18 – Different Lenses – Different Energy Densities – Different Uses (25:49)

The Lightblade Learning Lab with Russ Sadler

The Lightblade Learning Lab is a series of videos that Russ did for Thinklaser Limited based on using the Lightblade 4060 Laser Cutting and Engraving Machine. Thinklasers Lightblade 4060 has a 400 x 600mm bed size and was supplied with a 60W EFR laser tube. In this session, Russ takes a look at laser focusing lenses, their properties, intensities and their different uses.

laser focusing lenses and their different uses
Laser focusing lenses and their different uses

Contents

  • Looking at the beam and where its energy is concentrated
  • Energy density
  • Size of beam when focused – grows with focal length of lens
  • Reason for using lenses with different focal lengths
  • The concept of ‘working length’
  • Using the focus ramp (see Video 10) to show how the energy density varies with the distance from the lens
  • Setting the height of the lens from the work and the power of the laser
  • Looking at the scorch lines on the ramp in close-up
  • Building up a picture of the beam’s profile with different lenses
  • Looking at the Watts per square millimetre
  • The damage threshold of various materials
  • The effect of speed and using the right lens
  • The reason that the Lightblade customer is provided with 3 lenses
  • Cutting parameters

My thanks go out to Tom at Thinklaser for giving permission to embed these videos on this site. If you are looking for a new laser machine from a quality supplier, then I would suggest you check out their website: www.thinklaser.com.

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Focus Ramp

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What can a laser cutter cut?

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Transcript For Laser Focusing Lenses

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00:14

Welcome to another Lightblade learning lab,

00:18

today we’re going to be talking about

00:21

lenses again but lenses are very

00:24

important because they focus the light

00:27

energy or the beam energy that we’ve got

00:30

coming out of this machine down into a

00:33

very small point which is where the work

00:36

is done the beam itself is as we found

00:40

out maybe eight nine ten millimeters

00:43

diameter, it’s quite large

00:46

most of the energy is concentrated in

00:48

the central section probably within the

00:50

central fifty percent of the beam, all the

00:52

energy that we can collect is

00:54

concentrated down through the lens into

00:57

a very very small area here I’ve got some of my

01:01

hairs which you can hardly see now if I

01:03

measure one of those hairs we can see

01:06

that it is point oh four point oh five

01:09

millimeters diameter double that up to 0.1 and

01:14

that’s the size of the laser beam

01:16

that we are probably going to get on the

01:19

best lens that we can probably find for

01:22

this machine and that’s a one and a half inch

01:24

Lens for this machine, now when you start getting up to a

01:28

two inch. two-and-a-half inch and a four inch lens

01:32

those numbers start growing considerably

01:36

maybe up to 0.2 or 0.3 even now 0.3

01:41

is still not very thick but it makes a

01:44

huge difference to the way in which the

01:47

power is delivered onto the surface now

01:50

look here I’ve got

01:53

a tool that I’ve just picked up out of my workshop

01:55

it’s got a very sharp point on one end

01:58

and a blunt section on the other end. Here

Transcript For Laser Focusing Lenses (Cont…)

02:02

I’m going to press as hard as I possibly

02:05

can on that card and wobble it around

02:08

with that end

02:12

how much of an impression have I made? Not

02:15

very much

02:17

now i’m going to press the same amount

02:19

of energy because I haven’t changed my

02:22

strength and I was doing exactly the

02:25

same thing with the other end

02:27

now there’s a pretty significant hole in

02:32

there

02:36

that’s nearly through to the other side

02:38

this one

02:39

well it’s hardly marked the surface there’s a

02:42

mark on there but I can feel that

02:44

it’s just the smallest dent and this is

02:47

a very soft card so I would’ve expected

02:49

to have caused more damage than that but

02:52

the reason why I haven’t caused more

02:54

damage is because the energy that I’m

02:56

able to put into that is spread over a

02:59

much much larger area where as when I do

03:04

it this way around the energy is

03:06

concentrated into that point

03:10

so the point is an energy concentrator

03:12

basically what we’ve got we’ve got a

03:14

much higher and you’ll hear me using

03:16

this term a lot the energy density the

03:21

energy per square millimeter is a lot

03:24

greater when i do that then it is when i

03:28

do that

03:29

and that’s the principle of why we

03:33

use different lenses and you say well ok

03:37

if we use a lens that produces a shape

03:43

like that a very thin hair like beam

03:48

we’ve got huge energy density we can do

03:50

a lot of damage to the product

03:52

why don’t we just use one of those? The

03:56

answer is in this very crude diagram that I’ve put

03:58

here we may be able to concentrate the

Transcript For Laser Focusing Lenses (Cont…)

04:02

energy in a very small area as opposed

04:05

to the larger area that we saw

04:08

demonstrated in my mechanical

04:10

demonstration but the 1.5 inch lens

04:13

focuses the light like this it’s

04:18

a very sharp shaped lens which goes down

04:23

to a sharp point but it goes down to

04:25

sharp point quite quickly and expands

04:28

again very quickly so it’s useful

04:31

working length is actually very short

04:37

now this one which is the 2-inch lens

04:40

okay it’s got a bigger footprint

04:42

but it’s working length where the energy

04:47

density remains reasonably constant is

04:52

larger and this one which is the

04:55

two-and-a-half inch has got an even

04:57

shallower beam shape and it has a

05:03

working length which is much longer

05:08

so you’ve got these strange properties

05:09

associated with the focused light you

05:13

can either have lots of energy over a very small

05:15

length or you can have less energy over

05:18

much longer lingth and this is the

05:21

trade-off you’ve got between lenses. Now

05:23

what we’re going to do today is i’m

05:27

going to try and turn these drawings into

05:32

a real picture in other words I’ve got

05:35

my focus gauge here which runs from the

05:39

focal point plus 4 millimeters to the

05:43

focal point minus 4 millimeters

05:50

ok now my intention is to draw a line

05:54

along here and hopefully if I get it

05:58

right

05:59

I will manage to get the focal point in

Transcript For Laser Focusing Lenses (Cont…)

06:01

the middle and so we can see that the

06:03

change of line thickness and power or

06:07

density, energy density changes as we

06:10

move along the line because at this

06:12

point here we’ve got a big footprint and

06:16

at this and we’ve got a big footprint

06:18

we’ve got a very small one in the middle

06:19

so that means their energy density at the

06:21

top here is very small, in the middle

06:25

it’s very high and we get less of a

06:28

change with these because although we

06:31

might be starting off at the same length

06:33

the same diameter we don’t get we come

06:36

down in a much shallower curve. Right now

06:38

I’m just going to do a quick pulse test

06:40

to make sure that my beam is

06:42

approximately in the right place to give

06:44

me good consistent power

06:51

that looks pretty good now I’m not going

06:53

to bore you to death with hundreds and

06:55

hundreds of results what I’m going to do

06:57

is to show you my method and then we’ll

06:59

work on from there, i will carry on

07:01

working and show you the end results now

07:04

what I’ve got here is my focus ramp

07:06

which takes me from zero in one

07:11

millimeter steps out to 4 millimeters plus and

07:14

4 millimeters minus. Now I’m gonna pop

07:18

that in there and actually what I’ve got

07:20

this time I’ve got some card and it’s

07:23

one millimeter thick card so it’s nice

07:25

and stiff so that when I pop it in here

07:28

there is no chance that it’s going to

07:32

flex in any way at all

07:36

in other words this surface here is

07:37

going to be as flat as the reference that

07:40

i can produce on here and i’ve also used

07:42

my metal surface here my flat metal

07:44

surface because i know that that is true

07:48

well I can see that I’ve got a 2-inch

07:50

lens here and a two and a half inch

07:53

lens so by default that one must be a

07:56

one-and-a-half inch lens and what we’re

07:59

going to try and do to put to start with

Transcript For Laser Focusing Lenses (Cont…)

08:01

that I’m going to turn this over the

08:06

other way like this and I’m going to try and

08:09

set this so that the power just burns

08:13

through somewhere around about the focal

08:17

point

08:17

ok so we’re starting off these tests with

08:20

a speed of a hundred millimeters per

08:24

second and a power of 25% now I’ve got no

08:29

idea what that is in terms of watts but

08:31

we’ll sort that out later, I’ve got the focal

08:34

point set to 7.5 millimeters

08:45

ok let’s look at our result

08:49

have we got what we’re looking for yes

08:51

we have now what we’ve got here we’ve

08:56

got something that runs from two and a

09:00

half plus to about 1 minus, it’s about a

09:07

millimeter too high so what we’ve got to

09:10

do to put that back in there again

09:12

instead of seven half millimeters we

09:16

really ought to drop that down to six

09:18

and a half millimeters same speed same

09:22

power

09:25

and there we go that we’ve moved it down now

09:27

we’ve gone from we’re roughly one and a

09:31

one and three quarters to two and a

09:36

quarter so we may well have gone too

09:38

much but we’re now going to do is we’re going

09:41

to change the power slightly because

09:43

this has cut through as you can see this

09:47

is cut through and what I want to do is

09:49

try and decrease that length now ah there we go

09:53

right up the middle

09:54

fifteen percent we’re just about making

09:56

it through just about making it through

Transcript For Laser Focusing Lenses (Cont…)

10:00

and would you believe it looks as though it’s

10:02

about minus one and just about +1 maybe

10:06

one and a half, so we still might be a

10:08

little bit out on centering but that’s not

10:12

bad

10:13

so we do a result on the front so we can

10:15

see what the dimensions are now this is a

10:19

two-and-a-half-inch lens test which is

10:20

typical for the other tests that just

10:22

been doing

10:24

so the first thing I’m going to do is to

10:26

set the power and the power has got to be

10:31

set up to 17%

10:37

now we do it this way just so that we’ve got

10:39

something to measure on the front

11:00

so that’s 10mm/s

11:03

focus is 5.5

11:06

here we are looking at the scorched line for the

11:09

2 inch focal length lens now this is the

11:13

line as it entered the bottom of the

11:16

ramp this is four millimeters below the

11:19

focal point now you’ll see that they

11:23

look like a couple of tram lines along

11:24

there but when we look a bit closer and

11:28

tip the card up you can clearly see

11:31

that it’s a V-groove cut by the laser

11:35

itself into the card but i’d like you to

11:37

look at the top of the V and I think

11:39

you can see there’s a very small almost

11:41

a filleted radius on the top of the V where

11:44

the power has dropped off quite

11:46

dramatically now I’m going to basically

11:50

for these measurement purposes I’m

11:53

going to ignore the little fillet radius

11:56

on the top of the V because that

11:58

basically has got no significant power

Transcript For Laser Focusing Lenses (Cont…)

12:01

it’s enough to scorch the edge but that’s all

12:03

it is doing so what I’m really

12:05

interested in is the powerful cutting

12:07

piece towards the center where the V is

12:09

going all the way down from the vertical

12:11

now what i’m going to do is measure

12:13

all positions between minus 4 and plus 4 to

12:16

get nine results and then we’ll take a

12:20

look at the shape of the line now I have a

12:23

glass graticule which is marked off in

12:25

point 1 millimeter divisions it’s

12:28

possible with this microscope to

12:30

reasonably accurately measure the

12:31

dimensions

12:32

I mean I’m once i get below point one of

12:36

a millimeter which is the division that

12:37

you can see on here you can easily

12:39

estimate to half a tenth which is

12:41

.05 point .025 well that’s a little bit

12:46

flaky but I’ve attempted it in a few places

12:49

well here we are measuring the dimension

12:51

right at the end which is the minus 4 position

12:54

then we’re moving along to the minus

12:56

three position quickly taking it all the

12:59

way through to zero over the course of four

13:02

millimeters I think you can see how

13:03

dramatically the the line has changed so

13:07

just in case it’s confusing with the

13:09

graticule in the way i’ll show you a

13:10

series of pictures we start from the

13:12

lens here at four millimeters and then

13:15

three millimeters

13:16

and then two millimeters and then one

13:19

millimeter and then 0 that’s how the beam width

13:23

is changing as it goes from four down to

13:26

zero the focus point, well after a lot of

13:28

tedious repetitive work we finish up

13:31

with this rather daunting array of

13:33

figures here now don’t get too upset

13:36

because it’s actually very very simple

13:39

let me just explain let’s start off here

13:41

at this two-and-a-half inch focal length

13:43

lens and what we find is we’ve got the

13:46

focus point here at zero and then we’ve

13:48

got plus or minus 4 millimeters above

13:50

and below the focus point now the beam

13:52

diameter translates to a beam area and

13:57

this is the area in square millimeters

13:59

in this next column and then what we’ve

Transcript For Laser Focusing Lenses (Cont…)

14:02

got across here are 10 20 30 all the way

14:06

through a 100 watts of power that could be

14:10

going through the lens so if we had as

14:15

in my case a 60 watt tube, that doesn’t

14:17

mean to say there was 60 watts going

14:18

through the lens but let’s just stay

14:20

with 60 watts now right at the focal

14:23

point we have the smallest possible beam

14:26

that we can get and in this instance it

14:29

was 0.18 of a millimeter now we

14:32

translated that diameter into an area

14:35

square millimeter area so if we squeezed

14:38

10 watts into 0.25 square millimeters we

14:45

will finish up with 393 Watts per square

14:49

millimeter it’s just a standard

14:51

definition and that is called energy

14:54

density now we’ve calculated the energy

14:57

density for different wattages for that

15:01

same spot size and you can see how the

15:03

energy density creeps up why do we and

15:06

why are we interested in energy density

15:08

every particular material will have a

15:13

threshold of damage, burning damage

15:16

because basically that’s what our beam

15:18

is doing its burning the material now

15:21

that threshold maybe a thousand watts

15:27

per square millimeter it maybe 10,000

15:29

watts per

15:30

square millimeter it’s difficult to say

15:32

because there is no information out

15:34

there on the internet which tells me

15:36

what this burning threshold is now for

15:40

this exercise I used card which had

15:43

some sort of substance or body to it, it

15:45

was one millimeter thick and it required

15:47

a noticeable amount of power to cut

15:51

through it the amount of energy required

15:54

to cut through that card will ultimately

15:58

be defined by the energy density

Transcript For Laser Focusing Lenses (Cont…)

16:01

itself that it can resist but i’m

16:05

suspecting that card the card that I

16:08

used is probably sitting here with an

16:11

energy density of probably somewhere in

16:13

the region only maybe four or five

16:16

hundred now if we were to draw a line a

16:19

vertical line through those three graphs

16:22

at roughly where my arrow is which is

16:25

about 500 watts per square millimeter i

16:30

suspect that at 500 watts per square

16:32

millimeter i would probably be somewhere

16:36

near the damage threshold for that card

16:39

but the reason why I want to bring that

16:42

to your attention is because each one of

16:45

these lenses and we’re looking at the 60

16:47

watt energy density for the lenses is

16:51

capable of exceeding the energy density

16:54

required to damage that card so that

16:58

means that any of those lenses i could

17:02

use to cut the card

17:04

ok something else rather interesting

17:08

about these pictures is the way that

17:11

they actually describe the type of

17:15

lens they are imagine these to be

17:19

three different types of knife one of

17:21

them very sharp like a scalpel this one

17:25

being a bit like a pain knife and this

17:29

one being more like a butter knife now i

17:33

think if i give you that analogy you

17:36

will clearly see that it doesn’t require

17:38

much effort to cut something with a

17:40

scalpel

17:41

it will require more effort to cut the

17:44

same thing with a penknife and a lot

17:47

more effort to try and cut it with a

17:48

blunt butter knife, that will help you to

17:51

visualize how and why you would want to

17:54

use a certain type of lens we’ve already

17:59

seen this sharp lens can cut with as

Transcript For Laser Focusing Lenses (Cont…)

18:02

little as 17 watts whereas in fact cutting

18:06

the same material with a blunt lens takes

18:11

54 watts and I hope that that clearly

18:17

describe the difference between these

18:18

sets of lenses now I, now I need you to

18:21

do a little bit of mental gymnastics

18:23

because i want to bring you back to the

18:26

fact that i used a hundred and ten

18:28

millimeters per second speed to do all

18:31

of these tests i could have used this

18:33

and lets call it a blunt lens which

18:36

takes 54 wats to cut through the

18:39

material at hundred and ten millimeters

18:41

a second i could have used 15 watts to

18:46

cut through the same material but i

18:50

would have had to do it at very very

18:52

much slower speed it certainly has more

18:55

power than the damage threshold of the

18:58

material so it will definitely cut the

19:01

material but it won’t cut it as

19:03

efficiently if we were to use the red

19:06

lens with 15 watts of power we would

19:10

probably have to run it maybe ten maybe

19:13

even less than that millimeters per

19:15

second to achieve a cut

19:17

where as cutting that same cardboard

19:19

with the sharp lens we can do it at a

19:23

hundred and ten millimeters per second

19:25

that again hopefully reinforces why you

19:29

would want to use the right lens for the

19:31

right job if you’re only ever cutting

19:34

thin materials

19:35

why would you ever go and use it two and

19:37

a half inch lens or a two inch lens when you

19:40

can slice through it at a much faster

19:42

speed with a one-and-a-half inch lens so

19:44

if you’re only using one of the half and

19:46

two millimeter thick material it’s a

19:48

no-brainer if you’ve only got a 60 watt

19:50

machine

19:51

so I hope these illustrations have

19:54

demystified why you were supplied with

19:57

three lenses and why you might want to

Transcript For Laser Focusing Lenses (Cont…)

20:00

seek a fourth lens in your armory

20:03

depending on the power of machine that

20:05

you’ve decided to buy so longer length

20:08

lens requires more powerful machines and

20:11

more powerful machines means you can cut

20:13

thicker materials so i hope this begins

20:15

to remove some of the mystery about why

20:18

you would want different types of lenses

20:20

ok now there was another very good

20:23

reason why I’ve approached the tests in

20:27

the manner that I did, i used a thickish

20:30

card i use a constant speed but what I

20:34

did was to vary the power to just get a

20:40

cut through the material for each one of

20:43

these three lenses technically what that

20:45

means is i was using the card as a

20:49

measure of the energy density that was

20:53

in the beam in other words i had to have

20:55

the same amount of energy density

20:57

available in each one of these lenses to

21:01

just cut through the card and what we

21:04

can do is we can look at the results the

21:06

backward way round

21:08

i was using seventy percent power which

21:10

when I look it up on my calibration

21:12

chart which I’ve got for the tube it was

21:15

approximately 62 watts now we assume

21:19

that we’re going to lose power through

21:21

the mirrors there’s three mirrors each

21:24

losing three percent let’s just assume

21:26

and a lens which could be another three

21:29

percent so we could be losing as much as

21:32

twelve percent through the transmission

21:34

system before we get down to the work so

21:37

the available power at the work could be

21:40

as little as 54 watts now I don’t

21:43

know this for a fact because I didn’t

21:45

measure it

21:46

54 watts divided by 0.25 square

21:49

millimeters gives us an energy density

21:52

of 2160 watts per square millimeter

21:56

when we do that same calculation for the

21:59

2-inch lens where we were able to use

Transcript For Laser Focusing Lenses (Cont…)

22:02

twenty percent power

22:04

which was 28 watts less the twelve

22:07

percent brought it down to 25 watts so

22:11

we divide 25 Watts by the area of the

22:14

beam at the focal point we get 2080 watts

22:18

and similarly when we do the same result

22:21

here for the one-and-a-half inch lens

22:23

we find ourselves using sixteen percent

22:25

power which is equivalent to 17 watts

22:28

less the twelve percent is 15 watts and

22:32

when we do that calculation again with

22:34

the area of the footprint we find that

22:38

we get two thousand three hundred watts

22:40

per square millimeter now I hope you can

22:43

see that it’s not perfect but it’s in the

22:48

right sort of region we’ve got a typical

22:51

we’ve got a typical energy density here

22:56

which we’re using to damage this material

22:58

which is a round about 2,100 2,200

23:01

watts per square millimeter now this

23:05

was never meant to be a perfect

23:07

scientific experiment and I never

23:08

expected to get results may be quite as

23:11

good as this but it does demonstrate

23:13

clearly that that material regardless of

23:18

the lens that we fire at it has got the

23:20

same damage threshold it would be good

23:23

to know what the damage threshold for

23:26

different materials is because then we

23:28

could exploit the lenses and can predict

23:30

what the lenses were going to be capable

23:32

of now you may consider that’s going to

23:34

be a bit of a futile waste of effort

23:36

considering the next subject that we’re

23:38

going to be looking at is cutting

23:41

parameters cutting parameters are

23:43

something that you do not predict

23:45

you laboriously sit down with your machine and

23:47

work through the variables and once

23:49

you’ve got an ideal set of data you log

23:52

that data and that then becomes your

23:55

cutting parameters until the next

23:57

session when we’ll be talking about

23:59

cutting parameters

Transcript For Laser Focusing Lenses (Cont…)

24:00

thank you very much for your attention

24:02

today and hope this has been of some

24:04

benefit to you

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Last updated August 26, 2021

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The information provided by n-Deavor Limited, trading as Laseruser.com (“we,” “us” , or “our”) on (the “Site”) is for general informational purposes only. All information on the Site is provided in good faith, however we make no representation or warranty of any kind, express or implied, regarding the accuracy, adequacy, validity, reliability, availability or completeness of any information on the Site.

UNDER NO CIRCUMSTANCE SHALL WE HAVE ANY LIABILITY TO YOU FOR ANY LOSS OR DAMAGE OF ANY KIND INCURRED AS A RESULT OF THE USE OF THE SITE OR RELIANCE ON ANY INFORMATION PROVIDED ON THE SITE. YOUR USE OF THE SITE AND YOUR RELIANCE ON ANY INFORMATION ON THE SITE IS SOLELY AT YOUR OWN RISK.

The Site may contain (or you may be sent through the Site) links to other websites or content belonging to or originating from third parties or links to websites and features in banners or other advertising. Such external links are not investigated, monitored, or checked for accuracy, adequacy, validity, reliability, availability or completeness by us.

WE DO NOT WARRANT, ENDORSE, GUARANTEE, OR ASSUME RESPONSIBILITY FOR THE ACCURACY OR RELIABILITY OF ANY INFORMATION OFFERED BY THIRD-PARTY WEBSITES LINKED THROUGH THE SITE OR ANY WEBSITE OR FEATURE LINKED IN ANY BANNER OR OTHER ADVERTISING.
WE WILL NOT BE A PARTY TO OR IN ANY WAY BE RESPONSIBLE FOR MONITORING ANY TRANSACTION BETWEEN YOU AND THIRD-PARTY PROVIDERS OF PRODUCTS OR SERVICES.


AFFILIATES DISCLAIMER

The Site may contain links to affiliate websites, and we receive an affiliate commission for any purchases made by you on the affiliate website using such links. Our affiliates include the following:

  • makeCNC who provide Downloadable Patterns, Software, Hardware and other content for Laser Cutters, CNC Routers, Plasma, WaterJets, CNC Milling Machines, and other Robotic Tools. They also provide Pattern Files in PDF format for Scroll Saw Users. They are known for their Friendly and Efficient Customer Service and have a comprehensive back catalogue as well as continually providing New Patterns and Content.
  • Cloudray Laser: a world-leading laser parts and solutions provider, has established a whole series of laser product lines, range from CO2 engraving & cutting machine parts, fiber cutting machine parts and laser marking machine parts.
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