27 – Understand How to Engrave Pictures – Part 1 (44:03)

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 will teach us how to laser engrave a picture.

How to laser engrave a picture
How to laser engrave a picture

Contents

  • Russ has already documented his learning process on the following RDWorks Learning Lab videos:
    • 104 – Dithered Graphics Not What They Seem
    • 105 – How Shapely are your Bitmap Dots
    • 106 – Going Dotty Part 1
    • 107 – Going Dotty Part 2
    • 108 – Going Dotty Part 3
    • 109 – Joining the Dots Part 4
  • Scanning two images – a comparison between two different techniques
  • Looking at the different ways they were produced in RDWorks
  • Outline fonts (vectors) and bitmaps
  • Running a simulation to how the laser would engrave them
  • Pixels
  • Resolution and PPI (pixels per inch)
  • Converting between PPI and Pixels per Millimetre
  • Increment in mm (0.0847)
  • Dots per inch
  • Bitmaps create dots on the laser, vectors create lines
  • Increment with vectors determines distance between lines
  • Example of a piece of Perspex with a bitmap image of dots
  • Common focal lengths and their theoretical spot sizes
  • Effect of power on dot size
  • For fine resolution we need a short focal length lens and very low power
  • Effect of speed – elongation of dot
  • Narrowing effect of speed on a more powerful pulse
  • Different shades of brown on paper with different powers
  • Doing the calculations to get the parameters for good clean dots:
    • it works out that a ‘golden’ number is 5 ms/pixel of the picture
  • Effects of different materials
  • Dot sizes in relation to different resolutions
  • Looking at how fast the laser can pulse, a test piece in Perspex to show this.
  • Profiles of individual pulses versus ones which are fired in quick succession.
  • The effect of the time it takes the beam to switch on and off.
  • The ‘pseudo greyscale’ effect.

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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Transcript for How to Laser Engrave a Picture

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00:13

Welcome to another Lightblade Learning Lab, now today I’m going to attempt the

00:19

impossible

00:22

now you may laugh at this but I’m going to attempt to tell you virtually

00:27

everything that I know about graphics and how we deal with graphics on this

00:32

machine not I can hear you’re laughing because you think that I probably know

00:36

about that much about graphics because I’ve purposely steered away from the

00:41

subject up to now. We’ve talked a little bit about it and as you can see I can

00:47

make a moderate attempt at getting a picture down onto a piece of material

00:52

and I can show you quite a few more examples but that does not mean to say

00:57

that I actually know what I’m doing that was a bit like shooting bullets in the

01:02

dark yeah I fiddled around with a few parameters and I’ve got some reasonable

01:08

results which is promising for you guys because it means you don’t need to know

01:12

very much to get some sort of result but the problem there was I was having to

01:19

fiddle around to get the parameters so I decided I will embark upon trying to

01:26

understand what graphics was all about now I have documented all my mistakes

01:33

all my learning processes that I went through to get the information that I’m

01:38

going to pass on to you today if you want to go into great detail then you

01:42

should spend sorry you might want to waste a bit of time and look here at my

01:48

other video channel but to save you the tedium of doing that

01:53

I’m going to attempt the impossible today and put something like about six hours

01:58

worth of video into maybe three quarters of an hour

Transcript for How to Laser Engrave a Picture (Cont…)

02:03

so sit back be patient and we’ll see what we can do to explain this

02:09

subject to you in fairly simple terms in a way that you will fully understand what

02:15

this machine can and cannot do in the way of graphics ok now I’m going to scan

02:21

two images on this page and I’m going to give you a clue they’re both going to be

02:28

scanned at the same speed and they’re both going to be scanned at the same

02:33

power, power is about 4 watts coming out of the tube minus the efficiency of all

02:40

the mirrors and the lens so there’s probably only about two and a half to

02:43

three watts arriving at the paper surface here so it’s a piece of white card

02:48

with two and a half watts going down onto the surface

02:54

now what I want you to do is the usual powers of observation to see if you can

03:00

determine the difference between scan one and scan two

03:47

well, what do you think do they look any different?

03:52

I think you’ll say the answer is no but believe me they are very different in

03:58

the way in which the machine has produced them that’s what the whole of

Transcript for How to Laser Engrave a Picture (Cont…)

04:02

this session is going to be about the difference between those two pieces of

04:07

text that actually look the same but they’ve got completely different

04:11

characteristics let’s go and have a look at what the

04:15

program is that I used to send those two pieces of information down to the Laser

04:20

machine. OK well here we are in RDWorks and you can see the two words text

04:28

on the screen now if we look up here at the top right hand corner you’ll see

04:34

that one of them is called BMP and the other is called nothing and that’s

04:42

because that second piece of text has been produced like this 50 millimeters

04:47

tall and I will change the font so that it doesn’t get confused with anything else

04:52

and we’ll turn it into a script font of some sort there we go like that not very

04:59

pretty but it demonstrates the point that I’m making when you create a piece

05:05

of text in RDWorks it comes in as an outline font now that outline is

05:13

basically a vector graphic outline and let’s just make it blue for a minute to

05:20

demonstrate put it onto a blue layer now that blue layer can either be set as a

05:29

scan layer which it is here or I can set it as a cut layer like that it doesn’t

05:39

look any different the only difference is the machine will treat it differently

05:43

this cut layer is exactly what it says if you cut that word text it will fall

05:48

out and leave a hole in your material if we

05:51

convert it to a scan layer as I said it looks no different at all but this time

05:57

something different will happen now you can see the other two pieces down here

Transcript for How to Laser Engrave a Picture (Cont…)

06:01

and there’s the big one at the top let’s just do a simulation and you will see

06:05

what happens now it’s going to scan from the bottom and when it gets to this one

06:13

it’s going to scan it lines across the page so the hollow text there basically

06:20

means it’s uncommitted you can have it cut or you can have it scanned now what

06:26

about the black text underneath well it’s come in as a bitmap you can’t

06:32

change that because that’s all to do with how you produce the text outside

06:37

this program and this piece of text has been produced in something like

06:43

Photoshop or some other art package where you might produce your drawings

06:49

and you import them into here as a bitmap or as a JPEG or a PNG or some

06:56

other form of basically a bitmap type file in reality when I engrave these

07:02

they both look the same but here as a text file as you can see it’s pretty

07:11

blocky around the edge now I might be talking

07:15

down to a lot of you guys but there’ll probably be people out there that do not

07:19

understand what a bitmap is in relation to a vector file which is the text drawn

07:24

above it okay well here we are close in on our two pieces of test text and I

07:29

don’t think you can really see any raggedness around them around the edge of

07:34

the bottom set of text it’s still pretty clean that is solid black text now

07:42

although this is a very largely magnified part of a picture I think

07:46

you’ll be able to see that there is an eye in that picture with a pupil and

07:50

iris around the outside it is not a solid color it’s made up of lots of

07:56

little pieces now those little pieces are called pixels and every one of those

Transcript for How to Laser Engrave a Picture (Cont…)

08:02

pixels on that screen there is mapped with an x and y-coordinate

08:09

so that the computer knows whether one pixel on the screen has got to be turned

08:14

black or white so there’s a huge amount of data that the computer

08:19

has to process to store a picture like this

08:23

now let’s just go and take a look what I mean by pixels and this strange word

08:30

called resolution the resolution of a picture is described in terms of PPI and

08:37

I’m going to draw this one at 300 PPI pixels per inch now that’s the standard

08:46

industry format for resolution and this particular picture is 300 PPI

08:53

horizontally and it’s also 300 PPI vertically makes sense because they’re

09:06

squares now when it comes to working in RDWorks most of you guys will have

09:16

your machines set to work in millimeters if you’re one of the very few people in

09:22

the world which probably includes America that uses inches still you will

09:28

otherwise have to use millimeters what we need to do is to establish what the

09:34

size of one of these pixels is okay now I’ve scribbled on this piece of paper

09:39

here a very simple calculation that you can carry out that will enable you to

09:44

convert from pixels per inch down to a size of a pixel so the first stage is to

09:51

take 300 pixels per inch and divide it by 25.4 now there are 25 point four

09:57

millimetres in an inch so what we’re really trying to do is to calculate the

Transcript for How to Laser Engrave a Picture (Cont…)

10:02

number of pixels in a millimeter and the answer to that is eleven point eight one

10:10

millimeters in a pixel so that’s pp millimeter pixels per millimeter now if

10:17

we want to find out the size of a pixel if we know how many pixels there are in

10:21

a millimeter if we divide the millimeter by eleven point eight one pixels it

10:27

tells us that we’ve got 0.0847 millimeters

10:31

is the size of that pixel there so that pixel there is 0.847

10:42

millimeters square at some stage when you’re setting up your scan parameters

10:47

you will be asked the question what is the increment and that will be asked

11:01

for in millimeters that’s the answer to the question Oh

11:06

eight four seven because you want to step down one pixel at a time in the

11:12

y-direction Right now, the Machine cannot produce square shapes it doesn’t

11:20

work in pixels per inch although pixels per inch is the thing

11:25

that drives the machine what actually comes out of the nozzle are round dots

11:32

you’ve seen it for yourself when you do a pulse burn down onto a piece of paper

11:38

if you set the focus right you get the very very fine dot the machine works in

11:46

dots per inch and the resolution of the picture that you’re trying to produce is

11:53

in pixels per inch you would say yeah but the machine is going to

11:58

automatically convert squares into dots as it runs across the page well you’re

Transcript for How to Laser Engrave a Picture (Cont…)

12:04

absolutely right every one of these squares tells the machine to do

12:11

something if we look at these pictures they do match up it decides whether it’s

12:16

going to put down a black dot nothing a black dot nothing black dot black dot black dot

12:22

now you might assume that that’s pretty logical most people don’t

12:26

understand that the machine can and does work in dots when it’s using a bitmap as

12:33

a driver I’m using that word a little bit carefully as a driver because there

12:38

is another thing that can be used as a driver now I’m just going to draw two

12:43

that look approximately like circles and this circle here is made up with pixels

12:57

and this circle here is not made up with anything at all this is a vector because

13:10

it has only got a ring around the outside and this one is a BMP it’s a circle made

13:18

up of squares how the Machine sees a bitmap every time, every time it sees one

13:24

of these pixels it sends out a pulse or not pulse or not not when we’ve got a

13:34

black picture like this which is filled in it’s filled in with a series of black

13:41

dots and it’s the collection of black dots that make the image look solid now

13:48

that’s not the same when we’ve got the vector drawing the laser doesn’t go and

13:52

put dots on here what it does it decides it’s going to scan backwards and

13:57

forwards across this image like this and as it goes across it’ll draw a line with

Transcript for How to Laser Engrave a Picture (Cont…)

14:03

the laser so we get a line that goes across there like that

14:08

and then it comes back and we get another line and then we get another

14:12

line we do not get dots we actually get lines that go backwards and forwards

14:17

across the image like this now I’ve purposely drawn these as lines with big

14:23

gaps between them but what you do this is where you set your increment and you

14:29

can set your increment so that the lines join up and that then becomes a solid

14:35

black shape as well but as you can see the mechanism by which the bitmap is

14:40

produced and the vector shape is produced are completely different

14:44

principles and that is a very very important thing that people do not know

14:50

or understand it took me a long time to find out this difference this is a piece

14:55

of perspex looking from the back of the perspex at

14:59

an angle so that what we can see is a black rectangle that I had produced and

15:06

the black, the black rectangle is obviously made up of every single dot in

15:13

the bitmap and you can see those dots on here now when you look at it from the

15:19

front it looks like a solid black shape but when you look at it from the back

15:23

you can clearly see that that solid black shape has been made up by dots

15:28

that join together now that is a very very important principle that you need

15:32

to understand about producing graphics well we’re not really going to talk much

15:37

more about this one because this one is very simple just lines and spacing

15:44

increments this is the one that’s very very interesting to explore because if

15:50

you want to put a photograph down onto a sheet of any material you will have to

15:58

use a bitmap image to start with because that’s what photographs are the cleanest

Transcript for How to Laser Engrave a Picture (Cont…)

16:04

way of producing a picture like this is probably to do it in Coreldraw and it

16:08

may will then give you a DXF output so that you can produce a vector file which

16:14

you can then engrave across and this is a nice simple way of producing block

16:20

graphics logos text all the complications and difficulties come with

16:26

doing something which is a bitmap and this is what I spent a huge amount of

16:32

time, trying to understand this took me quite a long time to find out and I was

16:39

quite staggered to find every dot in the bitmap can produce a signal for the

16:45

laser beam to pulse this picture depends upon the size of the dot itself and the

16:53

resolution of the picture now there are many different focal lengths of lenses

17:00

that you can use in these machines the most common ones are one and a half inch,

17:05

two inch, two and a half inch and four inch now

17:13

every one of those lenses has something called a theoretical spot size now that

17:19

is the smallest size dot you can produce with that lens and a two inch lens has

17:28

got one which is a little bit bigger and a two and a half inch lens has got one

17:32

that’s even bigger still and the 4 inch lens has got one that’s even bigger this

17:38

is about 0.075 millimetres which in old money is point zero zero three, three

17:49

thousandths of an inch and this one is point one zero zero which in old money

17:55

is point zero zero four thousandths of an inch and so they go on and they get

Transcript for How to Laser Engrave a Picture (Cont…)

18:02

bigger and bigger as the lens gets bigger this is probably the lens that

18:07

most of you will have the Lightblade machine is fitted with one of these now

18:13

in terms of size you can see that it is very very little difference now I don’t

18:20

know whether you can see that but that’s two of my grey hairs there, that hasn’t

18:25

helped my bald patch has it? The size of that hair is roughly point zero zero

18:31

three, three thousandths of an inch or 0.075 millimetres so that just gives you

18:36

an idea of just how small the spot size can be and this at point zero zero four

18:44

you would not be able to see the difference between a 3 thou and a four thou

18:48

hair so don’t get too concerned if you’ve got a two inch lens in your

18:53

machine because it is not going to be a disaster when it comes to doing bitmap

19:00

graphics it makes a big difference to the area the energy density you

19:05

concentrate 60 watts into that size and then put 60 watts into that size the

19:11

energy density there is almost well it is twice as high as it is in this one so

19:16

in terms of cutting power this change makes a big difference

19:22

but in terms of Engraving power it makes no difference at all

19:28

okay now I’ve just drawn up a few little features on here having decided this was

19:35

roughly the thickness of a human hair and we don’t have to worry about it in

19:38

terms of resolution ability to produce a dot what we have got to worry about now

19:44

is power because we can produce these dots at very very low powers but as soon

19:50

as we start putting power into the dot the dot starts doing this gets bigger

19:56

and bigger and bigger now that’s not what we want if we want fine resolution

Transcript for How to Laser Engrave a Picture (Cont…)

20:03

pictures we need the smallest dots that we can possibly get ultimately what

20:07

we’re trying to achieve is a picture made up of dots different density areas

20:13

just as you see them in a newspaper photograph if you look at it very

20:18

closely you’ll see that is exactly that a series of dots and in fact we’ve

20:22

already seen that today here’s how the eye can be fooled much much further away

20:27

and you wouldn’t even see these dots you just see what looks like a photograph a

20:33

grayscale photograph and that’s one of the magic tricks that that happens

20:39

between the eye and the brain it has this ability to heal over all these

20:43

little imperfections and produce a lovely picture for you to look at so we

20:49

need the smallest dots we can get and to achieve that we really are looking for a

20:56

small focal length lens and very low power now how does speed affect it? Well up

21:04

to about a hundred millimeters a second the dots really don’t change too much

21:10

away from Circular but if you start going 150 200, 300 and 400 millimeters a

21:17

second then here’s what happens your dot turns into a sausage a short sausage and

21:25

then the faster you go the longer the dot becomes but this black set of

21:33

pictures that I’ve drawn here is not quite right because

21:35

there’s another phenomenon that occurs in parallel with that now if you use a

21:40

larger power to start with you get a bigger dot and if you start then

21:47

increasing the speed of that larger power here’s what happens you get a

21:52

sausage but the sausage is smaller than the dot and as you increase the speed

21:57

the sausage gets narrower and narrower and so you get back much more towards

Transcript for How to Laser Engrave a Picture (Cont…)

22:03

the original dot size here so consequently if we start looking at how

22:08

these lines sausages now sit on this picture here bear in mind we get a pulse

22:16

for every pixel these are dots but we’re still going to get a pulse here a pulse

22:25

here and a pulse here so that means we’re going to get a dot there a dot

22:30

there and a dot there the dots are going to start joining up and they’re no longer

22:35

going to be seen as dots like that what we see is that now that looks much more

22:47

like the vector scan but it isn’t because it’s still driven by dots and

23:01

those dots that drive this are producing some really weird effects now it would

23:09

be very very nice if we could produce dots and our picture look like this but

23:15

when we actually burn dots into a piece of paper what we get is something like

23:21

this now we’ve got brown dots slightly brown dots and lots of other different

23:31

grades of brown dots in the background but hey if I was producing a proper dot

23:36

graphic like this all these dots would look the same it would look like the

23:44

measles they would all be nice and black and at the same quality

23:48

the question is why are they all different shades of brown, different

23:54

power the shade of brown tells us the amount of power that’s gone into the dot

23:58

how much the paper has scorched when we’re doing dot graphics like this we

Transcript for How to Laser Engrave a Picture (Cont…)

24:03

have the power set to say eleven percent and eleven percent

24:10

Max and min there should be no variation in the grade of the dots they should all

24:16

be exactly the same little black dots but they’re not and the question is why

24:22

not now just coming back to this, this was done fifty millimeters a second very

24:31

slowly but it’s done with thirty percent power thirty-five or forty watts of

24:37

power coming out of the tube and this is a resolution of 100 pixels per inch

24:44

that’s what this picture was done at if we start going faster than that we lose

24:49

our individual dots and we get lines where the sausages are all joined

24:54

together so a hundred pixels per inch at 50 millimeters a second which enables us

25:00

to produce this lovely pattern here single dots is what I would class almost

25:06

as the Holy Grail that we’re looking for we’re looking for the ability to produce

25:10

good clean single dots to try and get my measles effect that I call it you know

25:16

we want nice uniform colored dots all over the picture so that we can produce

25:21

the same sort of picture that we produce with an inkjet printer just two colors

25:25

black and nothing so the background is left bare and every one of these little

25:31

burns is exactly the same color now that’s what we’re aiming for and this

25:36

looks as though it’s going to give us the parameters for achieving that so

25:41

here we’ve got 100 pixels per inch divide that by twenty five point four

25:45

means we’ve got three point nine four pixels per millimeter and so here we’ve

25:50

got fifty millimeters per second as the speed that we’re running it so if we

25:55

want to work out how long it takes for one millimetres worth of travel we take

Transcript for How to Laser Engrave a Picture (Cont…)

26:01

one millimeter we divide it by 50 milli we divide it by

26:05

50 and that basically gives us an answer of point zero two seconds per milliliter

26:12

20 milliseconds if we now take the 20 milliseconds in other words that’s how

26:18

much time it takes to travel a millimeter but we know that in a

26:22

millimeter we’ve got three point nine four pixels so if we take the time it

26:28

takes to travel one millimeter and divide it by the number of pixels in

26:33

that millimeter we get approximately let’s just call that four for example

26:38

four pixels per millimeter it’s nice simple calculation and we get

26:43

approximately five milliseconds per pixel so that’s the amount of time that

26:48

it takes to actually put one of these pulses down now if we start messing

26:57

around with anything faster than 5 milli seconds per pixel then which will start

27:03

producing something which is getting closer and closer to this so at the end

27:10

of the day the crucial thing is how much time we’re spending putting a pixel down

27:16

and that depends on the resolution of the picture and the speed at which we do

27:23

it if we want to remain at 5 milliseconds per pixel which really is

27:28

what I class is almost a golden number then we can either change the resolution

27:35

of the picture from say a hundred pixels per inch to 150 pixels per inch now that

27:40

we’re looking we can increase the resolution and make it a finer quality

27:43

picture but to do so what we’ve got to do is we’ve got to reduce the speed for

27:48

50 millimetres a second to 33 millimeters a second to allow the same

27:54

amount of milliseconds per pixel consequently we could also do it the

27:59

other way around we could make the picture much much coarser down to 50

Transcript for How to Laser Engrave a Picture (Cont…)

28:04

pixels per inch and then we could push the speed up to our hundred millimetres

28:09

a second which would still give us five milliseconds per pixel what I’m going to

28:14

do is to show you this information in a slightly

28:16

different way so Just remember we’ve got five milliseconds that we calculated

28:21

for 50 millimeters a second, a hundred PPI so for that resolution that’s how much time

28:28

we have for every pixel now what you see here is a relative set of resolutions

28:35

this is a hundred pixels per inch 150 300 and 600 PPI these are all drawn to

28:43

scale and what that means is the pixel size for 100 PPI 0.25 4 millimeters it

28:49

then drops to 0.169, 0.084 and 0.04, 0.04 is just over the thickness

28:57

of a human hair for every pixel so just to keep that in perspective now what

29:04

that means is as we’ve already seen at a hundred PPI we can calculate if we run

29:10

this at 50 millimeters a second and every one of these is run at 50

29:15

millimeters a second we’ve already calculated on the previous sheet that it

29:20

takes 5.08 milliseconds for a Travis across one pixel now if we start halving

29:29

or we go to 150 pixels per inch that changes to 3.39 milliseconds as

29:37

opposed to 5 and when we get to 300 pixels per inch it’s down at one point

29:43

six nine milliseconds and at 600 millimeters a second it’s down at 0.85

29:49

milliseconds so we’re getting incredibly short periods of time allowed for every

29:55

pixel now what I’m trying to illustrate with this drawing is that the time

Transcript for How to Laser Engrave a Picture (Cont…)

30:01

allowed for every pixel is getting less and less and less as you increase the

30:07

resolution you might think you’re doing a great job of increasing the definition

30:12

of your picture but what you’re actually doing is allowing less time for the

30:16

laser beam to burn in one spot and therefore the color of the spot will be

30:22

less dense now that’s true if you’re using

30:27

something like white card or an organic material like wood leather but of course

30:33

that won’t apply if you’re using glass or slate or granite or something like

30:39

that we get a completely different effect so let’s stay with white paper

30:43

card or wood where we get grades of brown depending on the power that we put

30:47

into the dot now things are starting to get quite complicated because although

30:52

I’ve broken this whole problem down into little individual elements like this

30:58

they don’t actually and can’t actually exist like this in reality typical dot

31:04

size for a two inch lens would be probably somewhere in the region of

31:08

about point two in practice I know theoretically it should be point one of

31:13

a millimeter but in practice when you start burning it you’ll find it very

31:17

difficult to get much less than point two of a millimeter so therefore when we

31:22

start looking at a point two millimeter pixel in here that’s what it looks like

31:29

now at 50 millimeters a second we’re talking about using small amounts of

31:36

power here by the way say eleven percent very very small amounts of power and we

31:41

get that sized pixel the best one that we can get now if we carry on using

31:45

eleven percent power and we move the resolution up to 150 PPI and we try and

31:51

put the same pixel on there that’s what we’re going to get we’re going to get a

Transcript for How to Laser Engrave a Picture (Cont…)

32:02

dot that overlaps with the next dot and then when we try and do it at 300 we’re

32:09

going to finish up with the same dot at the same dot but look what happens is

32:16

covering more and more pixels but remember the machine is driven by the

32:21

pixels so consequently what will happen is we shall get the first circle here to

32:28

match the first pixel and then the second pixel will drive another dot and

32:33

so we should get another dot here and then

32:36

third pixel will drive yet another dot and look I think you can see very

32:42

clearly that we’re getting dots overlapping on dots overlapping on dots

32:46

so we’re getting multiple multiple multiple burns and the finer the

32:51

resolution the more burns we get in X and of course then we start stepping

32:56

down in Y so we need to get multiple burns in X and in Y we’ve got a few

33:01

parameters that are getting mixed up here we’ve got the dot size which is

33:07

remaining constant even though the resolution is getting smaller and

33:12

smaller and finer so we’re getting more and more overlaps which technically

33:17

means we probably could be getting more darker burn but of course counteracting

33:23

the darker burn we’ve got the opposite effect and that is the fact we’re

33:27

getting less time for each one of these burns

33:30

so although we technically might have a darker burn because we’re getting

33:34

multiple burns we’re getting substantially less power

33:38

because there’s less time for the dot in one spot now you can’t second guess what

33:46

the result of this is going to be I’m just telling you what the problem is

33:50

that you’re facing what we should do is find out how these things affect a

33:56

picture in the next session when we start building pictures at different

Transcript for How to Laser Engrave a Picture (Cont…)

34:00

resolutions but for the time being what I’m trying to do is to explain to you

34:05

the problems that we’re going to face so that you can understand all the details

34:10

associated with producing graphics now you can see that we’ve got less and less

34:16

time here for our burn to take place but what we haven’t considered is whether

34:23

the machine itself can operate this fast for putting a pixel down and so I’ve

34:33

done some work on both of my machines to see how good they are at responding to a

34:40

demand for a pulse from each one of these pixels and we know that this one

34:46

works because there’s the picture and so I’ve done some more work at 50

34:52

millimeters a second and 100 PPI with a different pattern and I’ll show you the

34:58

results of that pattern what you’re seeing here is the penetration of the

35:04

beam into a piece of perspex now this is looking at it from the back so that we

35:12

can see the penetration into the perspex itself look you can see these little

35:16

Peaks so we’re looking at this from the backside at about 45 degrees so that we

35:22

can see the penetration and what I’ve done I’ve generated a pattern here which

35:28

is basically one pixel and this was all done at a hundred pixels per inch the

35:33

resolution so we’ve got one pixel and then we’ve got a four pixel gap two

35:38

pixels for pixel gap three pixels and a four pixel gap and we did that all the

35:42

way up to ten pixels and so that’s one two three four as it says on here I have

35:48

the tenth one is missing what we find is that the rise time the time that it took

35:55

for the beam to form that Green Line is virtually nothing this was nine pixels

Transcript for How to Laser Engrave a Picture (Cont…)

36:01

wide and what’s happened we’ve got some very strange effect taking place here

36:06

and then once the beam switch is off at about here we’ve got this pink line

36:10

which shows the decay of the beam the time that power actually takes to

36:16

disappear and so you can see that that pattern is exactly the same on every one

36:22

of these patterns here whether or not we’ve got one or nine pixel groups so

36:30

the rise and fall times are very consistent now the reason I’m showing

36:34

you this is because just one of these pixels that I’m showing you on here is

36:42

this so these are the single pixels that we can see here these pixels here in the

36:49

background are single pixels and they’re the same pixels as you see here now what

36:56

I’m now going to do is to just try and explain

37:01

what we see here with these funny shapes now on this pattern here I’ve just drawn

37:10

at just a nominal pixel pattern the black pixel nothing black pixel nothing

37:15

black black black nothing black black nothing black nothing black okay now what

37:21

we’ve got here is a little picture of the beam switching on and off and that’s

37:28

what that black pattern is in the background there that’s the maximum

37:32

power that we could expect the beam to achieve and this is theoretically what

37:38

we’re expecting to see we get a pulse here it is creating a black dot and then

37:45

we get no pulse so we get nothing then we get another black pulse and so it

37:50

runs and here where we’ve got a group of three we’ve got pulse pulse pulse we’ve

37:57

already proved that and I keep coming back to this picture so you don’t forget

38:01

it we’re driving this laser with individual pixel pulses and I’ll just

Transcript for How to Laser Engrave a Picture (Cont…)

38:08

remind you of this picture here Green is the beam on and pink is the beam

38:15

switching off and you can see how much drag or how much lag delay time there is

38:20

in switching the beam off the beam switches on and it never makes it to

38:26

this maximum that we were trying to go for the maximum power why not well

38:30

because it has to switch off and it takes a long time to switch off so

38:34

therefore it starts switching off early and it never makes it to this maximum

38:39

position and we can see that clearly on here look there is a one pixel there’s

38:46

two pixels and two pixels joined together are higher than one pixel three

38:52

pixels joined together are higher than two pixels four are higher than three

38:58

but then that’s where we start to stabilize out once we get to four pixels

39:02

we’ve achieved a uniform height so basically it takes almost four pixels to

39:08

reach the ideal depth the maximum depth now I haven’t quite shown that on this

39:12

diagram here but what we’ve got we’ve got

39:15

a rise time and a fall then we’ve got nothing a rise time and a fall

39:19

nothing then we’ve got a rise time and a fall but because we’ve got a pixel right

39:25

next to it it doesn’t get a chance to fall right the way back to zero so

39:29

consequently we rise again in the same time and we can climb the power a little

39:36

bit higher towards the target that we’re aiming for and then it falls and it

39:40

doesn’t drop down again as far and eventually after in this particular case

39:44

I’ve drawn it three pixels I’ve reached the maximum so and then we drop down

39:50

with two pixels we get to a different level with one pixel we get to this

39:55

level so basically what I’m saying is if we’ve got one pixel we achieve this much

Transcript for How to Laser Engrave a Picture (Cont…)

40:00

burn if we’ve got three pixels we can achieve this much burn and if we’ve got

40:07

two pixels we can achieve this much burn now that’s a variable amount of burn

40:14

depending on the grouping of the pixels it’s getting quite complicated the way

40:19

in which all these factors are interacting together but the point I’m

40:24

really making here is that this piece here is a constant so the amount of time

40:32

it takes for the beam to go up to a target value it’s aiming to get to 11%

40:37

remember but it never got to 11% because we didn’t give it enough time it had to

40:43

switch off this is why I keep laboring the point about these times and

40:47

milliseconds because 5 milliseconds here’s what the picture looks like now

40:55

there’s only 3 milliseconds we would have even less time to go up and come

41:00

down and goodness me look what happens at 0.85 milliseconds we get hardly a

41:06

chance to start before we’ve got to stop so what I’m really saying is here the

41:13

response time of the beam itself to turning its power on and turning its

41:20

power off to different levels can have quite a significant effect on the amount

41:25

of burn we get and the density of the burn that

41:30

we get on these pictures and remember earlier I was saying to you I was trying

41:37

to find an explanation why our dots were not all dark black well here we are

41:43

we’ve got different groups of dots here which operate at different powers where

41:50

we’ve got groups of three four and five dots up here we’ve got a dark burn but

41:55

where we’ve only got single dots we’ve got hardly any burn at all and

41:58

then we may have double dots and triple dots that isn’t what I was expecting but

Transcript for How to Laser Engrave a Picture (Cont…)

42:04

that’s what we’re going to get if we drive the resolution too high and if we

42:11

run the speed too high remember the resolution is going to

42:16

affect the time but of course this is at fifty millimeters a second so if I

42:23

increase that to 100 millimeters a second I would have half as much time on

42:28

each one of these and if I ran it at 200 millimeters a second I have a quarter

42:34

as much time so time is a most important factor because it’s all related to the

42:43

response capability of the power supply and the laser tube to produce a sharp

42:49

dot now that was hard work and I hope you’ve understood that it is the

42:55

explanation that I’m putting forward for this strange pattern of variable burns

43:00

you will see in the next session when I start doing some real pictures how this effect

43:07

this strange effect which I’m calling pseudo grayscale because technically

43:12

these should all be black all the same density but they’re not the same density

43:18

so we’ve got degrees of grayness in here but we’re not using grayscale this is

43:25

what I call a pseudo grayscale system well I’m going to leave you now where

43:30

we started this session off that there being a filled vector piece of text and

43:37

that being a bitmap piece of text now as I said to you they don’t look any

43:42

different but believe me I think after this session you can understand that

43:47

they are done in a completely different way now this is a very simple exercise

43:54

in the next session we shall deal with real photographs where I think you’ll be

44:00

quite surprised at some of the results we get

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