water, so comparing cyan and PC will give a good comparison of the spectra of cyan and diluted (faded) cyan. The cyan and PC have "Lab" color values and corresponding "HSB" values of:
L a b H S B
Cyan 39 -18 66 208 100 77
PC 65 -45 -53 197 100 98
Note that cyan's "Brightness" was greatly increased when the ink was diluted. When yellow and red were diluted, they didn't change very much in brightness, but became less saturated. Magenta changed in brightness from 75 to 87 and also became less saturated. On the other hand, the cyan lost almost all of its light blocking ability (77-->98 on brightness). It may not be so much that only cyan fades, but that we are more sensitive to a change in brightness than either saturation or hue. Another factor that may also be at play is that cyan inks are less likely to block UV than yellow (but about the same as magenta). Is it possible that yellow's ability to block the short wavelengths extends into the UV spectrum and that yellow may help to protect against UV fading by partially blocking the UV?
The same reasoning extends to "lighter" (photo) inks (PM and PC) and why they are also reported to fade faster than the "darker" inks (cyan and magenta). These photo inks were necessary to get a high quality print when only large drop sizes were available, but is it possible that a 1 or 2 pl drop size has made the photo inks unnecessary? This would reduce ink usage by 1/3 (http://www.nifty-stuff.com/forum/viewtopic.php?id=466) and increase the life of our prints at the same time. Has anyone done a direct comparison between one of the new 4 color (plus pigment black) 1 pl Canon printers with an i8500 or i9900 to see if the lack of photo inks is compensated for by the 1 pl drop size?
It has been noted that when black inks fade that they often take on a red color cast. The attached image shows the spectra of several colors swatches that may help to explain why this happens.
The first set of data compares Formulabs dye and pigment black inks, showing several points of interest:
1. The pigment ink is darker across the entire spectrum, giving a darker color. The pigment ink is very effective at all wavelengths.
2. The dye ink loses effectiveness at the longer wavelengths (far reds), but this is at the far limits of our vision and may not even be seen.
3. The dye ink is also slightly less effective at the shorter wavelengths (violet/cyan/blue), and this gives a blue cast to the dye black.
The second set of data shows cyan and PC. For now, just look at the far right of the cyan spectrum - it is far better at blocking the reds than the black. For a test, a mixture of 2/3 black&1/3 cyan was painted and measured. The third set of data shows that the ability of the blend to block the reds is better than the black alone, but at the expense of reducing the blocking ability at the shorter wavelengths. I am not suggesting that cyan should be added to black ink, but there may be other dyes that could extend black's blocking ability further into the reds without reducing its effectiveness at the short wavelengths. The reason that this is important is because of what happens when the black fades. It is unknown whether an ink's blocking ability is uniformly reduced at all wavelengths as it fades or not, but (since we have no real information on this) let's assume that a faded ink acts like a diluted ink so that we can guess what might happen when the black fades.
The bottom set of data shows the spectra for black when the ink is at full strength and diluted to 50% and 25%. Just like we saw with the diluted inks in the previous post, the effect of dilution (and therefore probably fading) is that the transition points are shifted sideways so that (in this case) shorter wavelengths aren't affected and only light at the long wavelengths (reds) will be increased. Obviously, more red light reflecting from the paper will give a red color cast to the image.
Another color that is often mentioned as having fading problems is cyan. It was previously shown that PC has about the same color as a 25% mixture of cyan ink and 75% water, so comparing cyan and PC will give a good comparison of the spectra of cyan and diluted (faded) cyan. The cyan and PC have "Lab" color values and corresponding "HSB" values of:
L a b H S B
Cyan 39 -18 66 208 100 77
PC 65 -45 -53 197 100 98
Note that cyan's "Brightness" was greatly increased when the ink was diluted. When yellow and red were diluted, they didn't change very much in brightness, but became less saturated. Magenta changed in brightness from 75 to 87 and also became less saturated. On the other hand, the cyan lost almost all of its light blocking ability (77-->98 on brightness). It may not be so much that only cyan fades, but that we are more sensitive to a change in brightness than either saturation or hue. Another factor that may also be at play is that cyan inks are less likely to block UV than yellow (but about the same as magenta). Is it possible that yellow's ability to block the short wavelengths extends into the UV spectrum and that yellow may help to protect against UV fading by partially blocking the UV?
The same reasoning extends to "lighter" (photo) inks (PM and PC) and why they are also reported to fade faster than the "darker" inks (cyan and magenta). These photo inks were necessary to get a high quality print when only large drop sizes were available, but is it possible that a 1 or 2 pl drop size has made the photo inks unnecessary? This would reduce ink usage by 1/3 (http://www.nifty-stuff.com/forum/viewtopic.php?id=466) and increase the life of our prints at the same time. Has anyone done a direct comparison between one of the new 4 color (plus pigment black) 1 pl Canon printers with an i8500 or i9900 to see if the lack of photo inks is compensated for by the 1 pl drop size?
