1 of 2
1
Bandpass Filter Simulation
Posted: 17 July 2008 06:26 PM   [ Ignore ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member

A tech support engineer at Omega Filters was incredibly cool and performed a computer simulation of bandpass filters for me targeting an 850 nm application. The simulation addresses the problems associated with light hitting the band pass filters at angles which I posted in this thread. The 850 CWL 10 FWHM is great when the blobs are near the center of the screen. However, as you move out toward the corners of the screen the blobs fade. This is because as light comes into the filter at angles greater than zero degrees the effective pass is shifted blue out of the range of the intended light pass. The 850BP10 simulation demonstrates this failure. To address the problem you can red shift your filter selection and increase the FWHM. This is what the 2nd simulation shows. The bold trace show a zero degree angle of incidence and the faint trace show 28 degrees.

Image Attachments
850BP10.jpg865BP55.jpg
Profile
 
 
Posted: 17 July 2008 07:11 PM   [ Ignore ]   [ # 1 ]
Avatar
RankRankRankRankRankRank
Joined  2007-04-08
Total Posts:  2539
Dedicated

rdagger, I thought this could be solved by getting a non “interference” bandpass filter.

 Signature 

MTmini, MTbiggie, & Audiotouch creator & Community Core Vision Co-founder

Follow on:
My Blog | Facebook | Twitter | Youtube

Profile
 
 
Posted: 17 July 2008 07:22 PM   [ Ignore ]   [ # 2 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member
cerupcat - 17 July 2008 07:11 PM

rdagger, I thought this could be solved by getting a non “interference” bandpass filter.

I don’t think there is such thing as a narrow bandpass filter that is not an interference filter.  According to the engineer I spoke with there really is no perfect filter.  The alternative to the interference filter is to use a combination of long and a short pass filters.  However, the second simulation is what he recommended.  Select a CWL slightly higher than your source wavelength and select a wider FWHM to compensate for light from the corners of your table.  I just purchased another BP filter using these guidelines and I will let you know how it works.

Profile
 
 
Posted: 17 July 2008 07:44 PM   [ Ignore ]   [ # 3 ]
Avatar
RankRankRankRankRankRank
Joined  2007-04-08
Total Posts:  2539
Dedicated

Interesting. Let us know how it goes.

I’m wondering why no one else has mentioned this happening since there’s a good number of people using bandpass filters (including myself). Mines from omega too, although it’s more than 10nm width.

 Signature 

MTmini, MTbiggie, & Audiotouch creator & Community Core Vision Co-founder

Follow on:
My Blog | Facebook | Twitter | Youtube

Profile
 
 
Posted: 17 July 2008 08:11 PM   [ Ignore ]   [ # 4 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member
cerupcat - 17 July 2008 07:44 PM

Interesting. Let us know how it goes.

I’m wondering why no one else has mentioned this happening since there’s a good number of people using bandpass filters (including myself). Mines from omega too, although it’s more than 10nm width.

A larger FWHM definitely diminishes the blue shift. Also, my webcam’s wide-angle lens exacerbates the effect. If I were using a regular lens, it may not be an issue.  Hopefully the new lens will maximize the brightness of my blobs.

Profile
 
 
Posted: 18 July 2008 05:02 AM   [ Ignore ]   [ # 5 ]
Avatar
RankRankRankRank
Joined  2008-06-04
Total Posts:  565
Elite

Thanks for the tech spec on the old band pass issue. My 940nm works fine but i have a standard none wide angle lens.  The filter has a tolerance of +- 2nm so i think this helps, Mine Was from Knight Optics in the UK. Very good informative service.

 Signature 

}---)))@

V JAY!  http://www.v-lux.net LASER tut http://nuigroup.com/forums/viewthread/6407/

Architectural Technician and Building Surveys

http://www.jpgainsfordassociates.com

Profile
 
 
Posted: 22 July 2008 09:36 PM   [ Ignore ]   [ # 6 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member

I just got my new bandpass filter.  It is 855 CWL and 40 FWHM.  I decided only to bump it up to 855 instead of 865, because I figured that the center of the screen gets the most use and should be a priority over the corners.  I will test it out tonight.  Omega Optical has great prices.  This filter was only $19.

Image Attachments
855BP40.jpg
Profile
 
 
Posted: 23 July 2008 02:23 AM   [ Ignore ]   [ # 7 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member

Here is a comparison test featuring the new bandpass filter. The first picture is without a filter. My InFocus DLP projector is emitting so much IR that you can not even see my fingers. I’ve placed markers in the corner of the screen to mark the edges of the acrylic. The two 850 nm lasers are visible on the right side of the frame. There is also a noticeable glare spot to the left of center. The 2nd pic is with my old 850 CWL 10 FWHM filter. It does a great job of cutting the unwanted light and most of the glare, but unfortunately the corner markers disappear because of the blue shift at wider angles. The 3rd pic shows the new 855 CWL 40 FWHM filter. It does not block as much light as the prior filter, but the corner markers are now visible and the results are pretty good.

Image Attachments
1 No BP DLP Hand.jpg2 BP850-10 DLP Hand.jpg3 BP855-40 DLP Hand.jpg
Profile
 
 
Posted: 23 July 2008 09:42 AM   [ Ignore ]   [ # 8 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member

Here is a video showing the 3 tests with the projector turned off.

Profile
 
 
Posted: 23 July 2008 12:36 PM   [ Ignore ]   [ # 9 ]
Rank
Joined  2008-07-07
Total Posts:  88
New Member

Hi,

i’m building a quite large FTIR table (113,0cmsX85,0cms) and using the following setup:

- Unibrain Fire-i BCL 1.2 Camera;
- Unibrain 1,9mms lens (no IR coating);
- 940 nm Vishay Leds (i’m gonna test Osrams SFH485 too).

I’m having some problems with detection and trying to find a suitable bandpass filter. I’ve been to the sites you guys mention here but felt a bit lost.
Can anywone advise me on a good filter for my setup?
tks.

Profile
 
 
Posted: 23 July 2008 03:55 PM   [ Ignore ]   [ # 10 ]
Avatar
RankRankRankRank
Joined  2008-02-12
Total Posts:  1120
Member

You probably want somthing along the lines of a 940BP40 for the vishay leds and for the Osram leds an 880BP30 is quite good

 Signature 

Company Website - Knode Research and Development
My Youtube

Profile
 
 
Posted: 24 July 2008 03:13 AM   [ Ignore ]   [ # 11 ]
Rank
Joined  2007-12-05
Total Posts:  14
New Member

I just got my new bandpass filter.  It is 855 CWL and 40 FWHM.  I decided only to bump it up to 855 instead of 865, because I figured that the center of the screen gets the most use and should be a priority over the corners.  I will test it out tonight.  Omega Optical has great prices.  This filter was only $19.

Hi rdagger! Thanks for your info. Again! they are very useful. It is very painful for me to find suitable BP filter in my city.

I have checked Omega Optical’s website, but I cannot find the item “855 CWL and 40 FWHM”. Moreover, no similar filter cost only $19. All I can see there at least $190 or above. Do you have any special discount?? Can you tell me more tips about that?

 Signature 

http://www.youtube.com/user/tilllasting

Profile
 
 
Posted: 24 July 2008 03:27 AM   [ Ignore ]   [ # 12 ]
Avatar
RankRankRankRank
Joined  2008-02-12
Total Posts:  1120
Member

royc you search for omegabob2 on ebay that is omega optical’s online ebay store there you will find cheap BP filters

 Signature 

Company Website - Knode Research and Development
My Youtube

Profile
 
 
Posted: 24 July 2008 06:47 AM   [ Ignore ]   [ # 13 ]
Rank
Joined  2008-07-07
Total Posts:  88
New Member

Taha,

tks for your help. I’ve searched for the references you gave me, 940BP40 and 880BP30, and i can’t find them.

I’ve found something similar, that seems to narrow, here: http://cgi.ebay.co.uk/IR-Optical-Filter-880BP10-12-7mm-diameter-metal-ring_W0QQitemZ310058221445QQihZ021QQcategoryZ109452QQtcZphotoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

Can you be more specific? i understand how to look for the info but i’m still a little bit lost.

I’ve looked in http://www.knightoptical.co.uk/ and in the ebay store above.

Another question, for the 1.9 wide angle lens i should by a 12.5mm 12.7mm filter right? do you think it’s gonna be dificult to attach that or are the filters standard?

Thanks again.

Profile
 
 
Posted: 24 July 2008 10:11 AM   [ Ignore ]   [ # 14 ]
Avatar
Rank
Joined  2008-06-10
Total Posts:  94
New Member
royc - 24 July 2008 03:13 AM

Hi rdagger! Thanks for your info. Again! they are very useful. It is very painful for me to find suitable BP filter in my city.
I have checked Omega Optical’s website, but I cannot find the item “855 CWL and 40 FWHM”. Moreover, no similar filter cost only $19. All I can see there at least $190 or above. Do you have any special discount?? Can you tell me more tips about that?

No special discounts.  Here is the link to the auction.

Profile
 
 
Posted: 24 July 2008 11:50 AM   [ Ignore ]   [ # 15 ]
Avatar
RankRankRankRank
Joined  2008-02-12
Total Posts:  1120
Member
behem0t - 24 July 2008 06:47 AM

Taha,

tks for your help. I’ve searched for the references you gave me, 940BP40 and 880BP30, and i can’t find them.

I’ve found something similar, that seems to narrow, here: http://cgi.ebay.co.uk/IR-Optical-Filter-880BP10-12-7mm-diameter-metal-ring_W0QQitemZ310058221445QQihZ021QQcategoryZ109452QQtcZphotoQQcmdZViewItemQQ_trksidZp1742.m153.l1262

Can you be more specific? i understand how to look for the info but i’m still a little bit lost.

I’ve looked in http://www.knightoptical.co.uk/ and in the ebay store above.


Another question, for the 1.9 wide angle lens i should by a 12.5mm 12.7mm filter right? do you think it’s gonna be dificult to attach that or are the filters standard?

Thanks again.

First of all you if you are buying from omegabob2 then you can email him on ebay for a filter the filter you are looking for and within a day or 2 he if he has the filter you are looking for he put it up on ebay for you. well i based the specifications on the trials i did with 4~5 different BP filters i got. i’ve found that BP40 or BP50 work the best.... and for wide angle lens then i would go for a 25mm or 26mm in diameter lens, you really dont want any barrel distorsion.

 Signature 

Company Website - Knode Research and Development
My Youtube

Profile
 
 
   
1 of 2
1