It is a fun trick at trade shows to put a pickle in the laser that can engrave stainless steel and aluminium. I ask people to guess what would happen to the pickle and get very different answers. Melt like a marshmallow, explode like an egg in the microwave, the most fantastic answers come along. But no one guesses the right answer; nothing happens at all. No spark, no plume of smoke and you don’t taste anything.
To understand why a pickle does not shrink and metal does, we need to go to the basics and ask what exactly materials can do with light. Even though Physicists and Chemists can make it very complex, the practice is fairly simple, material has only 3 options how it wants to deal with light.
- Reflection (reflecting like a mirror)
- Transmission (letting through like a window)
- Absorption (absorbing like you feel the sun prickling your skin)
The 3 examples are clear and easy to imagine for everyone. But also a little short of the mark. Try looking outside from a lit room after dark, the glass looks like a mirror! And the torch of your mobile against your finger makes your whole finger glow red (transmission). The very best mirrors reflect 99.99% of a specific light, no material does 100% of the possible choices. And these Euro-sized mirrors cost hundreds of euros.
White light does not actually exist but is a mixture of all the colours of the rainbow.
Literally, a rainbow is created because raindrops ‘pull apart’ white light. A glass of water in the windowsill or a prism can also do this. The other way around could also be possible, if all colours of light are put together, white light is created. Every screen takes advantage of this! A green, red and blue pixel together makes white.
Back to the rainbow, if I were to put a green piece of paper on this rainbow, all the colours except the green disappear. No more red or blue can be seen. This is because ‘green’ absorbs all colours from white and reflects only green light. And if there is no green in the light? Then nothing reflects and it looks black. After all, without reflection there is no light to see and without light it is dark. Just as ‘white light’ does not exist, neither does ‘black’. It is just the absence of light.
Now we replace the ‘white’ sunlight to make the rainbow with a lamppost.
One of those old-fashioned yellowish Sodium lamps, not that modern LED. With this light, 2 thin yellow lines remain of the rainbow. (a) Why exactly these 2 lines has to do with quantum mechanics, for now assume so.
Because the same process can also be reversed when white light is passed through sodium gas, in exactly the same place where the 2 luminous lines were before, there are now 2 holes in the rainbow. Sodium absorbed light here. Each material has its own unique ‘barcode’. This way, we know what materials the sun or stars are made of.
Back to lasers
Back to lasers. A laser makes light of exactly 1 colour and, thanks to the rainbow, we know that Sodium has 2 absorption spots. If the laser light is a different colour from where Sodium absorbs then our laser does nothing at all. After all, sodium allows this colour, and therefore all light energy, to pass through. With a yellow laser, Sodium absorbs all the light and thus all the energy and converts it into heat. With enough energy (heat), Sodium will melt, evaporate or turn to plasma.
If we look at the absorption rate of aluminium and iron (main component of stainless steel) for the exchange laser (Fiber, wavelength is 1064nm), it can be called good. For iron better than aluminium, but for both more than enough to convert light into heat and thus melt, evaporate or plasma. With gold (Au), silver (Ag) and copper (Cu), this laser has more difficulty. While steel (Acier) goes easier.
A pickle is 95% water, and certainly pure water does very little with the light from the metal laser. And where gold reflects the unused light, water simply lets the light through (transmission). And that last 5 per cent of pickle? A lot of trace elements with mainly Sodium. With enough current, a pickle gives off yellow/orange light. If we measure that light, it gives exactly the same wavelength as the Sodium street lamps!
This means that a pickle just passes the light of the metal laser. The laser does not ‘see’ the pickle which is why I always put a metal plate under the pickle for the trick, I know the laser light goes right through it and quietly starts engraving the bottom of the casing! (1)
Absorption
Absorption is an important factor in determining which laser is needed for best results. After all, without absorption, the light does nothing and the laser cannot do anything. An absorption rate may be viewed roughly as a percentage. At 0.20, 20% of the light energy is converted into heat. Looking purely at the diagram, the shortest wavelength would be best. With this data, it is easy to determine whether another laser, with more or less power, can achieve the same result. A 60Watt with 10% absorption is just as good as a 30Watt with 20%. Include the price in the equation and we will know which laser is the most interesting financially.
The most common lasers are the Fiber and CO2 lasers. As explained for the pickle, the absorption rate for a Fibre laser is very low for water and non-existent for sodium. For engraving most common metals, it is very suitable, also because increasingly powerful lasers are being developed.
The CO2 laser produces light with a wavelength of 10,604nm, or 10.6um for short. This is light in the infrared and infrared is better known as heat. An infrared lamp is also called a heat lamp and is widely used for chicks, reptiles, keeping food warm and sore muscles. A CO2 laser is actually a very precise infrared lamp.
So this means that humans, animals and food can convert infrared light into heat, i.e. absorb it. And that is what a laser needs, and for organic materials, a CO2 laser is quite suitable. Basically anything that comes from nature. From stone, glass, wood and leather to vegetables, fruit and eggs.
*pleasant fact, a Fiber laser transports the laser light from the laser source to the mirrors via a fibre optic cable, given the absorption rate of CO2 on glass, a Fiber laser can never be a CO2 laser. The fibre-optic cable is made of glass, hence the name, and is said to absorb all light energy.
At Lion Laser Systems, in addition to the CO2 (Match/Merlin) and the Fiber (Fiber/Alpha Metal), there is a 3rd type of laser, the UV. With an ingenious trick, it is possible to make the wavelength of the Fiber 3 times shorter, 355nm. (1064/3=354,67) This short wavelength gives a very high absorption on metals, and even though UV lasers are nowhere near as powerful as a Fiber, the absorption compensates for this enormously.
The UV laser can handle virtually any material, even organic materials absorb UV radiation. The reason one gets tanned in the sun is indirectly due to UV radiation. Like burning, which strangely enough has nothing to do with the huge amount of infrared (heat) radiation the sun also gives off. But behind a pane of glass, you don’t tan, and indeed, engraving on glass goes very well with a UV laser.
Plastic engraving also goes very well with a UV laser. Outside in the sun, garden chairs lose their colour and car headlights turn matt white. With the sun, this can take years, with a UV laser, all those years are shot at the plastic in a millionth of a second and the same thing happens. Only with extreme precision and accuracy. Transparent plastic becomes matt-white and coloured plastics lose their colour where it is possible, with the right settings, to decolourise the plastic either white or black. However, the latter has more to do with the properties of the plastic than the absorption rates. Even though that remains important, after all, without absorption the laser can do nothing.
Although the Lion-UV is a fantastic all-rounder and can handle almost every material imaginable seemingly effortlessly, it is only for plastics that it is the first machine we test on. It is usually more financially interesting to take the Fiber for metals and the CO2 for organic materials.
Legend
(1) Electric Pickle Demo: Watch video