IR Cutoff filter

[Mark Thorson]

maxfoo wrote:

Its a red masking film sold in art supply stores. You can get a 30"x40"
sheet for about $6.00 (us). Used to use it to make pcb long ago...

I doubt it would be a stable IR filter. Its chromophores
aren't designed for long-term stability.

 

[Guy Macon]

You can make a cheap optical low pass filter for testing purposes
out of photographic film negative. Take some Kodak Kodacolor ASA
100 film, remove it from the roll, expose it to light for a minute
or so, then wind the film into the roll again. Tell the person
doing the developing that you just want negatives and that you
overexposed it on purpose (you don't want them trying to lighten
the negative/darken the print!) The processed color negatives can
be used as a low-pass optical filter. It should stay stable for
about as long as negatives last unless you hit it with a lot of
light (direct sunlight or a laser). It's fragile and easy to
scratch so I ususlly use two layers with the scratchable sides
facing each other.

Also see:
http://members.misty.com/don/irfilter.html
http://www.amasci.com/amateur/irgoggl.html
http://www.x-raycameras.com/kodak_wratten_87_range.htm


--
Guy Macon, Electronics Engineer & Project Manager for hire.
Remember Doc Brown from the _Back to the Future_ movies? Do you
have an "impossible" engineering project that only someone like
Doc Brown can solve? My resume is at http://www.guymacon.com/

 

[Paul Mathews]

There are many kinds of 'modulation'. The simplest, in concept,
involves sampling just before a pulse, sampling during a pulse, then
subtracting the first sample from the 2nd. This works wonderfully
well, and it can be done with analog or digital electronics, in
hardware or software. Of course, it's best to optimize the optics
first. One of the easiest ways to reduce the ambient contribution is
to narrow the field of view, using lenses and/or baffles, to just what
you need and no more. Perhaps contrary to intuition, lensing can
actually reduce ambient photocurrent if it narrows field of view (and,
if your optics don't 'look' into the sun or an image of the sun).

Paul Mathews


"Leeper" <leeper@molalla.net> wrote in message news:<DqGdnamMeq77GgndRVn-iQ@scnresearch.com>...

I'm trying to reduce the IR response, so that the sunlight IR doesn't mask
the
visible light I am trying to measure.

Modulation and all that will not work, I need to know the amplitude of each
Red, Green, and Blue device.

I'm using a reversed biased BPW34, and a the speeds I need to measure at,
it's 50pf (or whatever the
datasheet said) is wonderful, it helps me maintain 400nS rise response, to
make amplitude measurements 100 nS later, of the R, G, & B sources rather
easily. It only lacks a filter.

Something like the SFH2430 (?), has too much capacitance and internal
resistance for the method I am
using.

The BPW21 isn't bad, but it's cost of 4.55 is a bit high for me, plus it is
not in a surface mount package.

I found one device from Hamamastu that might possibly work, it's marginal,
I'll have to wait for it to
arrive to actually test it.

Any advice is greatly welcomed, thanks.

"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405012037.1fb9c829@posting.google.com...
John Popelish <jpopelish@rica.net> wrote in message
news:<409268B9.3E2EE77@rica.net>...
Leeper wrote:

I'm looking for a IR cutoff filter that is quite a bit cheaper than
what
Edmund Optics sells. OCLI wants 21 dolars, I'd like to find something
cheaper. I need to pass wavelengths from 420-660nm, though if it
attenuated
light on each end a bit, it would still be fine for me.

I'm putting it in front of a silicon photodiode, and need to kill the
IR
response on this part.

Can anyone recommend some material/manufacturer? (I know devices like
the
BPW21 already have this, but I need to use a different part).

By far, the cheapest way to do this is to pick a photo diode that
incorporates such a filter. What diode are you using. Perhaps
someone knows of a similar one that includes the filter.

As to filter materials, Schott KG series filter glasses are made for
this purpose.
http://www.noao.edu/kpno/filters/sgt/131t_sm.gif
The BG series has more red attenuation.
http://www.noao.edu/kpno/filters/sgt/123b_sm.gif
http://www.noao.edu/kpno/filters/sgt/123t_sm.gif

I agree with John, with this addendum: If you're trying to improve
signal to noise ratio by reducing photocurrent due to ambient light,
this often proves to be an elusive goal. The reason is that shot noise
is proportional to the square root of bias current. So, you will
typically need to reduce ambient photocurrent by more than the square
of the passband transmittance of any filter. An example will make
this clear. Supposing that you reduce ambient photocurrent by a
factor of 2, for a shot noise reduction of about 40%. If your
passband transmittance is 60%, you will not have gained anything.

Paul Mathews

 

[Scrim]

"Mark Thorson" <nospam@sonic.net> wrote in message
news:4095CE6E.FA165DAC@sonic.net...

maxfoo wrote:

Its a red masking film sold in art supply stores. You can get a 30"x40"
sheet for about $6.00 (us). Used to use it to make pcb long ago...

I doubt it would be a stable IR filter. Its chromophores
aren't designed for long-term stability.


Unless I've slipped up here, strange that it should be red - kind of implies

it lets that end of the spectrum through. An infra red blocker whose range
extends into the visible will have a lack of red in its passed light, which
makes it look blue.

Scrim

 

[Gordon Couger]

----- Original Message -----
From: "John Popelish" <jpopelish@rica.net>
Newsgroups: sci.materials,sci.optics,sci.electronics.design
Sent: Sunday, May 02, 2004 7:49 PM
Subject: Re: IR Cutoff filter


: Leeper wrote:
: >
: > I'm trying to reduce the IR response, so that the sunlight IR doesn't
mask
: > the
: > visible light I am trying to measure.
: >
: > Modulation and all that will not work, I need to know the amplitude of
each
: > Red, Green, and Blue device.
: (snip)
:
: Have you looked at any photo diodes that have narrow band visible
: filters built in like:
:
http://www.photonics.com/spectra/newprods/XQ/ASP/newprodidnl.6444/QX/read.htm
:
http://www.photonics.com/spectra/newprods/XQ/ASP/newprodidd.3114/QX/read.htm
:
Or only sampling the AC of the switched light source and letting the DC
component the sun light fall out you only get the LED reading with none of
the back ground noise from the sun. Do a 180 degree phase shift at the
sensor and amplify to signals exactly the same and combine them at the end
and any thing you have left is the common mode error. Go back and find the
schematic for an old Instorn amplifier using vacuum tubes to see how this is
done.
Gordon

Gordon Couger
Stillwater, OK
www.couger.com/gcouger