tom@syssoft.com (Rodentia) (03/14/91)
I have been playing with infra-red transmitters and receivers, and
seem to be having a lot of trouble with range. I have an audio
amplifier connected to an IR receiver (transistor) in the following
manner:
+9---390ohm---audio input(+)---IR receiver---audio input(-)---gnd
With that, I can hear my remote from about four feet away (with a
fast dropoff in amplitude). In fact, I get similar results if I
remove the power from the receiver. My TV definitely can handle
eight feet plus, so what are they doing better? What is cheap to
do with lensing, and what helps electronically?
I know the remote does some modulation. What travels the best?
I was experimenting with pulse width modulated speech transmission,
but it drops off as fast as the remote does. I was trying to avoid
demodulation (as I could with PWM), but now I'm willing to add some
hardware to the receiver.
It appears that fast rise and fall times are characteristic of my
PWM signal. Could a differenting op-amp going to a set-reset flip
flop give me the sort of gain control I need? Or is frequency
information more amplitude independent? Any other tricks to get
better sensitivity out of the IR receiver?
BTW, I did read a Forest Mimms article out of Modern Electronics,
but that was just a high (adjustable) gain amplifier, and I was
looking for something that wouldn't need adjustment based on
distance. Thanks in advance.
--
Thomas Roden | tom@syssoft.com
Systems and Software, Inc. | Voice: (714) 833-1700 x454
"If the Beagle had sailed here, Darwin would have | FAX: (714) 833-1900
come up with a different theory altogether." - me |rick@ofa123.fidonet.org (Rick Ellis) (03/18/91)
On <Mar 13 19:30> Rodentia writes:
R> With that, I can hear my remote from about four feet away (with a
R> fast dropoff in amplitude). In fact, I get similar results if I
R> remove the power from the receiver. My TV definitely can handle
R> eight feet plus, so what are they doing better?
They modulate the IR at about 40khz (usually) and use a very narrow band
detector. The detectors ring badly but are good for detecting the leading edge
of a burst.
--
Rick Ellis
Internet: rick@ofa123.fidonet.org
Compuserve: >internet:rick@ofa123.fidonet.org
BBS: 714 939-1041
--------------------------------------------------------------------------bame@hpfcbig.SDE.HP.COM (Paul Bame) (03/19/91)
> I have been playing with infra-red transmitters and receivers, and > +9---390ohm---audio input(+)---IR receiver---audio input(-)---gnd > With that, I can hear my remote from about four feet away (with a > fast dropoff in amplitude). In fact, I get similar results if I > remove the power from the receiver. My TV definitely can handle > eight feet plus, so what are they doing better? What is cheap to > do with lensing, and what helps electronically? You're probably being swamped by ambient light to which your sensor is sensitive - probably some visible in addition to the IR. An IR filter would help but modulation (plus a filter) is the trick your TV uses. There are several IR ICs on the market though they are mostly for digital data. If you can make do with a digital data stream of <= 1KHz you can use the Radio Flak "IR receiver module". The fact that you can remove power from your receiver with about the same effect probably means you aren't biasing the phototransistor receiver (or transmitter?) properly. -Paul Bame bame@fc.sde.hp.com N0KCL
jws@cica4.mlb.semi.harris.com (James W. Swonger) (03/19/91)
You'll get the most benefit from three things: (1) Rejecting all non-signal inputs (2) Concentrating IR signal from a larger area (3) Increasing receiver gain To reject non-signal inputs, you want a colored filter (like the red-black square on the TV) which passes IR only, along with an input stage which rejects DC, 60 and 120Hz (basically anything well below operating freq.). Having rejected the DC/LF components, you can go to a much higher gain configuration - the 390 ohm collector resistor you have is probably selected to keep the output from railing under high DC illumination but your phototransistor outputs >current< proportional to light input so your gain is going to suck; with IR-pass filtering you should be able to increase that value or better yet blow off that configutation and use the phototransistor with an op-amp and go for some gain; AC couple the photo output and make a high-pass filter/gain stage, then run that through a Schmitt trigger. As far as concentrating the IR data input, you need to make a trade-off between concentration (physical gain) and acceptance angle. If you can take a very directional input sensitivity then you can use a big lens and get >10X gain from physics alone - at the cost of a more limited field of view. The transmitter can also benefit from a good lens - you want a fairly tight beam (though not so tight that you have to be a sharpshooter...).
dennisg@felix.UUCP (Dennis Griesser) (03/26/91)
On <Mar 13 19:30> Rodentia writes: R> With that, I can hear my remote from about four feet away (with a R> fast dropoff in amplitude). In fact, I get similar results if I R> remove the power from the receiver. My TV definitely can handle R> eight feet plus, so what are they doing better? In article <2766.27E40773@ofa123.fidonet.org> rick@ofa123.fidonet.org (Rick Ellis) replies: E> They modulate the IR at about 40khz (usually) and use a very narrow band E> detector. The detectors ring badly but are good for detecting the leading E> edge of a burst. OK, so let's talk detector quality. Specifically noise immunity. Like half the people on the net, I have been experimenting with the IR detector module made by Sharp, sold through Radio Shack. There are occasional bursts of optical noise that fool the receiver module into believing that it has seen an IR signal. I can deal with this. The incidence is low. The big problem is electrical noise on the power supply! I have a goodly amount of X-10 stuff in my place. Some of the transmitters are stronger than others. Whenever I press a button on one of the stronger transmitters, the IR module reports seeing a pulse. I have yet to analyze the duration and pattern of the signal. It merely shows up in the pulse-trapping setting of my logic probe. Obviously, some of the 121 kHz carrier of the X-10 leaks through the power supply that I am using for the IR module, and fools it into believing that it sees an optical signal. I have verified this by operating the IR module from batteries; the noise problem goes away. This is a bit spooky, since I get the IR module power from the AC line by running it through a transformer, full-wave filtering it, smoothing it with several hundred microfarads, and passing it through a 3-terminal regulator (LM 309K). Of course, I have a few 10 mfd tantalum caps sprinkled around the digital section, and on the order of one .1 mfd monolithic per TTL package. And did I mention the Corcom line filter in the primary of the power supply transformer? Still the noise gets through. Any ideas, gang?
dennisg@felix.UUCP (Dennis Griesser) (04/02/91)
In article <159639@felix.UUCP> I complain about X-10 signals interfering through the power supply with the Sharp IR receiver modules sold by Radio Shack. John Whitmore was kind enough to reply via E-mail, but my mailer can't respond to him. Since the discussion is perhaps interesting to other experimenters, I'll take the liberty of replying here... > This is very odd indeed; the full-wave rectifier in your > power supply should only turn 'on' the diodes near the AC peaks, > and the X-10 burst is near the zero crossing (not the peak), > so should never get past the diodes (let alone the filters). Yes and no. Each X-10 burst is sent three times, to coincide with the zero crossings in other phases of the AC line. So we might well get an X-10 burst at (almost) any time. Also, different X-10 transmitters produce signals of different amplitudes. At home, I have observed the following progression: maxi_controller < CP-290 << TW-523 If the bursts are strong enough (say 100v p-p), it doesn't matter when it hits, it will still get through the bridge. Perhaps I should try a MOV. > The LM309, however, is a terrible excuse for a regulator > at high frequencies; the gain of its internal error amp is low at high > frequency (and you get a millivolt of AC out for each volt of AC in > at circa 1 MHz). The plot shows 30 dB rejection at 100 kHz (which I > understand is the X10 frequency). You can double that with an > RC filter ahead of the regulator (it would be something like > your 10 uF capacitor with a 16 Ohm resistor), which might be a help. The X-10 frequency is actually spec'ed at 121 kHz, but most writers just round it to 120 kHz. This is probably close enough to 100 kHz to make your figure from the plot at least ballpark. Good idea for the R/C. I'll try it in a couple of days. Can you give me a formula for attenuation as a function of R/C/f? [Give me a break, I'm a software type.] Otherwise, I'll try that 16-ohm/10-mFd combination. > Maybe it would pay to use smaller current-carrying capacity > in your full-wave rectifier; the big ones have a lot more stray > capacitance than the smaller ones. I have actually tried two power supplies, both exhibiting the same problem. The one that I use more frequently has a bridge rated at only 1.5 A. Doesn't seem all that high. In the meantime, I have experimented with outlandishly large values of electrolytic filter capacitor in front of the 3-terminal regulator. A couple thousand mFd extra eliminates the interference from the CP-290, but lets the TW-523 through. The latter starts to get better around 47000 mFd and is eliminated around 80000 mFd. In addition to cost and general lack of style, the huge-capacitor approach is tough on the bridge rectifier when you turn it on. There's got to be a better fix out there. [Thanks again for your response, John. Anybody else care to step up to bat?]
heck@babcock.cerc.wvu.wvnet.edu (Michael Adam Heck) (04/05/91)
Just thought I'd offer another solution...instead of trying to filter the supply to the whole circuit, why not just filter the supply to the module. This requires a smaller capacitance for the same level of filtering. I don't know offhand how much current the module pulls but I'm sure it is probably small compared to the rest of your circuit. Maybe try this... +5v | R | _________________________ | |+ _________ --- |module | ___ | | | _________ gnd | | gnd As long as R is small enough to avoid any apreciable voltage drops due to the steady-state current of the module, the module should still function properly. C will supply any instantaneous current needs of the module as well as filter the noise from the 5 volt supply. I've seen this used in the low-level stages of high-gain amplifier modules to filter noise from the more sensitive stages of the circuit. Anyway, just a thought I had. Hope it can help. Mike
jgk@osc.COM (Joe Keane) (04/10/91)
In article <160142@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >[Thanks again for your response, John. Anybody else care to step up to bat?] OK, i'll take a swing. The problem is that high-frequency signals are coming off the power line into the circuit. The reason they're coming through is that there's a low-impedance path at those frequencies. I don't think that increasing the filter capacitor does much good at blocking these signals. I mean, it probably works with large enough capacitors, but it's not the right way to do it. It increases the peak current through the transformer and rectifier, and also makes your power supply less `friendly' in terms of generating harmonic currents. I think the right solution is to add a series choke (inductor). Chokes are good idea for a power supply, in order to smooth out current flow into the filter capacitor. The problem is that they're expensive and take up space, so people often leave them out. Then you're just relying on the leakage inductance of the transformer, which often isn't very large. It doesn't matter whether you put the choke in the primary or secondary, it just means a different value for inductance and current. Use whatever's cheaper. I don't know how much power your equipment draws, so i can't give exact numbers on what to do. I would say that a choke in the primary would be something like 10 millihenries at 1 ampere, while in the secondary it would be 100 microhenries at 10 amperes. You can get these pretty cheap at some places, or you can just make your own. Again these are just somewhat conservative order-of-magnitude estimates, you'll have to see what works. Note that, especially with a large filter capacitor, the peak current can be a lot higher than the average current if the rectifier only conducts during a small portion of a cycle. The choke should reduce this problem somewhat, but you should make sure the choke is rated for the peak current. -- Joe Keane, amateur electronics hacker jgk@osc.com (...!uunet!stratus!osc!jgk)
asd@cbnewsj.att.com (Adam S. Denton) (04/11/91)
In article <4729@osc.COM> jgk@osc.COM (Joe Keane) writes: >In article <160142@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >>[Thanks again for your response, John. Anybody else care to step up to bat?] > >I think the right solution is to add a series choke (inductor). Yup! >inductance of the transformer, which often isn't very large. It doesn't >matter whether you put the choke in the primary or secondary, it just means a Its best location is after the filter capacitor and diode. Then, install another filter cap after the choke -- making a `pi' filter. You will find this very effective. Use the highest inductance you can use given the constraints of 1) physical space 2) DC resistance of the inductor windings vs. load current and tolerable voltage drop (and core saturation if the supply is hefty) I often just take apart little audio xformers garnered from old surplus and re-wind them with 24-guage enamel wire -- usu. 150-200 turns. Has about 2.5 ohms of resistance. I make both filter caps equal in value. (Old car radios are a good source for these inductors too.) The inductance you need depends on the frequencies you want to block. If you're blocking 120Hz, the more, the merrier. If it's high-frequency noise, you can do with a little less. Compare the inductive reactance with the capacitive reactance of the post capacitor. Watch out for series-resonance! (Usually not a problem because the Q is so low, since the load looks like a low-value resistor.) >Note that, especially with a large filter capacitor, the peak current can be a >lot higher than the average current if the rectifier only conducts during a >small portion of a cycle. The choke should reduce this problem somewhat, but >you should make sure the choke is rated for the peak current. You DO NOT want to `filter' pure AC. You'll just be introducing a voltage loss. You DO want to filter the AC component of DC. Thus, the choke goes after the diodes (and initial filter cap), not before. Also...the peak current is mostly a function of the initial filter cap. Adam Denton asd@mtqua.att.com
tonya@hpldsla.sid.hp.com (Tony Arnerich) (04/13/91)
Felix spoke of needing horrendously large capacitance to get rid of some noise coming through his power supplies - 80000 uF, for instance. Wow - there's a better way. Most people think that a uF is a uF is a uF. Not so. There is a frequency dependence that in fact is staggering for electrolytics. In fact, they begin to be inductors, given a high enough freqency. Parallel inductance is a poor low-pass filter. The way they say to do it in "CMOS Cookbook" is to parallel capacitors of varying size and type: ~2000 uF electrolytic ~ 10 uF (tantalum?) ~ 1 uF (ceramic?) ~ 100 pF (mica?) Forgive me if I don't have the sizes and type right - get the book. The point is that at high enough frequencies, the "smaller" capacitors in fact have a higher capacitance than the "big" ones and carry the brunt of the filtering load. 120 kHz could very easily be "high" to an electrolytic. Also, there's no substitute for DISTRIBUTED capacitance throughout your circuit if you want to kill noise propagation in a digital design. Analog? Beats me. I work hard in digital circuits to *suppress* the analog. ;-) tonya@sid.hp.com
jgk@osc.COM (Joe Keane) (04/17/91)
In article <4729@osc.COM> i write: >Note that, especially with a large filter capacitor, the peak current can be a >lot higher than the average current if the rectifier only conducts during a >small portion of a cycle. The choke should reduce this problem somewhat, but >you should make sure the choke is rated for the peak current. In article <1991Apr10.235418.10741@cbnewsj.att.com> asd@cbnewsj.att.com (Adam S. Denton) writes: >You DO NOT want to `filter' pure AC. You'll just be introducing a voltage >loss. You DO want to filter the AC component of DC. Thus, the choke goes >after the diodes (and initial filter cap), not before. I stand by my original statement. It's more important to filter the current before the capacitor. The key phrase here is `harmonic current'. A rectifier and capacitor connected directly to the mains would produce large harmonic currents. Somewhere in this path you need some impedance to smooth out the current. Like i said before, cheap power supplies rely on the resistance and leakage inductance of the transformer. With reasonable filtering before the rectifier, the high harmonics aren't generated in the first place. This is a lot easier than trying to filter them out later. With large rectifiers, series inductors are mandatory. Large AC-DC converters have elaborate filtering schemes, with series inductors and multiple tuned shunts, and these are all on the line side. The power company would have a bird if you hooked up a large rectifier and capacitor directly to the power line. I don't know what specifically `large' would be though. Assumedly, the voltage on the filter capacitor is subsequently regulated. A regulator can deal with some 120 Hz hum, and if that's all there is, very little will get through to the output. That's because it can respond well to slowly changing voltages, and 120 Hz is fairly slow. However, high-frequency transients will shoot right through, or around, by capacitive coupling. -- Joe Keane, not a real EE jgk@osc.com (...!uunet!stratus!osc!jgk)
asd@cbnewsj.att.com (Adam S. Denton) (04/18/91)
In article <4739@osc.COM> jgk@osc.COM (Joe Keane) writes: >In article <4729@osc.COM> i write: >>Note that, especially with a large filter capacitor, the peak current can be a >>lot higher than the average current if the rectifier only conducts during a >>small portion of a cycle. The choke should reduce this problem somewhat, but >>you should make sure the choke is rated for the peak current. > >In article <1991Apr10.235418.10741@cbnewsj.att.com> asd@cbnewsj.att.com (Adam >S. Denton) writes: >>You DO NOT want to `filter' pure AC. You'll just be introducing a voltage >>loss. You DO want to filter the AC component of DC. Thus, the choke goes >>after the diodes (and initial filter cap), not before. > >I stand by my original statement. It's more important to filter the current >before the capacitor. Try it and see. I have designed and built many power supplies and switchers. You are right technically, in that a series inductor will do some filtering, but that same inductor will be much for effective in a pi filter. Try it. If it doesn't turn out that way, I'll eat my hat. >The key phrase here is `harmonic current'. A rectifier and capacitor >connected directly to the mains would produce large harmonic currents. >Somewhere in this path you need some impedance to smooth out the current. >Like i said before, cheap power supplies rely on the resistance and leakage >inductance of the transformer. With reasonable filtering before the >rectifier, the high harmonics aren't generated in the first place. This is a >lot easier than trying to filter them out later. Oh...I thought your goal was the most noise-free DC output from which a circuit could be powered. If your goal is the slowest-changing currents thru the rectifiers, then of course you'd want the L there. But if you want the smallest ripple (at any freq) at the supply output, which is usually what you want, then move the L to the DC portion. >With large rectifiers, series inductors are mandatory. Large AC-DC converters >have elaborate filtering schemes, with series inductors and multiple tuned >shunts, and these are all on the line side. The power company would have a >bird if you hooked up a large rectifier and capacitor directly to the power >line. I don't know what specifically `large' would be though. Actually the rectifer would release its smoke, and thus cease to work. :-) The L's and RC filters in switchers etc. in series with the diodes are there for very different reasons than the need you implied in your 1st post. They are there to limit absolute voltages, absolute currents, and to limit the derivates of both with respect to time. These constraints are seldom necessary in a low-frequency (120Hz), low-voltage supply. Most importantly, they are not there for `filtering' per se. They are there to ensure the V, I, dV/dt, and dI/dt ratings of the rectifiers and othere components are not exceeded. Otherwise, device failure will occur. >Assumedly, the voltage on the filter capacitor is subsequently regulated. A >regulator can deal with some 120 Hz hum, and if that's all there is, very >little will get through to the output. That's because it can respond well to >slowly changing voltages, and 120 Hz is fairly slow. However, high-frequency >transients will shoot right through, or around, by capacitive coupling. This is true. Especially troublesome are the RF components radiated directly by the diodes themselves. Often diodes in, e.g., TV main supplies have series ferrite cores (i.e. inductance) and parallel capacitors to limit the slewing of the current. Yes, you can probably do without a series choke if there is a post-regulator for LF. But put that choke in, and you'll see the HF disappear also. Try it! Adam Denton asd@mtqua.att.com Disclaimer: Presently, I am *not* wearing a hat.
pa1@tdatirv.UUCP (Pat Alvarado) (05/03/91)
In article <159639@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >OK, so let's talk detector quality. Specifically noise immunity. > >Like half the people on the net, I have been experimenting with the IR >detector module made by Sharp, sold through Radio Shack. There are occasional >bursts of optical noise that fool the receiver module into believing that it >has seen an IR signal. I can deal with this. The incidence is low. > . > >Any ideas, gang? Could the detector also be picking up levels of IR from ambient light? Most IR detector units cover the IR detector with a filter that is supposed to filter IR levels from ambient light, so the quality of the detector may be affected by the amount of IR from ambient light that is allowed to penetrate the filter. This could very well be the case where the detector is placed where a lot of sunlight is present. Thanks, -- ||| Pat Alvarado | v Teradata Corporation | tdat!pa1@suntzu.sun.com /\ /\ 100 N. Sepulveda Blvd. | uunet!edsews!hacgate!tdat!pa1 /// \\\ El Segundo, Calif. 90245 | pa1@tdat.teradata.com
csmith@plains.NoDak.edu (Carl Smith) (05/04/91)
In article <237@tdatirv.UUCP> pa1@tdatirv.UUCP (Pat Alvarado) writes: >In article <159639@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >>Like half the people on the net, I have been experimenting with the IR >>detector module made by Sharp, sold through Radio Shack. There are occasional >>bursts of optical noise that fool the receiver module into believing that it >>has seen an IR signal. I can deal with this. The incidence is low. >>Any ideas, gang? >Could the detector also be picking up levels of IR from ambient light? Yes. I am the one who posted about a week ago about the remote control extender that I built using the Sharp GP1U52X receiver that Radio Shack sells. I included a "transmit" LED that flashes when the receiver thinks it is picking up and retransmitting a signal. With two 60 watt light bulbs on in the room, about 10 feet from the receiver, the receiver will output short pulses of noise once every 2 or 3 seconds. This doesn't cause any problem, since the VCR in the other room doesn't react to the retransmitted pulses of noise. If you are interfacing it with a computer, these noise pulses could be a problem... It IS the lights, cause when I turn them off, the false detections quit. They quit completely when the room is near pitch black. Any light at all seems to cause an occasional glitch. >Most IR detector units cover the IR detector with a filter that is >supposed to filter IR levels from ambient light, so the quality of the >detector may be affected by the amount of IR from ambient light that is >allowed to penetrate the filter. This could very well be the case where >the detector is placed where a lot of sunlight is present. I popped the dark plastic piece out of one of my remote controls and placed it over the front of the GP1U52X reciever, and the "glitches" dissapeared completely. I noticed no decrease in reception distance. But I have not been able to find a source of this IR filtering plastic. But I haven't looked to hard... :) > >Thanks, > >-- > ||| Pat Alvarado | > v Teradata Corporation | tdat!pa1@suntzu.sun.com > /\ /\ 100 N. Sepulveda Blvd. | uunet!edsews!hacgate!tdat!pa1 >/// \\\ El Segundo, Calif. 90245 | pa1@tdat.teradata.com Carl D. Smith Jr. csmith@plains.NoDak.edu Electrical and Electronics Engineering North Dakota State University, Fargo, ND
frerichs@adsl (dfRERICHS) (05/06/91)
In article <159639@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >OK, so let's talk detector quality. Specifically noise immunity. > >Like half the people on the net, I have been experimenting with the IR >detector module made by Sharp, sold through Radio Shack. There are occasional >bursts of optical noise that fool the receiver module into believing that it >has seen an IR signal. I can deal with this. The incidence is low. > > >Any ideas, gang? What you are seeing is NOT optical noise coming into the detector... it is EM inside the unit. It is very important to tie the case of the detector/demod to ground (it seems they didn't bother at Sharp). Failure to do this makes for a noisy environment for the circuit inside and causes noise bursts on the output pin. djf
rrw@naucse.cse.nau.edu (Robert Wier) (05/07/91)
I might be able to add a note to this. My students this semester
designed an ir control interface for an HO model train layout -
they used volume up and down for speed, channel up and down for
direction, and power on/off for emergency stop (all on a standard
tv ir controller).
They had it working flawlessly UNTIL the big day for the demo, which
was on "Engineering Education Day" back in late February. That
day, they couldn't get it to work at all. We suspect that it was
the lasers zapping across the hall and the many amp electric motors
running in the power lab next door producing all sorts of electrical
hash running around in the air and on the power lines. We didn't
realize that the case on the Sharp ir receiver wasn't grounded at
that point....
- Bob Wier
-------------- insert favorite standard disclaimers here ----------
College of Engineering
Northern Arizona University / Flagstaff, Arizona
Internet: rrw@naucse.cse.nau.edu | BITNET: WIER@NAUVAX | WB5KXH
or uucp: ...arizona!naucse!rrwhillman@newsserver.sfu.ca (Steve Hillman) (05/07/91)
I tried the suggestion of grounding the case of the Sharp IR detector and it worked beautifully. All of the spurious output vanished completely, even with multiple incandescent lights on. -- Steve "Skillman" Hillman "Everyone generalizes" hillman@whistler.sfu.ca skillman@tz.wimsey.bc.ca
tom@syssoft.com (Rodentia) (05/07/91)
In article <10162@plains.NoDak.edu> csmith@plains.NoDak.edu (Carl Smith) writes: >In article <237@tdatirv.UUCP> pa1@tdatirv.UUCP (Pat Alvarado) writes: [stuff deleted] >But I have not been able to find a source of this IR filtering plastic. >But I haven't looked to hard... :) > >Carl D. Smith Jr. >csmith@plains.NoDak.edu >Electrical and Electronics Engineering >North Dakota State University, Fargo, ND Try a piece of exposed (dark) color film negative. That should help. (suggestion courtesy of "Modern Electronics" (or whatever it's called nowadays), Forrest Mims III column) -- Thomas Roden | tom@syssoft.com Systems and Software, Inc. | Voice: (714) 833-1700 x454 "If the Beagle had sailed here, Darwin would have | FAX: (714) 833-1900 come up with a different theory altogether." - me |
dth@cs.brown.edu (Dzung T. Hoang) (05/07/91)
For an inexpensive IR filter, try developing unexposed slide film--the positive variety, not the normal negative type. Since the film has been unexposed to visible light, but has been exposed to infrared in the form of heat and maybe in the photo lab, it should work. I read about this a long time ago somewhere. ---------------------------------------------------------------------------- Dzung T. Hoang dth@cs.brown.edu ----------------------------------------------------------------------------