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astrojohn
March 7, 2012, 18:55:48
While I'm waiting for my DBK21AU618 to arrive, I've been bench testing my DFK41 AU02.AS (weather has been too windy).

First, my DFK41 is a great little camera. But, here's my question: the camera is supposed to have an IR-cutoff filter which I presume makes it virtually blind to light longer than 700nm or so. I tested it with an ordinary IR TV remote which I presume uses a GaAs LED emitting light somewhere between 850 and 950 nm (depending on the LED used). The DFK41 is quite sensitive to this wavelength. When I placed my Astrodon NIR-blocking filter (which cuts off at 700nm) on the camera, it no longer sees the remote.

So, does anyone know why the DFK41 is sensitive to NIR when it's supposed to have an IR-blocking filter? The funny thing is that my Canon DSLR is also very sensitive to the remote 's LED as well so I'm curious about the performance of the IR blocking filters in general. My Astrodon filter apparently passes next-to-nothing in the NIR.

Stefan Geissler
March 8, 2012, 09:08:53
Hi,

For the sensitivities, I would like to point to the filter specification at http://www.theimagingsource.com/en_US/products/optics/filters/
The IR Cut filter in the camera is equivalent to the 486 filter. The sensitivity ends at 750nm. However, we rely on the filter manufacturer and did not test this on our own.

JKr
March 8, 2012, 10:45:50
I think it is quite common for built-in IR-cut filters to be a little "leaky". I did a few tests with my camcorder (Canon HV20) a long time ago, and it did let through a small amount of IR light. If I put my UV-IR filter, no IR is let through. If I remember correctly, the signal from the leaked IR was tiny compared to the signal from the visible spectrum, and had absolutely no effect on the color balance (when comparing images with and without using the additional UV-IR filter). I think the leaked IR light was only detected at exposure settings that would saturate the detector in the visible wavelenghts. Perhaps the same is true for the DFK.

astrojohn
March 8, 2012, 16:53:20
Hi,

For the sensitivities, I would like to point to the filter specification at http://www.theimagingsource.com/en_US/products/optics/filters/
The IR Cut filter in the camera is equivalent to the 486 filter. The sensitivity ends at 750nm. However, we rely on the filter manufacturer and did not test this on our own.



Thanks for the info...great information!

I've been following a DMK group on Yahoo and one of the writers commented on the NIR sensitivity of the DBK21AU618 and the inability to get decent planetary color rendition w/o the NIR-blocking filter (but the vendors all say the DBK IS the way to get good planetary color - so we'll see who's right). He also wrote that using a NIR-pass filter gives him a good luminance image so I'll give that a try also.

JKr
March 9, 2012, 08:29:37
Thanks for the info...great information!

I've been following a DMK group on Yahoo and one of the writers commented on the NIR sensitivity of the DBK21AU618 and the inability to get decent planetary color rendition w/o the NIR-blocking filter (but the vendors all say the DBK IS the way to get good planetary color - so we'll see who's right). He also wrote that using a NIR-pass filter gives him a good luminance image so I'll give that a try also.

That's expected, since the DBK does not have an IR-filter; you have to use your own filter if you want natural colors. Only the DFK models have inbuilt filters.