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#31
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In article , Roger N. Clark (change username to
rnclark) wrote: At one time I worked on a project finding black coatings for cameras and other EO sensors. It is amazing how hard it is to get a truly black black. Even the famed 3M Black Velvet was a 2% reflectance. The only thing we found was a coating by Martin Marietta that was close to 1%. Not only isn't it black, it's yellow, or blue or red ... Black, like white seems to be an imaginary concept. Carbon lamp black is probably the darkest diffuse reflector. It's reflectance in the visual range is about 1.6%: The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level corresponding to this part of the chart is deemed not to have come from the chart at all but from light scattered in the lens, and the circuitry is adjusted accordingly. Rod. |
#32
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"Roderick Stewart" wrote in message om... The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level Why not just test with the lens cap on? |
#33
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"Roderick Stewart" wrote in message om... The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level Why not just test with the lens cap on? |
#34
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In article , Roger N. Clark
Carbon lamp black is probably the darkest diffuse reflector. It's reflectance in the visual range is about 1.6%: The color of the lamp black varies with the source of the paraffin. North Sea Brent produces a different color than West Texas Light, produces a different color than .... This just for plain-ole black paint. 1.6% is quite a bit of reflectance - 1.8 oD, 6 stops down from white. -- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/ |
#35
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In article , Jjs wrote:
The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level Why not just test with the lens cap on? Test what? The superblack box is for setting flare correctors, which of course won't do anything if there isn't any flare. All you can test with the lens cap on is black balance, clamp balance ("pulse cancellation") and black shading. Superblack has to be visible simultaneously with a large area of high brightness in order to set and balance the flare correctors. Rod. |
#36
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In article , Jjs wrote:
The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level Why not just test with the lens cap on? Test what? The superblack box is for setting flare correctors, which of course won't do anything if there isn't any flare. All you can test with the lens cap on is black balance, clamp balance ("pulse cancellation") and black shading. Superblack has to be visible simultaneously with a large area of high brightness in order to set and balance the flare correctors. Rod. |
#37
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"Roderick Stewart" wrote in message om... In article , Roger N. Clark (change username to rnclark) wrote: At one time I worked on a project finding black coatings for cameras and other EO sensors. It is amazing how hard it is to get a truly black black. Even the famed 3M Black Velvet was a 2% reflectance. The only thing we found was a coating by Martin Marietta that was close to 1%. Not only isn't it black, it's yellow, or blue or red ... Black, like white seems to be an imaginary concept. Carbon lamp black is probably the darkest diffuse reflector. It's reflectance in the visual range is about 1.6%: The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level corresponding to this part of the chart is deemed not to have come from the chart at all but from light scattered in the lens, and the circuitry is adjusted accordingly. Ah - so that's where my licence fee goes...... |
#38
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"Roderick Stewart" wrote in message om... In article , Roger N. Clark (change username to rnclark) wrote: At one time I worked on a project finding black coatings for cameras and other EO sensors. It is amazing how hard it is to get a truly black black. Even the famed 3M Black Velvet was a 2% reflectance. The only thing we found was a coating by Martin Marietta that was close to 1%. Not only isn't it black, it's yellow, or blue or red ... Black, like white seems to be an imaginary concept. Carbon lamp black is probably the darkest diffuse reflector. It's reflectance in the visual range is about 1.6%: The BBC's Test Chart No 57 included (includes?), for the purpose of setting the flare-correction circuitry, a "superblack" rectangle that is actually a box a few inches deep, lined with black velvet. A small rectangular aperture at the front opens onto the surface of the test chart. Any light striking this rectangle enters the dark box and has to undergo several successive reflections by the black velvet before emerging, thus giving the rectangular opening an effective reflectance much lower than would be achievable by any actual surface. Any value of the output signal above dark current level corresponding to this part of the chart is deemed not to have come from the chart at all but from light scattered in the lens, and the circuitry is adjusted accordingly. Ah - so that's where my licence fee goes...... |
#39
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In message ,
"jjs" wrote: That should be slightly different, as the black rectangle will pick up light not meant to be focused in that area (the stuff that lowers contrast). Why not just test with the lens cap on? -- John P Sheehy |
#40
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In message ,
"jjs" wrote: That should be slightly different, as the black rectangle will pick up light not meant to be focused in that area (the stuff that lowers contrast). Why not just test with the lens cap on? -- John P Sheehy |
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