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Measurung dynamic range...



 
 
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  #11  
Old August 8th 06, 12:12 AM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
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Posts: 1,818
Default Measurung dynamic range...

Bill Funk wrote:
On Mon, 07 Aug 2006 12:27:11 -0700, "Roger N. Clark (change username
to rnclark)" wrote:


Volker Hetzer wrote:

Roger N. Clark (change username to rnclark) wrote:


A good way to make a light trap is a cone painted with
glossy black paint. The reflections from the painted surface
go deeper into the cone.

So the camera looks into the pointy end?
Otherwise it seems to me that the light gets reflected back.


Yes. Think of an ice-cream cone. Look at the inside,
and paint it glossy black. A machined metal (like aluminum)
works very well.

Roger



Wouldn't machined (or even moreso, polished) metal reflect much more
light?
Why did you pick glossy black?


You need something to absorb the light. Each reflection
absorbs ~95% of the light, and the remaining gets reflected
further into the cone. If you use flat black, each reflection
scatters some light out of the cone. A polished aluminum
surface would reflect most light so most light would never be absorbed,
and eventually, after reaching the bottom, would reflect back out.
Flat black gives you both controlled absorption and controlled
reflection.

Roger
  #12  
Old August 8th 06, 02:13 AM posted to rec.photo.digital
Bill Funk
external usenet poster
 
Posts: 2,500
Default Measurung dynamic range...

On Mon, 07 Aug 2006 16:12:21 -0700, "Roger N. Clark (change username
to rnclark)" wrote:

Bill Funk wrote:
On Mon, 07 Aug 2006 12:27:11 -0700, "Roger N. Clark (change username
to rnclark)" wrote:


Volker Hetzer wrote:

Roger N. Clark (change username to rnclark) wrote:


A good way to make a light trap is a cone painted with
glossy black paint. The reflections from the painted surface
go deeper into the cone.

So the camera looks into the pointy end?
Otherwise it seems to me that the light gets reflected back.

Yes. Think of an ice-cream cone. Look at the inside,
and paint it glossy black. A machined metal (like aluminum)
works very well.

Roger



Wouldn't machined (or even moreso, polished) metal reflect much more
light?
Why did you pick glossy black?


You need something to absorb the light. Each reflection
absorbs ~95% of the light, and the remaining gets reflected
further into the cone. If you use flat black, each reflection
scatters some light out of the cone. A polished aluminum
surface would reflect most light so most light would never be absorbed,
and eventually, after reaching the bottom, would reflect back out.
Flat black gives you both controlled absorption and controlled
reflection.

Roger


If you're really after a light trap, wouldn't a baffle, where any
reflections send the light to something that absorbs light be better
than any reflective surfaces?
--
Bill Funk
replace "g" with "a"
  #13  
Old August 8th 06, 05:28 AM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
external usenet poster
 
Posts: 1,818
Default Measurung dynamic range...

Bill Funk wrote:
On Mon, 07 Aug 2006 16:12:21 -0700, "Roger N. Clark (change username
to rnclark)" wrote:


Bill Funk wrote:

On Mon, 07 Aug 2006 12:27:11 -0700, "Roger N. Clark (change username
to rnclark)" wrote:



Volker Hetzer wrote:


Roger N. Clark (change username to rnclark) wrote:



A good way to make a light trap is a cone painted with
glossy black paint. The reflections from the painted surface
go deeper into the cone.

So the camera looks into the pointy end?
Otherwise it seems to me that the light gets reflected back.

Yes. Think of an ice-cream cone. Look at the inside,
and paint it glossy black. A machined metal (like aluminum)
works very well.

Roger


Wouldn't machined (or even moreso, polished) metal reflect much more
light?
Why did you pick glossy black?


You need something to absorb the light. Each reflection
absorbs ~95% of the light, and the remaining gets reflected
further into the cone. If you use flat black, each reflection
scatters some light out of the cone. A polished aluminum
surface would reflect most light so most light would never be absorbed,
and eventually, after reaching the bottom, would reflect back out.
Flat black gives you both controlled absorption and controlled
reflection.

Roger



If you're really after a light trap, wouldn't a baffle, where any
reflections send the light to something that absorbs light be better
than any reflective surfaces?


Yes, that is what you want, but there is no surface that
absorbs all light. So you must make a trap that any
light that does get reflected does so in a direction
away from your view. The is what the cone accomplishes.
The angle of the cone must be steep enough for any
reflected light to get reflected further into the cone.
If, for example, light was reflected 5 times, and the
reflectance was 0.02, then the light reaching the bottom
would be 0.02*0.02*0.02*0.02*0.02 times the original
light level. The more specular the reflection of the
glossy paint, the less light will be scattered back
to you. In practice, it works very well.

Roger
  #14  
Old August 10th 06, 06:01 AM posted to rec.photo.digital
Dave Martindale
external usenet poster
 
Posts: 438
Default Measurung dynamic range...

"Roger N. Clark (change username to rnclark)" writes:

Yes, that is what you want, but there is no surface that
absorbs all light. So you must make a trap that any
light that does get reflected does so in a direction
away from your view. The is what the cone accomplishes.
The angle of the cone must be steep enough for any
reflected light to get reflected further into the cone.
If, for example, light was reflected 5 times, and the
reflectance was 0.02, then the light reaching the bottom
would be 0.02*0.02*0.02*0.02*0.02 times the original
light level. The more specular the reflection of the
glossy paint, the less light will be scattered back
to you. In practice, it works very well.


If the sides are steep enough and clean enough that the reflection
always goes further towards the bottom of the trap, it doesn't matter
how reflective the walls are. No black paint is needed.

For a demonstration, buy a pack of double-edged razor blades. Look at
the cutting edges, and note that they have shiny ground and polished
metal just behind the cutting edge. Now get the blades very clean,
wiping off any excess oil that they were packaged in, and stack them in
a pile. Carefully line up all the blades in the stack and clamp them
in that position with a C clamp, a nut and bolt, or whatever you want.

Now look at one of the two edges of the stack of blades that is made up
of cutting edges. It will be very, very black. The cutting edges
stacked beside each other form very deep narrow "V" shapes, and
virtually all light that strikes these edges of the stack falls into one
of these V slots (since the cutting edges themselves are so thin that
they have near-zero area). And even though the reflectivity of the
metal is high, probably 80 or 90%, each time the light reflects some of
the energy is absorbed and the rest is sent deeper into the V, never to
return back into the world.

One of my professors had assembled a stack like this, and liked to show
it to people. Apparently a V-groove absorber like this is good for
absorbing laser beams that have substantial power, since the device is
all metal and the dumped energy just heats the metal. You don't want to
get it hot enough to warp the metal, but it can handle more power
without damage than black paint would.

Dave
  #15  
Old August 10th 06, 07:51 PM posted to rec.photo.digital
Bill Funk
external usenet poster
 
Posts: 2,500
Default Measurung dynamic range...

On 10 Aug 2006 07:39:56 -0700, wrote:


Since we've got a good discussion going on light traps, I'll try to
describe the one I am working on based on a pringles can. I clean out
the can, and save the top to make a baffle. I cut off the bottom (but
save it to put back on later). I do this to make inserting the lining
and cone easier. I am using a sheet of stickybacked artificial black
felt from a craft store (Michaels in my case). I first line the inside
of the can with a rectangle of the felt. Then I cut a cone out of the
remainder of felt and glue it to the bottom of the can. I then glue and
tape the bottom back on the can. I paint the underside of the top of
the can flat black. I glue a piece of my whitest card stock to the top
surface of the top, with rubber cement. I then cut a one inch square
in the top, and blacken the edge of the cutout with a black magic
marker, and replace it on top of can. The white card gives me a white
reference.

The first one I made was okay, but the workmanship was lacking, from
too many cut and try operations. I will be photographing the building
of the second one, with thoughts of sending the manuscript to one of
the photo magazines.


Ok, but I'm missing the point here.
Why would this be interesting to a photography magazine?
--
Bill Funk
replace "g" with "a"
  #16  
Old August 14th 06, 05:19 AM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
external usenet poster
 
Posts: 1,818
Default Measurung dynamic range...

Dave Martindale wrote:

"Roger N. Clark (change username to rnclark)" writes:


Yes, that is what you want, but there is no surface that
absorbs all light. So you must make a trap that any
light that does get reflected does so in a direction
away from your view. The is what the cone accomplishes.
The angle of the cone must be steep enough for any
reflected light to get reflected further into the cone.
If, for example, light was reflected 5 times, and the
reflectance was 0.02, then the light reaching the bottom
would be 0.02*0.02*0.02*0.02*0.02 times the original
light level. The more specular the reflection of the
glossy paint, the less light will be scattered back
to you. In practice, it works very well.



If the sides are steep enough and clean enough that the reflection
always goes further towards the bottom of the trap, it doesn't matter
how reflective the walls are. No black paint is needed.

For a demonstration, buy a pack of double-edged razor blades. Look at
the cutting edges, and note that they have shiny ground and polished
metal just behind the cutting edge. Now get the blades very clean,
wiping off any excess oil that they were packaged in, and stack them in
a pile. Carefully line up all the blades in the stack and clamp them
in that position with a C clamp, a nut and bolt, or whatever you want.

Now look at one of the two edges of the stack of blades that is made up
of cutting edges. It will be very, very black. The cutting edges
stacked beside each other form very deep narrow "V" shapes, and
virtually all light that strikes these edges of the stack falls into one
of these V slots (since the cutting edges themselves are so thin that
they have near-zero area). And even though the reflectivity of the
metal is high, probably 80 or 90%, each time the light reflects some of
the energy is absorbed and the rest is sent deeper into the V, never to
return back into the world.

One of my professors had assembled a stack like this, and liked to show
it to people. Apparently a V-groove absorber like this is good for
absorbing laser beams that have substantial power, since the device is
all metal and the dumped energy just heats the metal. You don't want to
get it hot enough to warp the metal, but it can handle more power
without damage than black paint would.

Dave

Dave,
While I agree with the razor blade stack produces a very good black
surface, it is not all that black, as there is diffraction from
the razor edges that cause scattered light. While certainly
more absorbing than the blackest paints, or carbon lamp black,
A better light trap is the single cone. The higher the reflectance
of the cone, the shallower and deeper the cone must be as you
need more reflections. Simple math: r^n where r is the reflectance
and n is the number of reflections needed to get to a given
reflectance level, e.g 0.000001. So you can do a good light
trap with a highly reflective surface, but the cone comes out
longer and is more expensive to make (been doing this stuff
for spectrometers and imaging systems for 20+ years).

Roger
  #17  
Old August 14th 06, 05:23 AM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
external usenet poster
 
Posts: 1,818
Default Measurung dynamic range...

wrote:

Since we've got a good discussion going on light traps, I'll try to
describe the one I am working on based on a pringles can. I clean out
the can, and save the top to make a baffle. I cut off the bottom (but
save it to put back on later). I do this to make inserting the lining
and cone easier. I am using a sheet of stickybacked artificial black
felt from a craft store (Michaels in my case). I first line the inside
of the can with a rectangle of the felt. Then I cut a cone out of the
remainder of felt and glue it to the bottom of the can. I then glue and
tape the bottom back on the can. I paint the underside of the top of
the can flat black. I glue a piece of my whitest card stock to the top
surface of the top, with rubber cement. I then cut a one inch square
in the top, and blacken the edge of the cutout with a black magic
marker, and replace it on top of can. The white card gives me a white
reference.

The first one I made was okay, but the workmanship was lacking, from
too many cut and try operations. I will be photographing the building
of the second one, with thoughts of sending the manuscript to one of
the photo magazines.


Why put white card stock at the entrance to your light trap?
The white card will scatter light into your field of view
(inside the lens), limiting your dynamic range. Of course,
that system scattering limiting dynamic range might be what
you want, but that would limit isolating the sensor
characteristics for example.

Roger
 




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