Inkjet Printing Industry — Understanding Resolution Units DPI, LPI, and PPI

The Essence of Resolution

Resolution is a unit used to measure the density of “dots” on a flat surface. These “dots” may refer to:

• Pixels in a digital image
  
  

• Laser imaging dots in CTP plate making
  
  

• Ink droplets produced by a printer
  
  

In the printing industry, the three most commonly used resolution units are:

• DPI (Dots Per Inch)
  
  

• LPI (Lines Per Inch)
  
  

• PPI (Pixels Per Inch)
  
  

 

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I. PPI (Pixels Per Inch)

PPI was originally used to describe the resolution of digital image files (i.e., image detail level). In recent years, however, it has gradually been replaced by the more intuitive concept of “total pixel count.”

For example, when describing camera or smartphone performance, we typically refer to:

• 8 megapixels
  
  

• 20 megapixels
  
  

• 100 megapixels
  
  

Why Is Pixel Count More Accurate Than PPI?

Pixels are digital, virtual points that do not have inherent physical dimensions.

For example, in Photoshop:

If the pixel dimensions of an image are fixed (e.g., 789 × 946 pixels) and “Resample” is turned off:

• Changing the resolution from 200 PPI to 100 PPI
  
  

• The file size (e.g., 1.12MB) remains unchanged
  
  

• The total pixel count remains unchanged
  
  

• The physical size doubles (e.g., from 9.99 cm wide to 19.99 cm wide, with proportional height adjustment)
  
  

Key Conclusion

Once the pixel count is fixed, resolution is inversely proportional to physical size:

Resolution × Physical Length = Pixel Count

This proves that:

• PPI alone does not reflect the amount of image information
  
  

• Pixel count must be combined with physical dimensions to evaluate actual image density
  
  

 

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II. DPI (Dots Per Inch)

DPI describes the density of physical dots produced by a device. It directly reflects hardware precision.

Examples:

• Monitor: 108 DPI = 108 illuminated dots per inch
  
  

• CTP plate setter: 2400 DPI = 2400 laser imaging dots per inch
  
  

• Inkjet printer: 1440 DPI = 1440 ink droplets per inch
  
  

DPI represents real, physical output precision rather than digital image information.

 

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III. LPI (Lines Per Inch)

LPI, or “screen ruling,” specifically refers to the number of halftone lines per inch in amplitude modulation (AM) screening.

1. Difference Between LPI and DPI

• DPI devices produce dots of fixed size (laser or ink droplets), typically arranged at 0°–90° orientations.
  
  

• LPI refers to halftone cells with fixed grid spacing, but each halftone dot varies in size to control tonal values.
  
  

• Halftone screening angles are more flexible, commonly using 15°, 45°, 75°, etc., to reduce moiré patterns.
  
  

2. Historical Background

The term LPI originates from traditional photographic plate-making processes, where screen ruling referred to the number of halftone lines in a screen.

Although similar to DPI in concept, LPI remains in use because it specifically describes the structural characteristics of halftone dot arrangement rather than device resolution.

Resolution Units – Summary Comparison Table