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DAVID-Laserscanner Forum • View topic - Impairment by IR filters - A further reason to prefer green
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Impairment by IR filters - A further reason to prefer green

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Impairment by IR filters - A further reason to prefer green

Postby WalterMo » Sun Aug 30, 2009 8:34 pm

Web Cam lenses are equipped with IR cut-off filters to reduce the increasing sensitivity of silicon photo sensors for IR. We will find the filter as a thin glass plate at the inner side of the lens.

Looking at the transmisson diagram of IR cut-off filters we see that they just have their cut-off where most of our red lasers are working. That means near 640nm. Taking into account that filters have tolerances (±10nm) it can be that a lot of the laser light will be reduced by the two possible types of filters (absorptive and reflective).

See my attached diagram. It came from Sunex.
http://www.optics-online.com/irc.asp

For the modification of my Logitech Pro9000
viewtopic.php?p=7805#p7805
and for these measurements I ordered cut-off filters from Sunex, the types IRC21 (absorptive) and IRC30 (reflective).

The measurements of these two types with a silicon photo diode (BPW20 in short current mode) at three different wavelengths gave these results:

At 650nm:
IRC21 = 22.4% transmisson
IRC30 = 32.2% transmisson

At 632nm (Helium Neon):
IRC21 = 35.8% transmisson
IRC30 = 94% transmisson

At 532nm:
IRC21 = 94.3% transmisson
IRC30 = 95.6% transmisson

Because the type IRC21 is cheaper than the IRC30 it is conceivably that manufacturers of Web Cams prefer the cheaper type for their cams. So it can easily happen that 78% or more of the light is lost!

But we cannot remove the filters if we want to get a colourfast texture. Who wants to see e.g. for red a weak pink? And the camara setting isn't able to compensate such distortions.

What can we do? We can simply use shorter wavelengths, e.g. red at 635nm or better green at 532nm and up to 95% of the light is saved.

Walter
Attachments
IR Filterkurven3.JPG
Light transmisson vs. the laser wavelength
Last edited by WalterMo on Tue Sep 01, 2009 12:46 pm, edited 2 times in total.
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Postby hal » Sun Aug 30, 2009 10:34 pm

Very interesting! Thanks for the infos :D
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Re: Impairment by IR filters - A further reason to prefer green

Postby accuteoptical » Fri Sep 11, 2009 11:31 am

Part No Description
IBG-630 T=50% 630±10nm
IBG-640 T=50% 640±10nm
IBG-650 T=50% 650±10nm
IBG-670 T=50% 670±10nm
IBG-680 T=50% 680±10nm
IBG-690 T=50% 690±10nm

The IR-cut filter(ICF) is a short pass (a lowpass filter) filter that blocks infrared light and transmits the visible light.


Image
IR filter,IR cut filter,beam splitter,beam combiner,www.accuteoptical.com
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Re: Impairment by IR filters - A further reason to prefer green

Postby MagWeb » Fri Nov 06, 2009 5:48 pm

Hi,

some experience and and questions I´m working on:

The IR filter covers all of the sensors - all green,blue and red ones on the chip. So that should mean, that the green, red and blue filtering on the chip itself is IR permeable. That should mean that all the sensors should be able to detect IR light if there is no filter.

I noticed building a PRO 9001, with the guidance of Walter, that a cam without IR filter (for it gets lost removing the original optics) shows a rather "pale" image also if the cam should see clear colors. Think that´s proving, that the for the human eye invisible IR is detected by all the sensors.
Now using a red laser gave a white line with a red corona. Reducing the saturation removed the red corona and I got a white line (which should be a result of all the pixels not an interpolated BW).

So, if I´m not interested in the colored texture wouldn´t it be better to remove the IR filter completely and use a beam near to IR?
(To get a right colored image, one could also attach a removeable filter on the outer side of the optics)

@Walter BTW: the dimension of the Pro For Notebook: board 24mm (chip centered in that direction) x 53.2mm (chip 43mm from the upper edge). This board is easier to abuse for a PRO 9001, for there are no higher parts around the censor and the USB cable is plugged in (not soldered).

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Re: Impairment by IR filters - A further reason to prefer green

Postby WalterMo » Fri Nov 06, 2009 8:22 pm

Hi Gunter,
Thanks for finding out the data of the Pro For Notebook. Perhaps I will exchange the now built-in TRUST WB-5400 with this camera board. Up to now this modified camera hasn't an IR filter built-in and I notice a very high sensitivity of the camera. But surely it's also a reason of the „large“ diameter of the Super-8-lens.

You are writing:
„The IR filter covers all of the sensors - all green,blue and red ones on the chip. So that should mean, that the green, red and blue filtering on the chip itself is IR permeable. That should mean that all the sensors should be able to detect IR light if there is no filter.“

I think all the sensors can detect IR. But these sensors aren't virgin. They have already red, green and blue filters in front. And who knows what IR absorpion they have. The very best seems to be to use a B&W camera for IR. But make a test with your IR TV remote control.

A red laser should be better for scanning if we use a monochrome camera. The sensitivity for silicon photo chips is rising from green to a maximum near 840nm.

But to attach an IR filter outside the lens is rather expensive because of the bigger diameter. And it must be equipped with a metallic holder.

Walter
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Re: Impairment by IR filters - A further reason to prefer green

Postby WalterMo » Sun Nov 08, 2009 5:19 pm

Attachments
SonyICX204AL.jpg
Spectral response of the Sony monochrome chip ICX204AL
SonyICX204AK.jpg
Spectral response of the Sony color chip ICX204AK
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