Infrared proximity sensor through glass - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: June 28, 2013, 08:17:00 am
content: We are trying to get an infrared proximity sensor to work through glass (boss wants it destruction proof) but it picks up the glass. So far an integrated transceiver with I2C bus etc has been used and it won't work through a double glazed window. Are we just trying the impossible or should infrared work through glass ?We're looking at using descrete transimitter and receiver now, are we wasting our time ?

author: Simon
author_rank: Unknown
timestamp: June 28, 2013, 08:30:36 am
content: We are using a VCNL4000:http://pdf1.alldatasheet.com/datasheet-pdf/view/425012/VISHAY/VCNL4000.htmlsupposed to be pretty good and is well used around.

author: Marco
author_rank: Super Contributor
timestamp: June 28, 2013, 08:38:29 am
content: So the proximity value is just plain constant regardless of what you put in front of the glass?

author: millerb
author_rank: Regular Contributor
timestamp: June 28, 2013, 08:39:19 am
content: Any chance of being able to fill the space between the sensor and the window with some kind of transparent potting or perhaps some way to fit a light pipe from over the emitter to the window?

author: Simon
author_rank: Unknown
timestamp: June 28, 2013, 08:40:57 am
content: Quote from: millerb on June 28, 2013, 08:39:19 amThat is what I'm starting to wonder, if it can be clear potted and do away with the glass or have a glass window potted in but that is complicating matters a bit

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 28, 2013, 08:43:02 am
content: The issue is reflection from either the front or back surface. You need to eliminate the air gap between the sensor and glass, e.g. with a clear soft silicone.You may still get issues with reflection from the outer surface.A way around this would be to use a sensor with source/emitter further apart, and put a wall between them

author: Simon
author_rank: Unknown
timestamp: June 28, 2013, 10:06:54 pm
content: won't stuff like silicone or resin for that matter change the emission and reception angles ?

author: DavidDLC
author_rank: Frequent Contributor
timestamp: June 28, 2013, 10:27:37 pm
content: The location of the sensor related to the glass is key, you need to know the angle of the transmision cone for the IR LED and the angle of the reception cone of the sensor, the refractive index of the glass has to be consider for the placement as well. You may want to ask for the integration guidelines to the vendor for the component you are using.http://en.wikipedia.org/wiki/Refractive_index

author: DavidDLC
author_rank: Frequent Contributor
timestamp: June 28, 2013, 10:31:56 pm
content: You need to determine where the cones interfere and also need to be carefull with the crosstalking in order to avoid false detections.This is a raw example of what you can do.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: June 28, 2013, 10:36:16 pm
content: Another recommedation is to make it to work without the glass first, just to make sure that the hole system is working, after that you start doing the calculations for the placement.

author: cthree
author_rank: Frequent Contributor
timestamp: June 29, 2013, 01:26:02 am
content: Because Science. Most glass is opaque to IR. It's designed this way on purpose, otherwise heat would go right through it and it would be impractical to use glass for windows.You need special glass.http://en.wikipedia.org/wiki/Transparent_ceramicsSome metals are IR transparent too, Germanium for example.

author: Unknown
author_rank: Unknown
timestamp: June 29, 2013, 02:55:04 am
content: Mythbusters actually used glass in an attempt to foil an infrared motion sensor. I don't remember if it worked, but it does block infrared, you could see it on the thermal camera. AFAIK there's no way around it.

author: Someone
author_rank: Super Contributor
timestamp: June 29, 2013, 03:04:01 am
content: Quote from: cthree on June 29, 2013, 01:26:02 amRubbish!http://www.chem.agilent.com/Library/brochures/5991-2392EN.pdfhttp://www.shimadzu.com/an/industry/ceramicsmetalsmining/chem0501005.htmTwo manufacturers of the equipment that measures this disagree with you.Quote from: Simon on June 28, 2013, 08:17:00 amCheck your angles and polarisation for reflections off the window, its hard but not impossible to fix.

author: Someone
author_rank: Super Contributor
timestamp: June 29, 2013, 03:08:06 am
content: Quote from: duskglow on June 29, 2013, 02:55:04 amOut in the thermal IR range, which is not where the proximity and photogate sensors work.

author: profmason
author_rank: Contributor
timestamp: June 29, 2013, 06:18:39 am
content: I have had good luck using sharp gp2 series proximity sensors behind glass in museum display cases for triggering exhibits. Typically incline them at greater then thirty degrees to the glass surface to avoid measuring the reflection. Range from 30 cm in.Cheers,Mmason

author: Simon
author_rank: Unknown
timestamp: June 29, 2013, 06:46:20 am
content: Ah, our problem will be the space, the glass has to be part of the bottom of the box and a 30 degree angle may not get the sensor looking in the right place but it might work

author: smashedProton
author_rank: Frequent Contributor
timestamp: June 29, 2013, 07:25:48 am
content: Glass is usually ok for infared. It attenuates uv a lot more, that is why feynman sat in a truck rather than wear welding goggles when there was an a bomb test. You should be able to null out the time offset caused by the index of refraction of the glass

author: smashedProton
author_rank: Frequent Contributor
timestamp: June 29, 2013, 07:28:52 am
content: Why can't acrylic work?

author: SeanB
author_rank: Super Contributor
timestamp: June 29, 2013, 08:00:43 am
content: Look at Mike's thermal camera videos, where the acrylic is a pretty effective filter.

author: Simon
author_rank: Unknown
timestamp: June 29, 2013, 08:06:24 am
content: We can't use acrylic anyway because everything has to be flame retardant etc.Is there a difference between infrared heat and light or is it the same thing that is talked about different depending on the circumstance ?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 29, 2013, 09:04:23 am
content: Quote from: Simon on June 29, 2013, 08:06:24 amIt's all about wavelength. Thermal imaging/temp sensing is in the 5-15uM range, IR LEDs are near-IR, typically just above visible red wavelengths - 780-980nm.For all intents and purposes, IR sensors like this will behave just the same as visible red light, the exception being the purpose-made black (typically acrylic) IR filter materials which have a sharp cutoff between visible red and near-infrared.Any normal material that passes red light will pass IR. May be some very minor differences in refraction, lens focal length etc but nothing you'll notice

author: Simon
author_rank: Unknown
timestamp: June 29, 2013, 11:57:08 am
content: so seems we have some confusion here between infrared light and infrared heat radiation and infrared light will act like visible light, right ?

author: dfmischler
author_rank: Frequent Contributor
timestamp: June 29, 2013, 01:17:34 pm
content: Quote from: Simon on June 29, 2013, 11:57:08 amIt is all infrared light. But the different wavelengths represent different energy levels (aka temperatures) and the temperatures we are usually interested in for thermal imaging are much longer wavelengths (i.e. cooler). Think about the brightness and color of light emitted by steel at high temperature and what that really means (e.g. bright cherry red steel is >800 degrees C). The atoms are so excited that some of the electrons jumping back to their base levels are giving off photons that energetic (i.e. with wavelengths that short) as the metal cools by radiation. Once it gets below some temperature (can't remember the threshold but certainly by the time you get below 400 degrees C) you can't see the emitted light anymore, but you can still feel the heat. The wavelength of the emitted radiation gets longer as the metal cools.Near-infrared light (that is, light of a wavelength almost short enough to be visible) will go through window glass pretty much the same as visible red light. Infrared light emitted by objects near room temperature have a much longer wave length and some (most?) of those wavelengths will not go through the glass.

author: Simon
author_rank: Unknown
timestamp: June 29, 2013, 07:02:20 pm
content: Yea i see what you mean, I mean i know that at the end of the day it's the same type of energy but if the range is that large it will manifest differently. so light should not have a problem or rather the wavelength near visible light won't. Well bugger me why the sensors won't work. Has anyone got any experience with integrated proximity sensors versus using separate emitters and receivers ?

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: June 29, 2013, 09:36:36 pm
content: Sorry about barging in on a polite debate, but felt I had to raise my voice on this subject.Simon,Judging from the datasheet, then your idea is in trouble. The detector seems optimized to work in the range from 1 to 100 mm, with a difference in signal strength over this range of about three orders of magnitude. Refer to Figure 3 on page 3 of the datasheet.Your problem is almost certainly that the detector sees the transmitter diode reflected in the *outside* air/glass surface. Each pane of glass obviously has two air/glass interfaces, and both will give reflections. Even using a transparent potting compound wouldn't do you any good, as there is no way to eliminate the outside air/glass interface. This problem is likely to get worse, if you are using thick, armored glass, as this would increase the distance between the chip and the reflecting surface.From figure 3 it seems obvious that even a weak reflection from the glass surface a few mm away will completely overwhelm any signal from the 50-100 mm range.Try holding the detector IC about 20mm from the glass surface, and see if it detects its presence. IE. do a detector test as shown in the test circuit, also on page 3.You have two options, both of which has already been mentioned and ruled out(?):1) Set transmitter and receiver at an angle to the glass surface. For a transparent solid there exist an optimal angle, the Brewster angle, where reflections in the surfaces are minimized or eliminated. This angle depends on the difference in dielectric constant between the involved substances. For air/glass this angle is about 42 degree, yet reflections are reduced in a broad range of angles about this value.2) Separate the transmitter and receiver, so the RX cannot see the internal glass reflections from the TX.Sorry about being the bringer of bad news...

author: Simon
author_rank: Unknown
timestamp: June 30, 2013, 07:06:08 am
content: Well if it is so it is so we can't change the world so need to find a way of making our design work in what we have

author: Simon
author_rank: Unknown
timestamp: July 08, 2013, 06:47:20 am
content: I'm starting to have a weird thought that might not work, as this is ovewr a white sink we could shine one infrared light into the sink and have peoples hands block the reflextions back to the sensors, although it is beginning to sound like a bad idea already.I'm having trouble finding plain simple IR receivers on RS to pick up a reflected or transmitted IR beam, any ideas ? can't remember where I saw them but someone gave me details for a simple cheap receiver with open collector/drain output (want to use it on my scalextric track)

author: IanB
author_rank: Super Contributor
timestamp: July 08, 2013, 07:20:51 am
content: Quote from: Simon on July 08, 2013, 06:47:20 amI have used many, many public restrooms with a proximity sensor under the tap that turns on the water when you place your hands there, or turns on the soap dispenser in a similar way. This is a solved problem.Rather than trying to reinvent a wheel, why not do a patent search for the solutions other people have already put into production and then try to find a variation on one of these that doesn't infringe (or simply license one of the working solutions)?(This is on a train again, isn't it? You want an automatically operating tap that conserves water and can't be left running?)

author: woodchips
author_rank: Frequent Contributor
timestamp: July 08, 2013, 11:15:10 am
content: Another possible cause of problems with window glass is that it is metal coated to reflect infra red back into the house, this is what raises the U value of double glazing to such a seemingly high value. If you just use glass, as with exhibition protection, then there is no need for it to be metal coated, so will at least try to work.

author: Simon
author_rank: Unknown
timestamp: July 08, 2013, 11:25:41 am
content: can anyone suggest some IR receivers that are simple to use and cheap, I am looking at the scalextric project again with IR in mind so thinking of an emitter and receiver and the car breaks the beam. something with open drain/collector output would do

author: Simon
author_rank: Unknown
timestamp: July 08, 2013, 11:43:42 am
content: I was hoping for something that did not use a "carrier" frequency as i wanted interfacing plain and simple

author: Simon
author_rank: Unknown
timestamp: July 08, 2013, 12:02:53 pm
content: I spotted a couple on RS, hopefully they do the trick.

author: Simon
author_rank: Unknown
timestamp: September 23, 2013, 02:20:39 pm
content: We are now looking at using non reflective perspex, is this a good idea ? I'm going around in circles trying to find a way of getting the IR light out without it reflecting and unless it looks through a cover it is going to be considered vulnerable.

author: Simon
author_rank: Unknown
timestamp: September 23, 2013, 02:22:25 pm
content: what about polarized plastic ? does such a thing exist ? (in sheets i mean, i know you can get the camera filters)

author: IanB
author_rank: Super Contributor
timestamp: September 23, 2013, 03:10:22 pm
content: There will always be reflections, that's not something you can avoid. What you need to do instead is design the system so the reflections don't upset the operation.As has been said, one way is to place a barrier between the emitter and the receiver so the internal reflections have no path from one to the other.

author: Simon
author_rank: Unknown
timestamp: September 23, 2013, 03:23:50 pm
content: Well I'm also looking at putting a foam mask on the glass, I have a tube glued to the pcb so that the transmission is directed so if there is a flaom mask on the glass it can poke into it stopping reflections

author: mamalala
author_rank: Unknown
timestamp: September 23, 2013, 03:26:47 pm
content: Quote from: Simon on July 08, 2013, 11:43:42 amIf this is to be used in any place where you have artificial or natural light, not using any kind of modulation/carrier scheme will get you in trouble sooner or later. Because then you end up with just a light sensor that can not differntiate what the source of that light is. Some reflected sunlight or somesuch will cause it to trigger.Heck, even _with_ a carrier plus modulated data, sunlight can sometimes play tricks. 20 or so years ago i came across a bunch of Sony TV's that randomly turned on or off, or changed the volume randomly, when direct sunlight was shining on the receiver...Greetings,Chris

author: Simon
author_rank: Unknown
timestamp: September 23, 2013, 09:11:38 pm
content: Thats was on my scalextric lap counter. this project has a proper setup - not designed by me but done properly i know, i just have to make it work mechamically

author: IanB
author_rank: Super Contributor
timestamp: September 24, 2013, 02:56:07 am
content: I attach a video of an IR proximity sensor working through a protective window. It's one I encounter every day.It is evidence that such a system must be reasonably practical to design as there are so many of them out in the world.Ways in which this could defend against reflections from the window:It could use a physical barrier between sender and receiver to block reflected signalsIt could look for changes in signal intensity so that it responds only to movementWays in which this could defend against false signals from other sources:It could modulate the basic IR frequency with some other specific frequencyIt could encode some pattern into the IR signal and only respond to this patternIt could respond only to rapid, high intensity changes likely to occur from movement close to the sensorMaybe IR detector modules such as the one seen here can be purchased off the shelf as a complete item?

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 24, 2013, 03:06:45 am
content: Quote from: Simon on September 23, 2013, 03:23:50 pmI told you about the method we use to minimize crosstalk, you ignore it ? I don't know. It works trust me, we sell millions of devices.We also use a gasket around the receiver/transmitter to minimize crosstalk when the source and the sensor are in separate packages. The polarizer will help with the crosstalk and detecion, we also use it in our designs.David.

author: Simon
author_rank: Unknown
timestamp: September 30, 2013, 03:43:49 pm
content: I resolved it eventually, seems shining the IR at a 45 degree angle works but I was getting some reflection hitting the housing and bouncing around so covered the inside of the box in black foam and it solved the problem mostly.