Two popular materials used for lenses are ZnSe and GaAs. ZnSe (Zinc Selenide) is the best material in the world for infrared optics, but the quality in China can be unpredictable. GaAs is more consistent in China as a raw material, but it is not as good. There are two types of ZnSe, CVD (Chemical Vapor Deposition) which is a process where an environment is maintained in a reactor to promote crystal growth, and PVD (Physical Vapor Deposition) where scraps are evaporated and then combined back into a solid. CVD ZnSe is best for CO2 laser optics. For more information on how to purchase, clean, remove and install a lens, scroll down.
Click on an Image Below to Purchase a ZnSe Focus Lens:
How to Purchase a Focus Lens:
Two pieces of information need to be known in order to select a focus lens: 1.) diameter of current lens and 2.) desired focal length.
Determine the diameter of the existing lens. There are one of two ways to do this: measure the existing lens or ask the seller/manufacturer. To measure the existing lens, it must be removed (follow the lens removal instructions below). Once removed, use a digital micrometer to measure the diameter of the lens. The lens is soft and can scratch easily, so take care not to scratch it when taking your measurements.
Determine the desired Focal Length. Laser quality lenses, are available in various Focal Lengths based on your needs for engraving and cutting. Below are four popular Focal Lengths for CO2 lasers. Most lasers ship from the manufacturer with 1.5" or 2.0" because they are fairly universal.
1.5" (38.1mm) Focal Length - smaller spot size, ideal for up to 600 dpi detailed engraving and fine cutting less than 2mm.
2.0" (50.8mm) Focal Length - multi-purpose, suitable for most applications, for 500 dpi engraving and cutting from 2mm to 5mm.
2.5" (63.5mm) Focal Length - excellent cutting lens, for 400 dpi engraving and cutting from 3mm to 15mm (depends on laser power).
4.0" (101.6mm) Focal Length - used for cutting thicker materials, larger spot size, for 200 dpi engraving, engraving of curved surfaces, and 7mm and greater cutting (depends on laser power). Lasers under 100W may not see much benefit from this lens.
These are suggestions not rules. The cutting ranges below are based on a single pass. Your results may vary depending on your alignment and your lens quality.
APC DPI and Dot Size Reference Chart:
Note: You will need to measure your actual beam size by placing a piece of tape over your third mirror (before the lens) and pulsing your laser to determine your beam size. Beam sizes are roughly as follows: 40/50w = 4mm beam, 100w = 6mm beam, 180w = 8mm beam, but are not guaranteed.
This video explains how to use the chart in details (Facebook Group access required).
Lower your bed and move the laser head towards the front center of the machine. This will make it easier to access the laser head assembly.
Remove the air-assist tubing from the air-assist fitted in the nozzle. Cup the fitting underneath with your thumb and place two fingers around the black tube on the blue or black ring that goes around the tube, and gently pull on the tube while compressing the ring.
Remove the air-assist fitting counter-clockwise until it is removed.
Place a soft cotton cloth, preferably lint-free, underneath the laser head.
Spin the cone-shaped nozzle clockwise (looking down on it) until it separates from the tube. It should be finger tight, but sometimes it can be stubborn to remove. In some cases you may have a knurled ring which makes this process easier. Some models may have a thumb/set screw which needs to be loosened prior to removing the nozzle.
Remove the lens retainer from the nozzle holding the nozzle and turning the retainer counter-clockwise (be careful not to let the lens fall out). Some machines may require the use of a snap-ring tool or a lens removal tool to remove the lens.
The lens should now be loose. Do not touch the lens! The oils on your fingers can damage the coating on the lens. Carefully slide the lens out of the tube onto the cloth. When handling the lens, hold it from the sides.
This video does a nice job showing you how to remove and reinstall the lens (your process may differ): Lens Removal and Installation.
Place the lens in the nozzle, FLAT side down. Again, FLAT side down!
Re-install the ring nut.
Tighten it until it is snug. Nothing more! Overtightening may damage the lens.
Screw the nozzle back on by turning the straight-knurled ring counter-clockwise, until it is snug, and tighten the thumb/set screw, if present.
The focus lens must be removed in order to clean it (follow the Lens Removal instructions above).
Use only 99% isopropyl alcohol or acetone and lint-free soft cotton wipes or swabs to clean the lens. It is also suggested to wear powder-free, latex gloves.
If you have compressed air available, it would be a good idea to hit it with a light blast of air first to remove any debris. Using a Q-tip, cotton ball, or lens wipe, apply the alcohol or acetone to the lens and gently swirl around until it is clean. Never use paper towels or wipes. ZnSe lenses are very soft and can scratch easily. Never use water based cleaning products on your lens. When you fire your laser, it will bake the water out, reducing the life of your lens. A high-quality lens, cleaned properly and often, will last years.
Reasons Why Lenses Fail:
Not frequently cleaning the lens.
Water in the air-assist.
Use a desiccant air line filter to remove water vapor.
Oil in the air-assist.
Use a coalescing filter on the air line to remove oil aerosols.
Air-assist was not in use.
Over-tightening the lens (or not using an O-ring).
Only tighten you lens to the point it does not move in the holder. ZnSe is a very soft crystal. Over-tightening will cause power loss, lens overheating, and beam distortion.
Using cleaning products containing water or dyes.
Poor beam alignment causing the laser head to overheat.
Dropping or chipping the lens during cleaning.
Use of cheap, non-laser grade ZnSe.
Poor coating or optic quality.