In choosing which type of laser to use, there are many variables which will effect the quality of the laser mark. Ultimately, it is a matter of experimenting with different lasers and different marks until an optimum mark can be achieved.

A. Wavelength of Beam - Opacity, Transparency & Reflectivity
Every type of laser emits a light beam at a characteristic wavelength. Nd:YAG is at 1.064 µm, while CO2 is a 10.600 µm. The different wavelengths react differently to some materials. Materials can be opaque , transparent or reflective to the different wavelengths. Most glass is transparent to YAG, but is opaque to CO2. Most metals reflect CO2, but are opaque to YAG. Plastics tend to be opaque to both types of lasers. In some cases, the choice of which laser to use will be made simply by which laser frequency works on the material in question.

In addition to the mechanical effects of the different wavelengths of laser beams, the different wavelengths have different optical characteristics in that the smaller wavelength (YAG) can be focused to a finer point.

B. Spot Size
Laser beams exit the laser unit in a small, round (generally) collimated beam that neither diverges or focuses. In order to be effective at transferring the maximum amount of energy, the beam must be focused to a point. Please see Sec. III,C for a discussion of the optics required to do this.

The fine point is called the "spot" and the different wavelengths can provide different spot sizes. Because it has a smaller wavelength, the YAG laser beam can be focused to a point as little as 1/20 the size of the CO2 laser.

The availability of different spot sizes raise several issues:

1. Power Density
   Power density is a function of how much energy is focused on a single point. The finer the focus of the beam, the more energy is transferred to the marking target. Because the YAG has an inherently smaller spot size, it will have the higher power density. In some cases, depending on the material being marked, high power density may be required to achieve a suitable looking mark. However, it is important to remember that soft materials need little power to mark and so a low power density may be actually be preferable for those materials.
2. Resolution of Text / Graphics
   Another consideration may be how small or well-defined the text or graphics must appear. The smaller spot size of the YAG gives it higher resolution. Some applications will need high resolution (e.g., marking in very small area) while others will benefit from low resolution (large mark area; easily legible). To help the reader visualize this difference, look at the following words: test test. These words are the same font and same point height. However, the letters of the first word are thinner than those of the second. This is the sort of difference seen by differently focused laser beams. Either type of laser can be defocused to increase the spot size and reduce resolution if required.
3. Speed of Marking
   The speed with which a material can be marked depends on many things, including the marking characteristics of the material, the power density of the beam, and the resolution required.

   If a material requires a great deal of energy to mark, then the mark speed must be slow to allow a sufficient amount of energy to hit any given point. Imagine a drill working on a piece of wood and then working on a piece of metal. The same drill will take much more time to effect the metal. In the same way, a laser beam may have to linger over a target to make an effective mark.

   In any case, the power density of the laser matters. A more powerful beam will work the same surface faster than a less powerful beam. Thus, with a higher power density beam the laser can work faster. However, with some materials the mark quality is improved by using a lower power beam and moving it slowly.

C. Similarities
The CO2 and Diode-pumped YAG lasers do have certain similarities. They share these characteristics:

• relatively small packages - allowing small overall system footprint
• air cooled
• require normal 110/220 VAC electricity (as opposed to three-phase)
• virtually maintenance free
• long service life (at least 15,000 hours)
• no external gas supply