The Basics of Your Laser

Laser Basics

Although we do not need to spend a lot of time on understanding how the actual laser machine works there are some things that need to be highlighted as they are important items that need to be considered when you are laser engraving.

Although I wanted to stay away from talking about laser tubes, more because it would be better told by an engineer, to know a laser is to know how the tube works.

Laser Tube Characteristics

The Laser Tube

The term laser is an acronym for Light Amplification by Stimulated Emission of Radiation. In its most simple explanation a laser is a device that converts electrical energy into optical energy. The light is amplified in the tube and is emitted from the tube traveling in a straight line at a single wavelength. The tubes are usually aluminum and are specially designed so that the gas stays sealed in the tube and does not leak out.

One characteristic of a laser tube is that they generate a lot of heat. Tubes need to remain below a certain temperature. If the laser tube gets to be to hot the laser tube will begin to lose its efficiency. To help maintain a stable operating temperature the outside walls of the tube have large heat sinks that conduct the heat away from the tube.

One thing to consider is that the CO2 laser tube is generally inefficient. This means that it generates a lot of energy but that a lot of this energy becomes non usable energy.

Bore and Mirrors

The bore has two purposes. The first is that it is where most of the optical energy is generated. It also provides the direction to the optical energy. Typically bores come in different shapes and sizes. The shape of the bore can greatly influence the shape of the beam. This can be important as beam shape can be advantageous and disadvantageous depending on your application.

The mirrors are mounted at each end of the bore. One of these mirrors is totally reflective and the other mirror is partially reflective. The mirror that is partially reflective is designed to allow some of the optical energy created in the bore to pass through and escape the tube. The optical energy that escapes is just the right size (wavelength) and moving in just the right direction (perfectly perpendicular to the mirror). The optical energy that escapes is the laser beam. The rest of the energy is reflected back into the tube and is not allowed to pass out of the tube.

Gas

The gas mixture in a sealed laser tube tends to be mostly CO2, with similar concentrations in nitrogen and helium gas. The actual mixture is kept secret by tube manufacturers.

RF (Radio Frequency)

All lasers are powered by radio frequency (RF) energy. The RF energizes the gas mixture. The energized gas becomes optical energy.

Different Types of Lasers

When we look at our industry there are 3 laser types that are prevalent. They are the Free Space Laser, the Slab Laser and the Waveguide laser. Each laser tube has its specialties and I will refrain from actively discussing the different tubes here.

Laser Machine Characteristics

DPI (Dots per inch)

As much as our laser combines a lot of unique technology we can look at it in its simplest form – as both a laser printer and or a plotter with a laser attached to it. Like a laser printer the laser engraver has different dpi settings. For example most laser engravers use a standard dpi setting of 500 or 600 dpi. What is dpi? DPI stands for dots per inch. If you engrave an image at 600 dpi there will be 600 dots in a linear inch of travel both horizontally and vertically. If you decrease your dpi to 300 than your laser produces 300 dots per inch in the vertical and the horizontal direction. One important point needs to be pointed out here. If you half your resolution from 600 to 300 than your engraving time is cut in half. Typically, as with laser printers, we find that the higher the dpi the better the quality of the engraving with lasers. This is true up to a certain point. Figure 24 shows a representation of how the laser lays down the dots at different dpi settings. Considering that the dots are always the same (as long as the power and the speed are the same) all that the different dpi settings do is cause the dots to be either closer or farther away from each other.

Figure 1

Figure 1 shows a representation of how the different dpi settings are spaced together.
The higher the dpi the more dots that are lasered in an inch of travel. The more dots transfers
into more time to engrave an image. 300 dpi engraves twice as fast as 600 dpi.

The closer the dots (remember that any item laser engraved is just a series of dots placed together) are to each other the better the quality the finished image will be. However if we look at the dots in the 1200 dpi representation you will notice that there is a lot of overlap. This overlap can be greatly exaggerated if we increase the power that our laser is delivering to the material. Thus what we find with the higher resolutions like 1200 dpi is that because of the tighter dot overlap our images tend to over engraved (too many dots) and will look worse than the lower dpi such as 600.

One other issue may become apparent is when you are looking at figure 25. Each of the sets of dots has 6 lines of dots. You should notice that the amount of space taken up by the dots is greater with the 200 dpi setting than the 1200 dpi. Thus the lower the dpi that we use the faster our machine will engrave. However there is a quality trade off. This quality will be dependent on the type of material that we use and the power and speed that we use. If we run a job at 600 dpi and the job takes 2 minutes to engrave, than running the job at 300 dpi will cause the job to run at approximately half the time or 1 minute. Note: I have always promoted that you should always try to use a lower dpi setting when you are engraving as it can greatly increase your through put. Considering that a lot of text we engrave is no bigger than an 1/8 of an inch no one is going to see the difference. The only problem we have with less dots is that you have less burning and most of the time you will have to increase your power to achieve a similar burn that you would get at the higher resolution.

Figure 2 shows the actual laser dots that are created by the laser machine. Most lasers create a dot that is more oval in shape. This creates a bit of a problem in cutting. The beam will be wide in the x direction and thinner in the Y direction – sort of like a paint brush.

Figure 2

Dot Gain

As we find with printing on paper dots created by the laser engraver are a certain size before the beam burns with the material. When the beam comes in contact with the material the dot tends to become bigger as the heat is absorbed by the material. The more power that is supplied to the dot the larger the dot becomes – this is because there is more heat which is absorbed by the material. I like to call this dot gain which is the same as we find with a printed dot on paper. A printed dot will be one size before it is placed on the paper. After the dot comes in contact with the paper the dot size becomes bigger. The more ink that is used the bigger the dot tends to become. Other variables such as paper will also influence the size of the dot that is created. Laser engraved dots are the same as the dots created on the paper. The more power that is supplied the bigger the dot becomes. The size of the dot is also influenced by the material that you are engraving.

It is this dot gain that can make or break a laser engraved image. Fine detail in an image can be greatly reduced when you apply to much laser power to your engraving material. Figure 27 shows an image that is created on some laserable black brass coated steel. The fine line around the outside of the image is perfectly engraved. However the bottom figure in Figure 27 shows that if we supply a little more power – even 3 more watts – the detail can be quickly obliterated.

Figure 3

Figure 4

So the trick when you are laser engraving is to practice until you come up with a power and speed setting that is perfect for both your laser and the material that you are engraving. You always need to take into account the dot gain that will be created by the laser. So create your samples and than compare those samples with different speeds and power to see what settings are the best.

Focal Length

The most important step that you can take to achieve good engraving on your laser is to make sure that you are in perfect focus. The laser beam functions the best when it is at its smallest point. If you remember back in your school days when you took a magnifying glass and moved it back and forth to focus the sun there was a point where you got a real small point. It was this fine point that burned the paper. If you moved the magnifying glass to far away than nothing would happen. This principle is the same as how the laser works. The laser machine is taking the laser beam and when it goes thought the lens it is being focused to a very fine point. The finer the point the smaller the dot sized that is engraved. Figure 28 shows the different lens that are available and what the dot (spot) size that each creates.

The most common lens (99% of the applications I come across) is the 2 inch lens. It creates a spot size of approx 5 thousand of an inch. For most people and most applications this is the only lens that you will ever need. The other lens are specialty lens and used for special engraving applications.

Because we are using a 2 inch lens in our laser than we need to be approximately 2 inches away from our engraving material. However we do not necessary have to be exactly 2 inches away. Figure 29 shows the beam actually straightens out for a short period before it starts to become larger. This straightened area is called the depth of field. Like a camera this area means the beam is still in focus. Each lens has its own depth of field. The bigger the depth of field the longer you can be out of focus. However as the chart indicates this comes at a cost in that the spot size of the laser is bigger. The 2 inch lens has a depth of field of .187 or a little more than 11/64ths of an inch.

Cleaning the System

One of your main enemies with the laser is dirt. The more dirt that is left in the machine the more issues you will have later on. Typically most manufacturers that I have seen have asked that you routinely wipe the machine down to keep it clean. I would suggest that you refer to your owner’s manual to find out what they suggest and what needs to be cleaned.

Also the optics (mirrors and lens) in the laser need to be cleaned. Dirty optics can rob you of power. I have seen machine lose half their power because of a dirty lens. So be diligent about cleaning your machine.

Figure 5

Table 1

Table 3

Note: some new optics have changed the above chart to smaller and more round dots sizes and shapes. Check with your manufacturer for up to date specifications.

The above table shows the each lens and how big the spot size is along with its best application.

Depth of Field

The depth of field is amount of distance that your laser is in focus. For example table 1 indicates that our depth of field for a 2 inch lens is .187 inches or a little over 11/64 of an inch or better still 3 sheets of Rowmark plastic. Thus you do have some room to work with when you making sure that you are in focus. Depth of field is the same as a camera. The amount of depth of field in a camera is how much of the image is in focus. The same can be said for the laser. If you are “are out of focus” than you are out of the .187 inch distance. Thus in theory to be out of focus is to be either above or below .187 inches from a 2 inch lens. Personally this is not a telling you to be careless with your focusing. What it is trying to tell you are that if your material is not entirely level or on a curve than you can have some leeway before your burning ability is greatly reduced.

You will also notice from Table 2 that the depth of field increase as the lens gets larger. Thus you can really do some items that are at different heights. I remember a customer that had to do some laser engraving on some large curved bases. The problem is that they where out of focus on a large part of the base. This caused inconsistent burning by the laser where one area was engraved deeper than another area. The use of a 4 inch lens greatly solved their problems. The larger spot size produced by the 4 inch lens was not an issue as they were engraving on wood.

So the rule here is that an increase in depth of field is offset by a wider or large spot size. Thus you need to make sure that the wider spot size justifies the large lens. The other thing to consider is that all laser beams are not created equal. Although each can create a certain spot size the burning ability of a laser machine is largely dependent on the shape of the beam and the rise and fall of the laser. Certain beams produce better spots for certain materials.

Speed versus Power

When I am training people on the laser machine I have always felt that you need to run the job as fast as you can. Time is money and the less time it takes to do something the better it is to your bottom line. For this reason I always run the machine at 100% speed. At 100% speed you can adjust your power accordingly. However what if you need more power? Well if you are at 100% power the only way that you can achieve a deeper burn is to slow the machine down. By slowing the machine down you are placing more power at the point of burn. Thus, we can say that the depth of burn is inversely related to the speed of the laser. The slower the speed of the laser the deeper our burn will be – depending on the material.

When it comes to light products like plastic we can laser engrave it a lot faster than we can say with wood.

Power Needed

One trick that I do is to create a rectangle in CorelDraw. Send the job over to the laser with a power and speed. Start engraving the material. Now adjust the power and speed on the laser (most machines allow you to adjust the power and speed while the job is running. So check out your laser manual). Typically I will adjust the power until I am just getting through the material. Once through I increase the power a little more. Once I have this setting I save it in my preference file in the laser driver.

Pricing

This is always the $50,000 question and because of certain laws I can not suggest actual pricing. But I can say that a lot people will price based on time. For example if you were to charge $100 per hour and the job took 6 minutes than you need to account for that 6 minutes and charge $10. The one thing to remember when you are pricing and that is that you will eventually need to replenish this machine. Your pricing should include a slush fund so that you can replace your machine when it gets to old.

Pricing is always subject and there can be a lot of factors when you are setting a price. The best thing I can suggest is if you are in doubt than run a sample. The more you know about the job the more likely you are that you will be able to deliver a more competitive price.

The purpose of this section is to give you a brief overview of the laser machine. If you know how your machine operates than you will be able to maximize your output. Knowing the machine will also make it easier for you to create the best quality images that you can on your machine.