Selecting lens for IR thermometer

On Sun, 19 Dec 2010 17:13:42 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sat, 18 Dec 2010 20:13:07 -0800, TheGlimmerMan
><justaglimmer@thebarattheendoftheuniverse.org> wrote:
>
>>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.
>
>Why not just sweep it across a point source of heat and see the
>response curve? 
>
>John


  IR black body calibration sources ("calibration ovens")are typically
made such that the heated black body is an Aluminum ingot placed behind a
matte painted sheet of Aluminum with a hole in it slightly smaller than
the ingot.  This keep fringe errors low near the edges of the ingot, and
cuts down on ambient convection currents wisping past your target when
you are trying to calibrate an IR device.

 The ingot is called "the target" and the "target size" equals the size
of the hole or "port" in the Aluminum sheet.

  He already stated what he wanted his targeting capability to be. 3mm.

  My test solves two problems at once.  Targeting capacity, and amplitude
when on target at a given temperature.