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Photography Question 
Ritwik Raj
 

Ghost image (?) in picture at high magnification


 
 
I am trying to take pictures of oil droplets injected into water with a Cooke SensiCam High Speed camera at high magnifications. I tried several lighting arrangements to remove my problem which is: I am getting a shadow-kind of stripe above the tip of the needle through which the oil is being injected.It goes away when I remove the needle from the field of view, but a similar thing then happens with the oil droplets as they come out of the needle. It is for an experiment I am performing and I need clean images.


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6/20/2005 12:09:51 PM

 
Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
gregorylagrange.org
  Could be diffraction or internal reflections from the water droplet.


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6/20/2005 12:36:40 PM

 
Ritwik Raj  
 
 
It doesn't seem to be internal reflection, as the stripe is there even when I am filming only the needle tip with no oil coming out.Plus it has a very strong directional nature and is only top to bottom.I'm attaching a photo to illustrate.


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6/20/2005 12:41:00 PM

 
Ritwik Raj  
 
 
the photo:


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6/20/2005 12:46:56 PM

 
Ritwik Raj   well...I can't upload it since the camera I have used isn't on the list.


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6/20/2005 12:48:07 PM

 
Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
gregorylagrange.org
  that's what I meant, reflection or diffraction of the needle in the water drop. You don't see it when the needle isn't sticking in the drop.
Light at a certain angle dosen't pass thru water surfaces, it gets reflected back. At least most of the light rays get reflected. Not 100%.


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6/20/2005 12:48:18 PM

 
Ritwik Raj   There's a little confusion,it seems.
I am getting the stripe even when there is no liquid coming out of the needle.Just the needle is causing the dark image on top of it.Once you remove the needle from the field of view,the stripe goes away too.It looks exactly as if the needle is as long as the whole illuminated frame.Someone suggested that it may be a problem of saturation of the pixels of the CCD array due to the uniform bright lighting that I am using.But even with a greatly reduced aperture,the stripe is visible, although the brightness of the image itself is diminished.


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6/20/2005 12:56:44 PM

 
Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
gregorylagrange.org
  I'm not saying that it's from the oil drops coming out of the needle. I'm saying that it's from the needle being in the water droplet.
light diffracts when it passes from one substance to another(air to water, water to air) and it also can reflect(not pass thru from water to air) if the angle is obtuse enough.
example-fish tank with water, shine a light from bottom of the tank upwards at a slight angle(15 degrees from vertical). As the light passes from the water to air, it bends a little. If you keep increasing the angle, moving the light closer to shining horizontal, at some angle most of the light is not going to pass thru the water surface, it's going to reflect off the water surface back down into the water.

Your using a small, spherical shape water drop, sticking a needle into it. Some of the light that is the image of the needle is going to strike the drop surface at obtuse angles, some of it is going to be reflected back and forth off the water surface.
another example-hold your thumb and index finger close together and look thru the gap at something in front, like your screen. Move your fingers closer together, there's a point where your fingers won't actually touch, but the gap will get dark as if they were.
combine you sticking a needle in a small water drop, and with the reflective and diffractive properties of water, and it's a shperical shape you're using, that can be the cause of this dark shadow, or needle reflection you're seeing.


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6/20/2005 1:17:46 PM

 
Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
gregorylagrange.org
  And after all that, maybe a polarizer filter would help.


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6/20/2005 1:18:54 PM

 
Ritwik Raj   I guess I am guilty of missing a piece of information: the actual setup.There is a plexiglass tank filled with water and the needle is stuck into the base of the tank, with oil being fed into it from outside, at the bottom.The light source, a 300W Tungsten filament ,is placed in such a way that the camera,the needle and the light are in the same line.There is also a translucent plastic sheet to diffuse the light, between the needle and the light, outside the plexiglass container.


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6/20/2005 1:45:11 PM

 
Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
gregorylagrange.org
  My bad, somehow thought you were doing something with a water drop.
It's probably a reflection off the sides of the tank. If it moves when you move the needle. Or it could be something to do with the high magnification making it able to pick up some distortion from the plexiglass.


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6/20/2005 8:37:42 PM

 
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