cross-polarization

[viper_0711]

hi

im experimenting a studio setup of fullbody scan.
i read some web site about cross polarization.
does someone have a detail suggestion on how it can be done properly to give 99% diffuse photos?
1) i try to put the polarizing films on the strobe and camera(with 90 degree rotated direction), however i still got some 40% specular reflection remains. the strobe is directly pointing to the subject. i'd like to check whats done wrong
2) obviously direct pointing the strobe to subject will produce harsh undesirable shadows.  before i come across cross polarization, i used to point the strobe to the white walls and make it a large reflector. can cross polarization works if reflected/diffuse lights like this is used?
 
thanks
viper

[meshmaster]

hi

im experimenting a studio setup of fullbody scan.
i read some web site about cross polarization.
does someone have a detail suggestion on how it can be done properly to give 99% diffuse photos?
1) i try to put the polarizing films on the strobe and camera(with 90 degree rotated direction), however i still got some 40% specular reflection remains. the strobe is directly pointing to the subject. i'd like to check whats done wrong
2) obviously direct pointing the strobe to subject will produce harsh undesirable shadows.  before i come across cross polarization, i used to point the strobe to the white walls and make it a large reflector. can cross polarization works if reflected/diffuse lights like this is used?
 
thanks
viper

Sorry to say that not much can be done.  Lights need to be at about a 45 degree angle from the camera.  The problem is that polarization works because it makes the photons directional.  The more "bounces" the photons have (off walls or other objects) the more scattered they will become and the polarization is vastly reduced. So the more lights you have, the more bounces you have from the rays the less effective the end result.  At the end of the day, large, even, diffuse light sources are most likely your best bet.

Cheers,

Joe

[viper_0711]

Thanks for the advise.

Im thinking to go diffuse reflected strobe off the walls and only place CPL on my cameras to cut a certain portion reflection out.

but im thinking of head scans since its less freeform than body poses, easier to control in small fixed region, and reflection on such closeup is a constant issue - im thinking if using cross polarization is good for this case.

if so, what will be the suggested setup to archive cross polar as well as most even lighting possible? lights direct illuminating from North-west-south-east? what do you mean by 45 degree to camera?

moreover, i tested that if i do cross-polar with camera(or eye viewing) directly pointing to the light. it works by being nearly dark for normal lights, and cut out 70-80% for strobes. so the Cross-polar working is confirmed, but now we need to to cut reflections instead of direct light. I cant get the exact theory understood becasue i imagine even lights coming out fr strobes polarized in one direciton. when reflected, it should be polarized in another direction and hence making the 90 degree orientation of camera CPL irrelevant. and i suspect thats why my cross-polar setup still wont cut all the reflection. see if anyone have an insight on this? is your 45 degree sugguestion relevant to this?

thanks

[meshmaster]

Thanks for the advise.

Im thinking to go diffuse reflected strobe off the walls and only place CPL on my cameras to cut a certain portion reflection out.

but im thinking of head scans since its less freeform than body poses, easier to control in small fixed region, and reflection on such closeup is a constant issue - im thinking if using cross polarization is good for this case.

if so, what will be the suggested setup to archive cross polar as well as most even lighting possible? lights direct illuminating from North-west-south-east? what do you mean by 45 degree to camera?

moreover, i tested that if i do cross-polar with camera(or eye viewing) directly pointing to the light. it works by being nearly dark for normal lights, and cut out 70-80% for strobes. so the Cross-polar working is confirmed, but now we need to to cut reflections instead of direct light. I cant get the exact theory understood becasue i imagine even lights coming out fr strobes polarized in one direciton. when reflected, it should be polarized in another direction and hence making the 90 degree orientation of camera CPL irrelevant. and i suspect thats why my cross-polar setup still wont cut all the reflection. see if anyone have an insight on this? is your 45 degree sugguestion relevant to this?

thanks

Hi,

My suggestion of 45 degrees between camera and lights is only an approximation, most likely 30 degrees is more like it... but you get the idea, there must be an angle between the camera and lights.  I've done a lot of testing with cross-polarization over the years and it is always problematic for many reasons, it will never work 100% especially in the context of 3d scanning...again, more lights, more problems due to multiple ray bounces.  Cross-polarization can be helpful under "some" circumstances, but it is far from a magic cure.  I maintain that it is simply best to use large, even, diffuse light sources.  If you still suffer from problems with specular highlights, adjust your lights or remove the specular problems in post production with photoshop.

Cheers,

Joe

[mala]

What polarising filters are you using,circular or linear?
Also worth pointing out that that cheap films do not polarise 100% of transmitted light
Remember that any light that does not pass through filter,most are 40% transmission max,will be reflected off the film and bounce around your studio as un polarised light...with something as powerful as a studio strobe this is very likely to happen.

It is possible to use polarising filters for face and body scanning effectively.
Google ICT lightstage and read their papers and also patents for further details.

HTH,
Mala

[viper_0711]

thanks for the advises. 

im using CPL on cameras. from its name i guess it means circular. the polarizing film is linear according to the spec. so web site said it both linear and CPL did the trick and just that CPL is more modern coating. i also doubt this so the test i did was to have the film placed in front of the light and the CPL in front of my eye/camera. rotate the CPL to a certain angle, yes it becomes completely black and i cannot see the light behind the film. so i assume my materials works.

however, as mentioned before, if my eye/camera is not directly looking at the light (as in the test) but on a subject that reflect the light. (as in all cross-polar example i saw on web), i still have no way to completely filter them no matter which angle i try.

i have googled and read through the light stage which involve cross-polarization but it didn't say in detail how it set it up.

the light stage seems use the same setup of cross-polar and the lights are from all directions, so i doubt there is a limited range of light angle as discussed above. and there should be a way to really effectively eliminate all reflection.

anyone have some insights?