Inkjet Printer Hints

By Donald Gudehus                            

Inkjet Printing:


The availability of low cost inkjet printers over the past few years has made it possible for amateur photographers and computer enthusiasts, as well as professionals, to print their digital images, created from scanned prints, digital camera exposures, and rendering software. Whatever the source of the digital files, it is important to keep in mind certain basic facts which relate to printable area, print quality, and printer quirks.

Print Size

The maximum useful printable area, measured in some linear unit such as inches, will normally depend on the number of pixels available in the file, and the desired viewing distance. The actual size of the print is determined by the "pixels per inch" specifier which can be set and stored in some cases, by the program used to print the file, e.g., Photoshop. Too few pixels in the file result in the print appearing pixelated, i.e., the pixels will be visible and the image will not have very much sharpness. On the other hand, too many pixels in the file, although not decreasing or increasing the apparent sharpness of the print, means that your files are unnecessarily large and that disk storage space will get used up faster. Also, larger files take longer for the printing application to open. It should be clear then that there is some optimum file size to strive for when making prints. Because one normally views a larger print at a greater distance (think of how one stands farther back when viewing larger paintings in a museum), it might seem at first thought that once the optimum file size is determined, one could just print any size print with that single file. This is not so for the reason that viewing distance and print size are not exactly proportional to each other. The actual relation is nonlinear in the sense that a doubling of print size is matched by a viewing distance which is less than twice what it was previously. Hence more pixels will be required for larger prints.

Apart from the general statements made so far, the actual required value of pixels per inch will also depend on the resolution capabilities of your printer. Many Epson printers are quoted to have a resolution of 1440x720 dots per inch. While this resolution can be achieved when printing a pattern which consists of dots that are fully black or fully white, in general the dithering which is required to display a range of intensity levels and a range of color, reduces the resolution by several times. Besides the printer resolution, the resolution of the human eye plays a role. The smallest angle most people can resolve is about one arc minute. At a typical viewing distance of 10 inches, this corresponds to a linear size of 0.0029 inches or 4.2 Epson dots in the X direction and 2.1 dots in the Y direction. With dithering included then, the human eye should just be at the limit of being able to see a slight trace of pixelation in a print. In order that the resolution of a print not be degraded from this best available level, the size of a pixel in the file should not span more than about 0.0029 inches, which correspond to a pixels per inch setting of 344 in the printing software. Therefore, a 4x6 inch print requires a file size of at least 1376x2064 pixels, a size compatible with the output of modern digital cameras. An 8x10 inch print is best held at a distance greater than 10 inches. The empirical relation d = 5.0 + 1.34W -0.016W2, where d the distance and W the width are in inches, gives a viewing distance of 14.70 inches for an 8-inch width . From this follows the linear size of one arc minute as 0.0043 inches, a pixels per inch setting of 234, and a minimum file size of 1871x2339 pixels. Please note that the above empirical formula is only valid up to a print width of about 30 or 40 inches.

Print Quality

Achieving a acceptable print depends on subjective and interpretive factors such as composition and subject matter of the original scene, and objective factors such as color balance, brightness, contrast, and distortion. In a 3D print, made with lenticular material, the amount of parallax in the foreground, key subject area, and background are additional considerations. Here, we will only consider some of the objective factors.

In making adjustments to your images, use a high quality monitor which has either a flat screen or a cylindrical curvature (e.g. Trinitron CRT). If the screen does not have an antireflection coating, be sure that there are no obvious specular reflections from room lights or windows visible on the screen. In fact, because diffuse reflections from room and other ambient sources will almost always be present to some degree, it is best to keep your shades drawn and/or room lights off. That way the blackest levels in your images will appear closest to their expected brightness levels (zero monitor output). If your files are to be viewed mainly on a particular operating system, be sure that the gamma of the monitor is set to the standard for that system, e.g., 1.8 for Macintosh, 2.2 for IBM architecture. Check your setup by displaying a known good file on your monitor to see if it appears satisfactory, or better yet use this grayscale/colorscale image to make more precise adjustments. If your file or the grayscale/colorscale image does not appear right, adjust the brightness and contrast of the monitor until the best overall results are obtained. Because the grayscale/colorscale image was created with a uniformly increasing level (from 0 to 255), its image on your monitor should gradually and linearly lighten from left to right. The grayscale/colorscale image can also be used to check whether colors are being displayed in their true hues on your monitor. Most monitors will either have an onscreen color temperature control or three potentiometers to make this adjustment.

Once you feel confident that your monitor is displaying your images accurately, you can make editing adjustments to the brightness and contrast of your file. Both brightness and contrast can be adjusted simultaneously by using the "curves" feature in Photoshop (Macintosh and Windos) and gimp (Unix and Linux). While separate "brightness" and "contrast" controls exist in Photoshop and gimp, the "curves" tool gives one many more degrees of freedom in adjusting the relative intensity values. When making this adjustment, try to achieve an output range which spans the available pixel values, i.e., 0 to 255. However, bear in mind that shadows are hardly ever close to a level of 0, and that some scenes may not have their brightest values near 255. You may find that dark areas of your frame that have had their pixel intensity values increased (say by a greater slope in the curves tool, or by increasing the contrast) now appear to be more noisey when viewed on the computer monitor. Usually this is only a concern for the perfectionist. One way to remedy the increased noise is to apply a gaussian blur filter, but since this could blur important information, the following approach is recommended. First be sure that the original file has a much larger number of pixels than will be needed in the final version. That is, scan your slide or print with a higher resolution setting, or set your digital camera to have a much higher number of pixels. After having adjusted the constrast to your satisfaction, apply the gaussian blur and follow this with a reduction in the image size. The noisy areas should now be much less noisy. Some working values to use here are an increase in pixel dimensions by a factor of two in each direction, a gaussian blur radius of about 0.7 pixels, and in the last step, a reduction in pixel dimensions by a factor of two.

Color balance is affected by the color of the ambient light at the time the original photo was exposed, and the relative color sensitivity of the photographic emulsion or detector. An easy way to correct the color balance is to identify a gray area in the scene, or some patch of known color, and adjust the relative color values in a program such as Photoshop or gimp so that the desired values of R, G and B are attained. For a true gray area, R, G, and B would be equal.

Distortion, especially barrel distortion, is common in digital cameras and wide-angle lenses. The plugins, Panorama Tools for Photoshop, and Wideangle by David Hodson for gimp, are good choices for correcting for this. This problem is most noticeable when you are photographing rectangularly bounded areas, such as paintings.

The whole purpose of this process is to print your file, however when it comes time to send your file to your inkjet printer, you may find that the print is too dark, too light, has the wrong contrast, or has the wrong color balance. This can occur in spite of the file or the grayscale/colorscale image appearing correct on your monitor. The cause of the discrepency is either the monitor, the printer, or both. Before deciding that your print actually needs an adjustment, be sure that you are viewing your print with sufficient illumination. Whereas viewing an image on your monitor required minimal ambient light, the requirements for viewing your print are just the opposite. This is because a monitor functions by emitted light, and a print presents its information by means of reflected light. The best light for viewing your print is skylight, so at this stage in the process, open up the blinds. If you have color management hardware and software, you can create corrections to compensate for imbalances between various devices (monitors, printers, etc.). If not, simply make a temporary adjustments to your file with your image editing software and try printing again. It is best to not make these changes permanent in the file, but just temporarily compensate at the time of printing. It may happen that the adjustment in brightness and constrast depends on the print size. For example, a file that appeared correct on a web browser required an adjustment of Brightness=+50, Contrast=-25 for a 4x6 inch print but only an adjustment of Brightness=+25, Contrast=-12 for a 7x9 inch print. This behavior may depend on the particular printer that you use.

The best way to control the size of your print is to use the "Resize" command in your image application to set the pixels per inch descriptor of the file so that, given the number of pixels in the image, the X and Y dimensions of the image are what you require for your print stock. Be sure to not change the number of pixels in the image or file, however.

3D photos require a series of exposures to be made at different positions along a straight line. The camera should always point in the same direction, rather than be angled toward the subject at each position, in order to avoid keystone distortion. The required linear offset between exposures depends on the lens focal length, film format, and some combination of two of the foreground, key subject, and background distances. The calculation is made easy with the Macintosh program Computrack by Donald Gudehus. After your series of exposures is interlaced, it can be printed on an inkjet printer and overlaid with a sheet of lenticular material. To make a permanent 3D print, your printer output should be laminated to the lenticular sheet with a layer of optically clear mounting adhesive (available from Micro Lens Technology, Inc.).

Inkjet Printer Quirks

Inkjet printers are remarkable devices, however the printer drivers sometimes behave bizarrely. One flaw which has crept into their design is that a single user-adjustable parameter can sometimes be entered in two different places in the program. In the case of the Epson Photo 750 running under Windows, the choice of whether the image is "Centered" is made in the Page Setup menu item, and again in the driver Print dialog. Also, in the Page Setup one finds "Fit to Page" and in the Print Dialog, "Maximum Printable Area". Such an approach represents poor software design.

If one is printing on 4x6-inch paper with the Epson, a setting of "Center on Page" in the Page Setup can be used with either "Maximum Printable Area" or "Centered" in the Print dialog, to produce a print that will fit within the serrated boundaries of the paper. Changing the Page Setup setting to "Fit to Page" causes the same file to be printed beyond the serrated boundaries of the paper. However when trying to print on letter-size paper, a setting in the Page Setup of "Center on Page" or "Fit to Page" will refuse to print with a setting in the Print Dialog of "Centered". Only a setting of "Maximum Printable Area" works.

Still another quirk occurs when using the Epson under Windows to make a print. Even though all parameters appear to be correctly set, the very first print may be off center and expanded to the wrong size. Successive prints are fine even though no changes in the settings are made. Rather than waste valuable time, ink, and paper on the first useless print, one can print a "minimal" file on ordinary laser printer paper, cut to the correct size, for the first print. The "minimal" file is simply a small file with a white background and four black patches in the corners. This file prints quickly, does not use much ink, and readies the printer for real printing. The black patches serve to verify on the paper that all is going as expected. A copy of this file can be downloaded here. The printer usually prints correctly however if one initially goes to the Page Setup menu choice, selects the appropriate printer, and sets its desired properties, e.g. centered.

Cleaning the Inkjet Nozzles

The following discussion pertains explicitly to the Epson inkjet printers, but may apply to some other brands as well. Inkjet prints are remarkable for their ready to use state as soon as they emerge from the printer. This comes about because of the fast drying inks contained in the cartridges. Unfortunately, there is a downside to this in that the inkjet nozzles can become temporarily clogged. The printer driver software as well as the printer operating buttons allow one to check the nozzle status by printing a Nozzle Check Pattern in which each of the many nozzles in the print head is briefly exercised. It is more practical to print this pattern on ordinary laser printer paper rather than the more expensive inkjet paper, since the pattern is easily interpreted independent of paper quality. If the pattern shows some gaps, the nozzles may be blocked with dried ink which then requires some corrective action. However, as noted below, it may not always be necessary to clean the black nozzle.

Both the printer driver and the printer operating buttons allow one to clean the print head with a Head Cleaning utility. The Nozzle Check Pattern should be printed after each cleaning cycle, but if after two or three attempts the nozzles remain clogged, further action must be taken. If you are unable to clear the print head with the built-in utility, Epson technical supports recommends taking your printer to an authorized service center to have the nozzles unclogged. Be warned however that if your printer is out of warranty, this service will run about $80 per hour plus the cost of possibly replacing the ink cartridges. They also may find it necessary to replace the print heads which would further add to the cost. Since the total cost could be half or more of what you paid for the printer, the remaining practical alternatives are throwing the printer away or cleaning the print heads yourself.

Actually, it is not too difficult to clean the print head nozzles yourself. To do this, first collect together some isopropyl alcohol, a third of a cup of warm water, a tweezers or forceps, a syringe, kitchen baster, or eye dropper, a new sponge, a large plastic garbage bag, some aluminum foil, a zip lock bag, and a bright light. With a sharp knife and a cutting board, cut off about a dozen pieces of the sponge into strips about one inch long by one-eighth inch by one-eighth inch. Spread the garbage bag out on the floor or a table and place the printer over it. Operate the printer so as to place the ink cartridges into the reload position (on the Epson one holds the load/eject button down for three seconds). Add about a sixth of a cup of isopropyl alcohol to the warm water. The Epson manual says that if you remove an ink cartridge from the printer, you can't reuse it, even if it contains ink. This is false; Epson technical support states that a cartridge can be removed for a short period of time. The important thing is that the ink not dry while the cartridge is removed. To prevent that from happening, proceed as follows: remove the cartridge of the color associated with the blocked nozzle, and immediately wrap it tightly with a piece of aluminum foil, and place it inside the zip lock bag. Now fill the syringe, baster, or eye dropper with a small amount of the water/alcohol mixture and squirt or drip the liquid onto the print head nozzle. The nozzle assembly is inside a plastic nipple-like structure which fits into an opening in the ink cartridge. Because the openings are microscopic in size and can easily be plugged or damaged, do not let any debris fall onto them, and do not touch the nozzles with the liquid applicator. The excess liquid will drain into a well which surrounds the nozzles. Wait a few minutes and then soak up the excess liquid with pieces of the sponge strips held in the forceps. Do not touch the nozzles with the sponge. Repeat this operation of squirting or dripping the liquid onto the nozzles, waiting, and cleaning up. When it appears that no additional dried ink is being dissolved by this operation, unwrap the ink cartridge and insert it into the printer.

At this stage the nozzles may be clear but the ink may not flow immediately. Proceed to alternately use the Head Cleaning Utility and the Nozzle Check Pattern until no gaps appear in the pattern. If gaps still appear after several cycles, try removing the cartridge again and repeat the entire procedure.

Note that when making color prints, only ink from the color cartridge is used (the color black in this mode is created by overlapping color inks). Therefore the black ink nozzle can remain clogged and the prints will be unaffected.

Is My Ink Cartridge Really Empty?

Inkjet printer firmware can notify the user when the ink level in the cartridge is getting low and when the cartridge is empty. However this system is not foolproof. In one case, an Epson Photo 750 reported that a new color cartridge was low on ink only hours after it was installed. The next day the printer decided that the cartridge was empty and refused to print. Noting, as mentioned above, that the instruction manual prohibition against removing the ink cartridge was false and in conflict with statements from Epson technical support, I briefly removed the cartridge and reinserted it. This caused the printer to think that the cartridge was full and the light went out. To get the ink flowing smoothly, I then gave the printer several Head Cleaning Utility and Nozzle Check Pattern cycles until the pattern was without gaps.