DPI vs. PPI For The Laser

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DPI vs. PPI For The Laser

What is Dots Per Inch? (DPI)
One of the most misunderstood concepts in our industry is the question of resolution. Unlike most laser printers, where a higher resolution means a better quality image, with laser engravers more resolution can actually be detrimental to the quality of the finished product. A laser engraved job produced at a higher resolution will take longer to engrave. Why? Let us show you.

First, we need to discuss the concept of both dpi as well as ppi:

DPI
What is dpi? It means dots per inch. Let us say we want to print a 1 inch box. If you print a one-inch box at 1000 dpi you will have one thousand dots both east – west (across) as well as north south (up or down) in the diagram. To give you a more “tonal” representation the dots can be made smaller so that you can see more “tones of black” or grayscale. This typically gives you a better quality output. The problem with laser engravers is that their spot size can only be made so small – approx. 0.03 thou of an inch with a high resolution lens. Added to the fact that when you burn a dot it is going to become bigger. If we lay down too many dots – which generally occurs at 1000 dpi – we get over burning and we get what is normally classified as a “blob.” Typically, we find that 99 percent of laser jobs can printed out fine at 250-500 dpi or less without sacrificing quality.

Another problem with a higher resolution is that it takes longer to print a job that is outputted at 1000 dpi than a job outputted at 500 dpi (See discussion on Dot Overlap Diagram for Various Resolution Settings). As mentioned earlier a job outputted at 1000 dpi lays down 1000 dots both vertically as well as horizontally in an inch. A job outputted at 500 dpi lays down 500 lines both vertically as well as well as horizontally. If you have only on head laying down dots in a vertical pattern then it takes twice as long to produce a job at 1000 dpi compared to 500 dpi. If your job takes 10 minutes at 500 dpi than it takes 20 minutes at 1000 dpi. Even an increase of a dpi setting to 600 increases the job from 10 minutes to 12 minutes – A 20% increase in time – and time is money.

I always try to do a job at 333 dpi first to see if the quality is acceptable. If it is acceptable then you can almost half your job time without sacrificing the quality of your final output. So remember although a higher dpi can give you better output, certain materials and job costs may mean that a lower dpi will suite your job set better. Once you try to run your laser over 500 dpi your finished product may not equal the extra time it takes to produce the image – you could even ruin your image by over burning it.

PPI
What is ppi? It means pulses per inch. Although not tide directly to dpi, PPI in some instances can be more important than dpi. It is tied directly to the output of the laser. Thus, if you are running a job at 1000 ppi — this means that the laser is pulsing 1000 times per linear inch of travel. Although adjusting ppi can help in giving you a better quality finished image, the main reason for controlling ppi isto help during vector cuts. For example, if I cut a line 1-inch long the laser will pulse 1000 times. However because a machine can travel at a number of speeds during a vector cut it must be able to adjust its “pulsing” to take this into account. For example, if we look at a straight line we see it consists of only two nodes. Because there are only two nodes, the laser machine can travel very fast. On a circle, though, there are a large number of node segments. These large numbers of nodes means that the laser machine cannot travel as fast. Thus, if the machine can not travel as fast it must adjust its pulsing to accommodate for the change in speed. If it did not then you would get over burning on say a circle as opposed to a square. This is a veryM important factor to remember. The quick test is to engrave out a square and a circle in 1/8-inch acrylic. If the pulses per inch are working right then you will notice that the edges are smooth on both the square as well as the circle.

Dot Overlap Diagram for Various Resolution Settings

1000 dpi at 1000 ppi

Single line of .003 dots at 1000 ppi

Single line of .003″ dots at 1000 ppi

 

Three lines of .003 dots at 1000 ppi and 1000 dpi
Three lines of .003″ dots at 1000 ppi and 1000 dpi

 

A 100% dot overlap. A single unengraved dot would be lost because adjacent dots would overlap and engrave the dot.

500 dpi at 500 ppi


Single line of .003″ dots at 500 ppi

Three lines of .003″ dots at 500 ppi and 500 dpi

 

 

Moderate overlap of dots. Approximately 20% of single dot could show through.

333 dpi at 333 ppi


Single line of .003″ dots at 333 ppi

 


Three lines of .003″ dots at 333 ppi and 333 dpi

 

 

No dot overlap. The areas between the dots will not be engraved.

Dot sizes apply under ideal conditions. Some materials will show a larger dot size than the actual beam diameter due to material melting and/or sub-surface reflections, which may result in less than ideal resolution. These diagrams apply to the 1.5″ (high-resolution .003″ spot size) lens available with all Universal lasers. The standard 2.0″ lens has a .005″ spot size, which will remove a larger area of material with each pulse and result in more overlap per pulse than the 1.5″ lens at the same resolution settings.