Re: Sony Mavica works with Wratten 87

[Todd Martin <todd@xxxxxxxxxxx>]

Hi all,

With all the recent talk of IR with digital and video cameras, I thought
I'd chime in...

Most of you probably know that the CCD imagers in the typical camcorder and
digital camera are sensitive to infrared wavelengths. In fact, until very
recently, most of "consumer" digital cameras used off-the-shelf camcorder
CCDs -- encumbered with the elongated pixels, low resolution, and slow
"read-out" methods intended for NTSC (or your favorite video standard).
Now, CCD manufacturers make CCDs with true square pixels, better color
masks, full frame read-out, higher resolution, etc. intended specifically
for digital cameras.

Incedently, most of the "new" work in digital image sensors is with
active-pixel CMOS devices. These are basically an area array of
photodiodes. This technology offers some great advantages to digital
imagery (some realized now, some promised for the future). CMOS sensors,
like CCDs, are made in silicon with similar sensitivities to light
radiation.

Both camcorders and digital cameras have to use an IR cut filter over the
CCD to prevent (unwanted) infrared energy from saturating the sensor. This
filter prevents (or reduces) "blooming" in the image. Typically, the filter
is part of the glass cover to the CCD chip. As no physical filter is
perfect, the IR cut-off filter is more like a "roll-off" filter. Some
filters have steeper roll-off than others making one camera "more
sensitive" to IR than another.

Also, any color CCD uses some type of patterned color mask directly on the
surface of the CCD to create images that have all 3 colors (3-chip cameras
are another story). The pigments in these masks also pass or block infrared
to varying degrees. Simply removing a IR cut filter from a CCD would have
unpredictable effects.

I know of two digital camera manufactures that are marketing infrared products:

Kodak has IR versions of it's DCS 4xx camera line:
They offered the original DCS line with a monochrome IR sensor (standard
sensor without color mask nor IR filter on window). Now they have a
specific model, DCS 420 IR:
http://www.kodak.com/productInfo/catalog/genInfo/dcsIRDigitalCameras.shtml

I see that they now have a "color" IR version for approximating false color
IR photos.
http://www.kodak.com/US/en/cgs/aerial/products/cameras/colorIR.shtml

I don't know much about the Kodak products -- as the camera's price tags (?
$10k - $15k or more ?) is above my, and my employer's, toy budget 8-).

Dycam makes a IR version of their own Model 4 camera called the ADC
(Agricultural Digital Camera). It was made to image two-color images that
approximate the satellite remote sensing red and near-IR (TM bands 3 and 4):
http://www.dycam.com/agri.html

I do have experience with Dycam's products. I evaluated the ADC for work
purposes. The resulting image, viewed as a RGB image, has and IR image on
the "green" channel and red image on the "red" channel. The blue channel is
blank (unless you over saturated the CCD). When I altered the image to have
IR on the red channel and red in the green channel, the resulting images
started to look like traditional false-color IR images.

Where as the ADC isn't as expensive as the Kodak hardware (~$1000 -- I
think), the ADC has some serious limitations: very low resolution and slow
image transfer times are the biggest.

I'm also aware of special "cameras" made for recording far-infrared images
to study heat profiles. These are very expensive and beyond this note...

I hope some of the above is useful to people on the list...

Have a good weekend,

~Todd








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