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#11
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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
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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
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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
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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
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Measurung dynamic range...
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#16
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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
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Measurung dynamic range...
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