Understanding DPI, NPI, and LPI for Inkjet Printheads in Manufacturing Applications

This is an overview of an exploration of inkjet printhead technology. If you’re integrating printheads into processes like 3D printing, coating deposition, or adhesive application, understanding their capabilities and how they work is essential.

This post examines the technical relationships between dots per inch (DPI), nozzles per inch (NPI), and lines per inch (LPI), focusing on their roles in achieving quality and detail with a Quick Look at drop volume as well. We’ll define these terms from the ground up, trace LPI’s analogue origins, and explain how they apply to your work, drawing insights from inkjet industry sources.

A Short History of LPI

Lines per inch originated in the 19th century with halftone printing, pioneered by William Fox Talbot around 1852 (Meggs, 1998). This technique broke images into dot grids, larger dots for dark areas, smaller for light. These are measured as LPI. Early newspapers used 60-100 LPI, offering basic clarity (Kipphan, 2001). By the 20th century, advancements pushed LPI to 300 for magazines, leveraging finer screens and greyscale for smoother gradients.

The digital era adapted this concept. Inkjet printheads replaced screens with nozzles, using DPI and greyscale to replicate LPI’s effect (Ritchie, n.d., p. 1). In manufacturing, this translates to controlling droplet patterns for functional outputs, like even adhesive layers or precise 3D structures.

Figure 1 -  Halftone Sample. You can see the effect of the half toning on the image, even though the dot grid (DPI) is regular. Image on the left is scaled to view.

How DPI, NPI, and LPI Interact in Inkjet Systems

Consider a printhead depositing a fluid on a substrate. Here’s how these factors govern the result:

NPI: The hardware printhead’s NPI, such as 360 NPI, sets the native addressability. Locations where droplets can land in one pass (Ritchie, n.d., p. 2). For a 3D printer, this might mean 360 base points per inch. However, effective DPI can exceed NPI through techniques like interleaving two 360 NPI heads to achieve 720 DPI (2 heads, 1 behind the other, offset in mounting by 1/2 a nozzle pitch) (Fig. 4, Ritchie, n.d.).

DPI: Output Precision DPI reflects the system’s ability to place dots, adjustable via encoder resolution or passes. A 360 NPI printhead might reach 720 DPI with enhanced media-axis control (Fig. 3, Ritchie, n.d.). In manufacturing, this means sharper edges on a 3D-printed part or denser coating patterns. There can be compromises in print speed 

LPI’s Influence is derived from analogue printing. LPI relied on dot spacing and greyscale. Inkjet can also use greyscale through variable drop sizes (e.g., 6-42 pL on Xaar Nitrox) to achieve an effective resolution, mimicking LPI smoothness (Ritchie, n.d., p. 4). A 360 DPI printhead with 8 grey levels hits 1018 DPI effective resolution, ideal for fine print detail.

So, for digital, NPI refers to the printhead ONLY. DPI refers to the image/output being printed (but can match the NPI of the printhead). LPI is analogue (therefore not really used in digital).

Figure 2: Dot size example (Richie, n.d., p.3)

Manufacturing Implications

In applications like 3D printing or adhesive deposition, precision and consistency are really important. A high-NPI printhead (e.g., 720 NPI) paired with high DPI can deposit micro-scale features—think conductive traces at 2400 DPI (Smith, 2015). However, Ritchie (n.d., p. 7) shows that 360, 600, and 720 DPI prints can appear similar at normal viewing distances, thanks to greyscale and dot placement accuracy (Fig. 12). 

Drop volume needs to be considered too. Small drops (6 pL or less, typically) excel for highlights, while larger drops (18+ pL at 360 dpi) ensure solid coverage (Ritchie, n.d., p. 6). For a coating, small drops might leave gaps, whereas larger drops guarantee opacity. Placement accuracy, prevents ragged edges or misalignment—crucial for functional layers/coatings.

Viewing distance also limits perceived quality. The human eye resolves 876 DPI at 10 cm, dropping to 300 DPI at 30 cm (Ritchie, n.d., p. 5). Excess DPI beyond this is redundant unless magnified, a key consideration for inspecting print, but not for functional parts.

HOWEVER - What is important to print is not what is important to function. 

Viewing distance and eye trickery are not important in functional applications. The function of the material and pattern is. So the key to using a printhead in manufacturing is how to operate a tool designed for print in order to create a functional output. This can change per application. 

For example. If we want a full coating from a lower resolution printhead, we must consider the print resolution (DPI) and the drop size to achieve that. As can be seen in Figure 2, we would need at least an 18 pL drop for those to join together to form a coating (for this model fluid example). Coatings typically have a minimum film thickness requirement for function (dielectric strength, scratch resistance), this will mean that a minimum volume of material is required. If that volume results in a coating thickness that is too thin, we need to increase drop volume. Though this comes at a compromise to the maximum frequency (and therefore line speed) that can be achieved.

If the coating is too thick, we must reduce the drop volume. But that may give us our gaps between nozzles. We therefore now look to a higher resolution printhead (or interleaving/scanning) to fill the gaps as we reduce the drop volume. However, for functional coatings, we may also have to consider another set of properties......

There is another process modifier we have not yet considered. The properties of the fluid being ejected. Mainly surface tension and viscosity. These will also have an effect on how much and in what way the drop spreads on the surface and dries/cures. And here we find another inkjet terminology headache. How drop volume translates to dot diameter is dependent on the fluid properties and the media being printed on. A higher viscosity fluid with a higher surface tension should 'spread' (flow) less on the same substrate than a low viscosity, low surface tension fluid. You can therefore achieve thicker films with lower drop volumes (or, fewer passes). Also, you may be able to achieve finer features with a higher viscosity fluid than a lower viscosity fluid. So for a conductive track, High viscosity printheads may be able to achieve things that other printheads can't (yet). 

Conclusion

For manufacturing, using printheads means understanding DPI, NPI, and LPI together. NPI defines the hardware’s potential, DPI delivers the output resolution, and LPI’s analogue principles guide effective quality via greyscale and dot control. Selecting a printhead type and performance requires balancing these factors with drop size, placement accuracy, and application needs, not just chasing high numbers (Ritchie, n.d., p. 8). This integrated approach should ensure your output, materials, coatings, or adhesives meet exacting standards.

A couple of end notes

Don't confuse DPI with PPI (Pixels per Inch). PPI is for the digital screen used to view and create the artwork to print. 

For really fine design details, it is important to remember that printheads are manufactured in DPI. Product designers may default to SI unit (metric) there will be a mis-match at the fine levels. For example, 1200 DPI is a pitch of 0.0212mm therefore a 1mm feature will be either 47 or 48 nozzle pitches (0.9964 mm or 1.0176 mm) making 1 mm difficult to achieve (fluid behaviour will influence at this level too!). This is a key understanding required within the software used to create the images from whatever artwork is used (CAD/Photoshop/Illustrator etc). Greyscale can be used to control these edge distances. However, for 3D printing, we must consider that we have a Voxel (Volumetric pixel) to fill rather than a singular planar image.