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.

Contents
- Russ has already documented his learning process on the following RDWorks Learning Lab videos:
- 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
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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
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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
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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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