Forums

Selecting lens for IR thermometer

Started by Nicholas Kinar December 18, 2010
Hello,

I am using an IR thermometer (MLX90614 from Melexis) to measure the
temperature of a surface. This device is in a TO-can, and I am using
the standard package version (cf. first page of the datasheet, option
code A).  I would like to place a lens in front of this IR thermometer
so that I can measure the temperature of the surface with a spot size
of 3 mm.

How do I select or calculate the required

(a) focal length of the lens;
(b) lens diameter;
(c) distance of the lens to the detector

I would like the lens to be situated at a distance of 5 cm from the
measurement surface.  I assume that I would require the lens to have
an IR response and appropriate coating, since according to the
thermometer datasheet, the spectral response of the IR thermometer is
limited from 5.5 um to 15 um by a filter.

Would similar principles also hold for the selection of a lens in
front of a photodiode?  Can anyone suggest how I might go about
selecting the lens?  Is a good reference work available?
Nicholas Kinar wrote:
> Hello, > > I am using an IR thermometer (MLX90614 from Melexis) to measure the > temperature of a surface. This device is in a TO-can, and I am using > the standard package version (cf. first page of the datasheet, option > code A). I would like to place a lens in front of this IR thermometer > so that I can measure the temperature of the surface with a spot size > of 3 mm. > > How do I select or calculate the required > > (a) focal length of the lens; > (b) lens diameter; > (c) distance of the lens to the detector > > I would like the lens to be situated at a distance of 5 cm from the > measurement surface. I assume that I would require the lens to have > an IR response and appropriate coating, since according to the > thermometer datasheet, the spectral response of the IR thermometer is > limited from 5.5 um to 15 um by a filter. > > Would similar principles also hold for the selection of a lens in > front of a photodiode? Can anyone suggest how I might go about > selecting the lens? Is a good reference work available?
High school optics is better than good enough. You must have done some simple ray tracing for thin lenses back when, no? The main problem is going to be what to make the lens out of. Germanium and silicon are the usual choices. You can also use a very thin Fresnel lens made of HDPE, as used in front porch lights. If you're doing this on a hobby budget, go down to the hardware store and get a $12 front porch light sensor and rip out the Fresnel lens. Those ones are segmented, so you have to pay attention where you cut it. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
On Sat, 18 Dec 2010 19:21:13 -0800 (PST), Nicholas Kinar
<n.kinar@usask.ca> wrote:

>Hello, > >I am using an IR thermometer (MLX90614 from Melexis) to measure the >temperature of a surface. This device is in a TO-can, and I am using >the standard package version (cf. first page of the datasheet, option >code A). I would like to place a lens in front of this IR thermometer >so that I can measure the temperature of the surface with a spot size >of 3 mm. > >How do I select or calculate the required > >(a) focal length of the lens; >(b) lens diameter; >(c) distance of the lens to the detector > >I would like the lens to be situated at a distance of 5 cm from the >measurement surface. I assume that I would require the lens to have >an IR response and appropriate coating, since according to the >thermometer datasheet, the spectral response of the IR thermometer is >limited from 5.5 um to 15 um by a filter. > >Would similar principles also hold for the selection of a lens in >front of a photodiode? Can anyone suggest how I might go about >selecting the lens? Is a good reference work available?
The $20 Fresnel lensed IR thermometer at Harbor Frieght can be micro adjusted (aimed) pretty easily, and at 5cm, you could easily point it only at the device in question. Place a piece of room temp (ambient) matte black painted aluminum in front of the target with like a 1cm hole in it at the target location. Make a set of successively smaller holed plates like that one. With that, you can see just how well your device is already pointing. With successively smaller holes in the ambient plate, that is. Also, your device under test has to be hotter than ambient to be a proper pointing determination target, etc.
On Sat, 18 Dec 2010 23:13:01 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>The main problem is going to be what to make the lens out of. Germanium >and silicon are the usual choices. You can also use a very thin Fresnel >lens made of HDPE, as used in front porch lights. If you're doing this >on a hobby budget, go down to the hardware store and get a $12 front >porch light sensor and rip out the Fresnel lens. Those ones are >segmented, so you have to pay attention where you cut it. > >Cheers > >Phil Hobbs
Why bother? Perfect, ROUND Fresnel lens already in place, not to mention the little sensor and circuit behind it. Jeez. $20 http://www.harborfreight.com/infrared-thermometer-93984.html The street light illuminator/dimmer jobs are wide scan format Fresnel that will see side to side well, but not up and down. This little job actually has correct optical characteristics. He said it was already "filtered" so no need to choose Germanium or Pyrex or other IR specific lens media. This is one mouse trap that there are already probably "best of class" versions made. So much so that they are practically ubiquitous like wristwatches.
TheGlimmerMan wrote:
> On Sat, 18 Dec 2010 23:13:01 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> The main problem is going to be what to make the lens out of. Germanium >> and silicon are the usual choices. You can also use a very thin Fresnel >> lens made of HDPE, as used in front porch lights. If you're doing this >> on a hobby budget, go down to the hardware store and get a $12 front >> porch light sensor and rip out the Fresnel lens. Those ones are >> segmented, so you have to pay attention where you cut it. >> >> Cheers >> >> Phil Hobbs > > > Why bother? Perfect, ROUND Fresnel lens already in place, not to > mention the little sensor and circuit behind it. Jeez. $20 > > http://www.harborfreight.com/infrared-thermometer-93984.html > > The street light illuminator/dimmer jobs are wide scan format Fresnel > that will see side to side well, but not up and down. > > This little job actually has correct optical characteristics. > > He said it was already "filtered" so no need to choose Germanium or > Pyrex or other IR specific lens media. > > This is one mouse trap that there are already probably "best of class" > versions made. So much so that they are practically ubiquitous like > wristwatches.
Because they're completely opaque in the thermal IR. Try using your Harbor Freight gizmo to look at a wood stove through a piece of Plexiglass. You'll measure the temperature of the plexi. Same with window glass. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Thanks Phil and TheGlimmerMan; thank you very much for your
suggestions and for putting me on the right track.  I suppose this is
simpler than I thought - no need to worry about Gaussian beam theory,
field of view, or the numerical aperture of the lens.  These were the
questions in my head when I made the initial post.

One other question: If I am to use a C-mount lens holder for the lens,
would I have to worry about the color that the lens holder is painted
if the sensor and lens assembly are taken outdoors?  I've noticed that
most of these C-mount lens holders are painted black, but this might
heat up if the thermometer and lens assembly is placed outside in the
sun.  Would it be beneficial to coat the outside of the lens holder
with white paint?  This would be the side of the lens holder facing
the surface to be measured.  (Paint containing barium sulfate or a
similar chemical might work.)  The lens would be inserted into the C-
mount holder and the holder then screwed on the end of the tube.  The
IR sensor would be situated at the end of the tube, aligned with the
optical axis of the lens.  What color should be the inside of the
tube?
> similar chemical might work.) =A0The lens would be inserted into the C- > mount holder and the holder then screwed on the end of the tube. =A0The > IR sensor would be situated at the end of the tube, aligned with the > optical axis of the lens. =A0What color should be the inside of the > tube?
So there are only two parts to this assembly: a lens holder and a tube with an appropriately-machined end that allows the lens holder to be screwed on the end of the tube. The IR thermometer is situated at the opposite end of the tube, mounted on a PCB. The tube is fastened to the PCB by a flange on the end of the pipe. The flange has holes, and it is secured to the PCB with mounting screws. An O-ring is placed under the flange to ensure that stray light does not enter the tube.
"Nicholas Kinar" <n.kinar@usask.ca> wrote in message 
news:88c7fef3-7620-4c36-aaae-cd292271c9a9@39g2000yqa.googlegroups.com...
> Hello, > > I am using an IR thermometer (MLX90614 from Melexis) to measure the > temperature of a surface. This device is in a TO-can, and I am using > the standard package version (cf. first page of the datasheet, option > code A). I would like to place a lens in front of this IR thermometer > so that I can measure the temperature of the surface with a spot size > of 3 mm. > > How do I select or calculate the required > > (a) focal length of the lens; > (b) lens diameter; > (c) distance of the lens to the detector > > I would like the lens to be situated at a distance of 5 cm from the > measurement surface. I assume that I would require the lens to have > an IR response and appropriate coating, since according to the > thermometer datasheet, the spectral response of the IR thermometer is > limited from 5.5 um to 15 um by a filter. > > Would similar principles also hold for the selection of a lens in > front of a photodiode? Can anyone suggest how I might go about > selecting the lens? Is a good reference work available?
How about using a reflecting parabola or a segment of one? tm
On Sat, 18 Dec 2010 23:34:24 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>TheGlimmerMan wrote: >> On Sat, 18 Dec 2010 23:13:01 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> The main problem is going to be what to make the lens out of. Germanium >>> and silicon are the usual choices. You can also use a very thin Fresnel >>> lens made of HDPE, as used in front porch lights. If you're doing this >>> on a hobby budget, go down to the hardware store and get a $12 front >>> porch light sensor and rip out the Fresnel lens. Those ones are >>> segmented, so you have to pay attention where you cut it. >>> >>> Cheers >>> >>> Phil Hobbs >> >> >> Why bother? Perfect, ROUND Fresnel lens already in place, not to >> mention the little sensor and circuit behind it. Jeez. $20 >> >> http://www.harborfreight.com/infrared-thermometer-93984.html >> >> The street light illuminator/dimmer jobs are wide scan format Fresnel >> that will see side to side well, but not up and down. >> >> This little job actually has correct optical characteristics. >> >> He said it was already "filtered" so no need to choose Germanium or >> Pyrex or other IR specific lens media. >> >> This is one mouse trap that there are already probably "best of class" >> versions made. So much so that they are practically ubiquitous like >> wristwatches. > >Because they're completely opaque in the thermal IR. Try using your >Harbor Freight gizmo to look at a wood stove through a piece of >Plexiglass. You'll measure the temperature of the plexi. Same with >window glass. >
Good thing neither are used as sight 'windows' on a wood stove. Yes, I made instruments that were meant to look through "windows". I made them that looked at molten glass gobs as well... through a glass window. Usually not though. :-) I made a 4" diameter tube that had a Gold mirror and a rifle stock and a rifle scope on it, and the power industry used them before imagers to look at transformers on the poles, and ground and to look at insulator strings on towers for excessive heat (of course). Saved a lot of up close transformer inspections. The "window" on the front of that tube had to be there to keep out dust. That window was "IR transparent" for the range we were looking at. It was mere plastic film held on by the tube cap's interference fit and stretch. But it had a narrow spectral bandpass capacity. The one(s) we sold to NASA with the 1000' focus had a Pyrex glass lens, IIRC. Looking for a burn through in the launch gas exhaust ports on the big pads. 10 foot spot at 1000 feet. It is a one inch diameter lens, and a 2mm diameter transducer window with a 2x2 mm bolometer 2 mm below that window in a can. We had solid Ge lens, but also had bolometers which had various "windows" right on the transducer, which is, of course, the cheapest route. Best of class. He stated his already has a filter on it. It is probably at the transducer level, and if the range he gave is wide, further filtration would also mean further attenuation of the signal, so hopefully he can compensate for that.
> The $20 Fresnel lensed IR thermometer at Harbor Frieght can be micro > adjusted (aimed) pretty easily, and at 5cm, you could easily point it > only at the device in question. > > =A0Place a piece of room temp (ambient) matte black painted aluminum in > front of the target with like a 1cm hole in it at the target location. > Make a set of successively smaller holed plates like that one. > > =A0 With that, you can see just how well your device is already pointing. > > =A0With successively smaller holes in the ambient plate, that is. > > =A0 Also, your device under test has to be hotter than ambient to be a > proper pointing determination target, etc.
Archie, I'm proud of you! Not a nasty aspersion or swearword in that whole post! Would you like this to be another addenda to your recent assessment or a good start on your next one?