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'[OT]: Light filters'
2000\06\24@121345 by Andrew Seddon

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This is basically a continuation of the auto darkening thread if anybody is
interested.

I have looked extensively but I really have no idea where to start having
had no training in this area. Basically I have a scan head that uses red
light to scan the image. What I want to do is make it so that you cannot see
the scanhead from outside the casing when it is not in use. I originally
thought of some kind of plastic that can have it`s opaque qualities altered
electrically, I know think there could be a simpler solution. Would it be
possible to use some kind of polarisation filter?? Or possibly a filter that
only allows red through. However would this show up as red on the casing,
bearing in mind that no light would come from the sensor as it is encased
but some could be reflected back from outside. Ideally I am looking for a
matt black effect when not in use but as close to that as possible would be
great.

Any help greatly appreciated.. PS Sorry for the wasted bandwidth I realise
this is severely off topic but if it`s any consolation the project has a PIC
in it!


Andrew Seddon

2000\06\25@142711 by rottosen

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Andrew Seddon wrote:
{Quote hidden}

You may want to try translucent red plastic to see what it looks like.
It may work good enough.

If not, try circular polarized red plastic. The polarization will reduce
the reflections that you are worried about. I have used this material
for LED displays. This was a long time ago so I don't know where you
would get any now.


-- Rich


'[OT]: Light filters'
2000\08\25@002647 by Tony Nixon
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Hi all,

Sorry for the off topic.

Does anyone know if there are light filters available that will only
pass a certain colour band? Eg. yellow only.


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2000\08\25@004712 by David VanHorn

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At 02:23 PM 8/25/00 +1000, Tony Nixon wrote:
>Hi all,
>
>Sorry for the off topic.
>
>Does anyone know if there are light filters available that will only
>pass a certain colour band? Eg. yellow only.

Kodak, Melles Griot, plenty of places.

What color are you looking for?

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2000\08\25@010827 by Tony Nixon

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David VanHorn wrote:
>
> At 02:23 PM 8/25/00 +1000, Tony Nixon wrote:
> >Hi all,
> >
> >Sorry for the off topic.
> >
> >Does anyone know if there are light filters available that will only
> >pass a certain colour band? Eg. yellow only.
>
> Kodak, Melles Griot, plenty of places.
>
> What color are you looking for?

I guess now that I think of it, it is a PIC subject.

I want to expand on the 'virtual car' project and make it follow a line
on the road. A PIC 16F873 now has control over the vehicle speed and
manages to get it up to 110KPH and hold it there +/- about 3 KPH. It
also mirrors the actions of a dummy steering wheel, but I would like to
automate this as well if possible, as it saves me the stress of having
to 'drive' it myself.

I have done some basic playing around with a video camera to see what
signal can be obtained with a yellow line contrasted against a bitumen
background. The idea is to use the shift in the yellow signal to decide
how far the vehicle is off center and use the steering servo to correct
any errors. There are no bends in the line.

I was hoping a filter of some sort would help with the contrast by
darkening all other colours.

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2000\08\25@014542 by Paul Drummond

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Tony, There are filters that you can aquire to filter colorbands...It
depends how sensitive your project is.
eg. Some color filters will let for example yellow through but infrared,uv
and other more extreme wavelengths will still pass through...If this is no
problem for the application at hand then l suggest looking in the RS
Catalogue as l think l can remember seeing something there.  If not decent
camera supply outlets may be able to help.
Cheers!

{Original Message removed}

2000\08\25@015614 by Mike Werner

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Tony Nixon wrote:
<snipped>
> I have done some basic playing around with a video camera to see what
> signal can be obtained with a yellow line contrasted against a bitumen
> background. The idea is to use the shift in the yellow signal to decide
> how far the vehicle is off center and use the steering servo to correct
> any errors. There are no bends in the line.
>
> I was hoping a filter of some sort would help with the contrast by
> darkening all other colours.

If you want to "intensify" one particular color, then use a filter of that
color.  Find a filter of as similar a yellow as possible, and put that on
the camera.  What the filter does is pass light of the same color as the
filter, and attenuate all other colors.

Something else to consider is use a black and white camera for this.
Putting a yellow filter over the lens of a B&W camera and pointing it at a
yellow line will *really* make the line stand out.  I'm not exactly sure
what bitumen is, but if it's what's generally referred to as asphalt here in
the States then it's generally a pretty dark black.  And with a B&W camera
with a yellow filter, that yellow line outta be damn near impossible to
miss.  Post-filter, it'll be almost dead white against a dead black
background.

Of course, this is all presuming that I'm remembering my intro photography
course correctly.
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2000\08\25@020802 by Peter Grey

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At 02:23 PM 8/25/00 +1000, you wrote:

Have a look at Edmund Scientific. They should have many such filters.

Good luck,

Peter
{Quote hidden}

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2000\08\25@035245 by Alan B. Pearce

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>Does anyone know if there are light filters available that will only
>pass a certain colour band? Eg. yellow only.

Try your local theatre suppliers for the cellophane (or whatever i is) they use
on stage lighting. A local drama group may even let you have a couple square
inches off a piece they have.

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2000\08\25@100029 by Reginald Neale

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  Tony said:

>I was hoping a filter of some sort would help with the contrast by
>darkening all other colours.


  You've got the right idea. As others pointed out, optimization
  will mean matching the response of your source/detector/filter
  combination to the extent possible. Edmund Sci is the most
  accessible filter vendor in the US. Are you stuck with existing
  yellow lines, or can you paint your own? A thin line of
  retroreflective paint and a beamsplitter detection setup would
  ignore just about everything except the line you want to follow.
  Contact me off-list if you need further info.

  Reg Neale

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2000\08\25@105155 by Terry Heng

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Tony, instead of painting the road, why don't you use shiny aluminium
(duct? air-con?) tape instead? You could do without the cam and go for
opto-reflective sensors. Much cheaper to bust up and less complicated. Just
a suggestion.

Terry



{Quote hidden}

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2000\08\25@135446 by Thomas McGahee

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Edmund Scientific sells them. Among other things they carry
a filter assortment that gives you a sample of a large
number of color filters. The sample set is very reasonably
priced, and then you can use it to determine exactly what
filter material you want to later order in quantity.

We used the filter assortment last school year in a project
that determined the color of an object based on the
percentage of color measured using a rotating wheel having
several color segments. The color assortment was quite good,
especially considering the low cost.


Fr. Tom McGahee

{Original Message removed}

2000\08\25@141940 by rottosen

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Reginald Neale wrote:
{Quote hidden}

A couple of comments:

 First, you may not be able to get a filter for the exact color of the
lines that are painted on the road surface. There are bound to be some
variations in the paint. This may mean that you want to use multiple
filters to make a wider bandpass to accept more color variation. This is
similar in concept to how IF filters are done in television receivers.

Second, while riding my bicycle along the white stripe along the
shoulder I have noticed that the paint is retroreflective. I think that
they sprinkle glass beads onto the paint after applying it to the road.
The yellow stripe may be done the same way. I don't plan on riding along
the double yellow center line to find out  ;-)

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2000\08\25@220644 by Mike Werner

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Richard Ottosen wrote:
>   First, you may not be able to get a filter for the exact color of the
> lines that are painted on the road surface. There are bound to be some
> variations in the paint. This may mean that you want to use multiple
> filters to make a wider bandpass to accept more color variation. This is
> similar in concept to how IF filters are done in television receivers.

If you are putting the filters onto the lens of a camera, it doesn't work
that way.  You will actually be *cutting* the amount of light that gets
through drastically, by actually making the effective bandpass narrower.

Think of the optical filters as fairly narrow notch filters with roughly
equal widths.  Putting multiple filters over a camera lens is like putting
notch filters with different center frequencies in series.  For all intents
and purposes you'll be attenuating *everything*.

To make up for the variances in the color of the painted line, use a weaker
filter.  That's the rough equivalent of using a notch filter with a wider
pass width.
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2000\08\25@234455 by Stephen B Webb

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> Richard Ottosen wrote:
> >   First, you may not be able to get a filter for the exact color of the
> > lines that are painted on the road surface. There are bound to be some
> > variations in the paint. This may mean that you want to use multiple
> > filters to make a wider bandpass to accept more color variation. This is
> > similar in concept to how IF filters are done in television receivers.
>
> If you are putting the filters onto the lens of a camera, it doesn't work
> that way.  You will actually be *cutting* the amount of light that gets
> through drastically, by actually making the effective bandpass narrower.

Even assuming you get filter to do an excellent job, you are going to have
to do some image processing to figure out where your line is (and how to
correct for it).  The amount of processing you need to do seems  to
require something more than a PIC.

You already have WAY more contrast than you need, so you can probably skip
the filter.  Run the image through an edge detection alg. (soebel, canny,
other...) and then filter based on edge orientation (with canny alg.)
(since you know that your line is in some orientation..)

Group edges into line segments and filter based on some minimum segment
length.

You can get info on all of this stuff at teh computer vision homepage...
http://www.cs.cmu.edu/afs/cs/project/cil/ftp/html/vision.html

Or I have code (C++) if it would help.


-Steve

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2000\08\26@090535 by Bob Ammerman

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Vision based seems very much like overkill for this. I would think a simple
line-following system based upon a couple of sensors would do the job very
well.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

{Original Message removed}

2000\08\26@115249 by Brian Kraut

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Go to your local disco and theatrical lighting supply store and ask for
for a gell sample book.  Voila, about 100 different color plastic
filters in 1 X 3" size.  Some of them even have spectral response graphs
for each sheet.

Tony Nixon wrote:

{Quote hidden}

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2000\08\26@161944 by Peter L. Peres

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>Even assuming you get filter to do an excellent job, you are going to have
>to do some image processing to figure out where your line is (and how to
>correct for it).  The amount of processing you need to do seems  to
>require something more than a PIC.

FYI afaik once upon a time this was done using analog S&H in four zones
(two bands with two sides each in the picture) and a small analog computer
that computed where the line was 'pointing to' and implemented a 'S'
approach to the desired vector using analog PID regulator technology. It
all depends on the field of view and the contrast achieved. In theory with
6dB contrast you should be able to get there. A normal B/W camera (that
sees the blacktop road and the line only - camera angle down at 60
degrees) will have 35 dB of contrast or more (almost 1:100). A PIC could
be programmed to generate the required apertures for S&H and do actual S&H
(assuming 16C711). It could output GPS NMEA-like data to simplify the
interface to the remaining system.

Of course 'real' image processing is more interesting, but do you really
want that ?

Peter

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2000\08\27@005249 by Roger Kadau

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< snip>
> I have done some basic playing around with a video camera to see what
> signal can be obtained with a yellow line contrasted against a bitumen
> background. The idea is to use the shift in the yellow signal to decide
> how far the vehicle is off center and use the steering servo to correct
> any errors. There are no bends in the line.
>
> I was hoping a filter of some sort would help with the contrast by
> darkening all other colours.
>
> --
> Best regards
>
> Tony

Have you checked out Glen Williamson's site ?
http://www.williamson-labs.com/home.htm

Has a section on lane tracking and if you check thoroughly you will find
some histograms on line color on asphalt.

There's also a graph heading, "color filters' transfer functions."

Also: NOT [OT]:
Just getting ready to assemble my ROMZAP. Thanks for all you've done for the
PIC community Tony.

Roger Kadau,
netServ

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2000\08\27@173801 by Gennette, Bruce

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Two thoughts -

How pure is the 'yellow'?  Although we may see the paint mix as yellow it
probably has a lot of green in it, possibly more green than yellow.  Can you
take a sample to a hardware store for a (rough) test of the actual colour on
their colour matching system?  Or to the (instrumental) analytical lab at
your uni.  The wrong filter could be a disappointing dead end.

Narrow wavelength band filters are available - we have a field use (hand
held) spectrophotometer for environmental measurement that uses a light
globe and a set of narrow band filters to 'select' wavelengths for analysis.
The machine is called 'The Analyst' and is made by an American firm.  Extech
Equipment Pty Ltd, P O Box 579,  Boronia, 3155 are the Australian agent,
maybe they can help you?

Bye.

       {Original Message removed}

2000\08\27@182821 by Tony Nixon

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Hi all,

Thanks for the tips.

I looked at some signals on the CRO from a color CCD camera and "on the
surface" it seems like a PIC with an inbuilt comparator should be able
to detect the change in light levels in the video signal as long as it
is in sync with the framing.

These CCD cameras are a lot tougher than I expected. I have had to use
two so far out of 7 high speed impact tests. One failed only because the
tiny lens cracked. Earlier on, it got pushed through the windscreen by
the dash board and landed in the grass about 20 meters away and still
worked ok.

The PICs are still humming along too. I've just managed to reduce the
PIC count from 4 to 2 and shrink 4 prototype boards into a single 4" x
4" PCB. It looks good on paper, but with two servo controllers, (one
high power), I can't really judge it until I make one.

It's got to be the best Nintendo game I'll ever play :-)

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2000\08\27@184044 by Tony Nixon

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Roger Kadau wrote:

> Have you checked out Glen Williamson's site ?
> http://www.williamson-labs.com/home.htm
>
> Has a section on lane tracking and if you check thoroughly you will find
> some histograms on line color on asphalt.
>
> There's also a graph heading, "color filters' transfer functions."

Thanks for the tips.

> Also: NOT [OT]:
> Just getting ready to assemble my ROMZAP. Thanks for all you've done for the
> PIC community Tony.
>
> Roger Kadau,
> netServ

No probs.

I uploaded again only a few days ago because I found a small bug. It's
amazing how these things hide until the software gets released.

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2000\08\29@164202 by Lance Allen

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On 25 Aug 2000, at 14:23, Tony Nixon wrote:

> Hi all,
>
> Sorry for the off topic.
>
> Does anyone know if there are light filters available that will only
> pass a certain colour band? Eg. yellow only.
>
>
> --
This will be difficult on a large scale but you could "illuminate"
coloured objects with narrow band light sources (the colour of
interest) and subtract pre and post illumination intensities.
The result is level sensed (digital comparation) and then multiple
sample averaged. This should yield very clean data, but the range is
intended to be only been 5 metres at no more than 5 kph.
I have set up a  mono single line CCD sensor but it should be
possible with a mono camera.

I confess to being stuck on the pattern recognition in the just
described method but I am an optimist.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

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