LASER Power and Slime-balls

I don’t yet have my new “toy” (LASER engraver). However, I have done a LOT of investigation into the current generation of low power “diode” LASER engravers. Small open frame designs with output power between 1.5 watt to about 6 to 7 watts. I will arbitrarily set this output “class” size to from 1 to 10 watts.

There is a marketing “trick” with this class of output diode LASER machines. It is very common for diode LASERs to be advertised, by stating in large typeface, their INPUT wattage. Typically, between 7 watts to 40 watts.

This advertising does seem to fool a small percentage of ignorant novice buyers. Remember ignorance is not the same as stupid. The reality is anyone with basic knowledge of electrical and LASERs will quickly see through this advertising ploy. The typical power supply for a 20-to-40-watt input LASER is 12 volts at 3 to 3.5 amps which is 36 – 42 watts. System output efficiency will always be under 20%. There are stepper motors and a computer that are also consuming some of that input power. Diode LASER efficiency is probably more like 15-20%.

Here is another thought. Incandescent lightbulbs were always sold by the power they consume. 40-watt, 50-watt, 100-watts.  Not how much light they produced. However, we now have output (in Lumens) on all lightbulbs. A better reference now that we are on the LED lightbulb standard. This shows that rating a LASER by input is a very lame throwback.

Yes, it is slime-ball advertising. Knowledgeable buyers will see this immediately by reading the fine print. Another favorite marketing ploy is to state LASER output in milliwatt (1/1000 watt). A 5000 mW output is 5 watts.

I have also seen some of these diode LASER modules marketed as (example) 15 Watt – 15000 mW at prices for the module (only) of >$500.00. It’s intended (with the high price) to appear to be a 15-watt input with a 15-watt output LASER. This is of course impossible and total fraud. This was on a China based vendor website. 

My point is this is not “safe” marketing as we expect in the USA.  It is very much Caveat Emptor (Buyer Beware) and customers must do the homework and understand exactly what is being offered. Good buys are possible but there will always be a risk.

Ok, so we should be over that output power hump. I am not an expert on LASER knowledge and operation skills. I do understand electrical power and energy system efficiencies.

Focus On This

One thing I have taken notice is the focal length and focus size of the LASER burning spot. Most applications lasers, whether cutting or engraving, require some degree of focusing the beam. The reason being that what actually does the cutting for you is not the raw power in the beam, i.e. 5 Watts, but it is the power density at the focal point, i. e. how many Watts per unit area.

5 Watts in a 1mm diameter (0.5mm radius) spot = 5watts / (A=pi®R^2) = 6.366 watts per sq. mm. 5 watts in a 0.1mm spot is 636.6 watts per sq mm.  This is called effective power density. A 5-Watt LASER is now a 637-Watt LASER in a 0.1mm spot!! Well, not exactly. It is still a 5-Watt LASER.

This begs the question of how is the advertised diode LASER output power being determined? True output power or ERP (effective radiated power)? Is it 5 watts per square centimeter or 5 watts per square millimeter? Area of a square millimeter is not the same as a 1mm circle which is 0.785 square millimeters.

One resource I referenced reports the output is measured by a thermal sensing power meter. This means total output power absorbed or measured as a quantity of heat over time. Spot size is not a factor except it must be wide enough not to damage (burn) the test meter. My calculations (above) are procedurally correct.

The point I am making is the smaller the spot the more burning effective a low power LASER becomes.

Another factor seldom mentioned is depth of focus. Projecting light is like photography. When projecting light to a focal point, the longer the focal length the deeper (narrower, longer) the reasonable sharp area of focus. Wide angle is only good for getting close with the lens. We have no adjustable F-stop in the LASER lens for depth-of-focus. We want all the light power as possible to pass through the lens aperture, called a fast lens in photography.

With LASERs we want long focal length to keep the lens as far away from the mess of the smoke and splatter of the burning process.

Moving the beam spot higher or lower than the focal point increases the spot size and reduces the power density. The depth of focus is what limits how deep a LASER can cut or engrave. A LASER focused on the surface ALWAYS has a wider and less powerful spot at the bottom of the cut.

Yes, there is a lot of math and science in truly understanding how a LASER functions. This post is only a hint of all there is to learn. Understanding the science is the difference between an operator and an expert. 

My goal is to study, practice, and move closer to the expert level…

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