Infrared proximity sensor through glass

[Simon]

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 ?

[Simon]

We are using a VCNL4000: http://pdf1.alldatasheet.com/datasheet-pdf/view/425012/VISHAY/VCNL4000.html supposed to be pretty good and is well used around.

[Marco]

So the proximity value is just plain constant regardless of what you put in front of the glass?

[millerb]

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?

[Simon]

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?

That 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

[mikeselectricstuff]

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

[Simon]

won't stuff like silicone or resin for that matter change the emission and reception angles ?

[DavidDLC]

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

[DavidDLC]

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.

[DavidDLC]

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.

[cthree]

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_ceramics

Some metals are IR transparent too, Germanium for example.

[duskglow]

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.

[Someone]

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.

Rubbish!

http://www.chem.agilent.com/Library/brochures/5991-2392EN.pdf
http://www.shimadzu.com/an/industry/ceramicsmetalsmining/chem0501005.htm

Two manufacturers of the equipment that measures this disagree with you.

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 ?

Check your angles and polarisation for reflections off the window, its hard but not impossible to fix.

[Someone]

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.

Out in the thermal IR range, which is not where the proximity and photogate sensors work.

[profmason]

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

[Simon]

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

[smashedProton]

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

[smashedProton]

Why can't acrylic work?

[SeanB]

Look at Mike's thermal camera videos, where the acrylic is a pretty effective filter.

[Simon]

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 ?

[mikeselectricstuff]

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 ?

It'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

[Simon]

so seems we have some confusion here between infrared light and infrared heat radiation and infrared light will act like visible light, right ?

[dfmischler]

so seems we have some confusion here between infrared light and infrared heat radiation and infrared light will act like visible light, right ?

It 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.

[Simon]

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 ?

[ElectroIrradiator]

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...