Figure 1 is the optical schematic of the Vario-Zooom variable-focus infrared thermometer (US Patent No. 7,355,178).

Optical Schematic of Variable Focus Infrared Thermometer

Figure 1
Optical Schematic of Variable Focus Infrared Thermometer

Visible Light Surface (VLS)
The VLS is a Lambertian surface that is exactly the same size, shape and texture as the Lambertian Infrared Detector Surface (IDS).  Visible wavelength “light” emanates from the VLS, either from direct quantum radiation (e.g., LED) or from re-radiation from a backlit diffuser. 
Infrared Detector Surface (IDS)
The IDS is a planer array of thermocouples connected with a square electrode pattern.  The light emitting/collection area of the IDS is the same size and shape as the VLS.
Beam Splitter
The beam splitter is a bi-chromic “hot mirror.”  It transmits visible wavelengths of light with little attenuation (≈10%).  However, for incident infrared light wavelengths of greater than 2 micrometers, it is highly reflective (R ≥80%).  Hence, the beam splitter passes the visible light from the VLS and reflects the incident infrared light from the optical system at an angle equal to its incident angle.

The normal centerline axis of the VLS must coincide with the system’s centerline axis, and the VLS is located at the system’s rear focus.

The beam splitter is normally located at a convenient place along the optical axis and inclined at 45° to the axis.

The IDS is located to the side of the optical axis in such a manner that its vertical centerline axis is positioned normal to the optical system centerline axis and intersects it at the Cardinal point where the beam splitter surface also intersects the system centerline axis.

If the achromatic objective lens (AO) has the same focal distance in the infrared wavelength band as in the visible band, then the distances of the IDS and the VLS from the cardinal point are equal.  If the focal distances are different, then the distances are proportional to the respective indices of refraction according to the formula:

Therefore, the differences in distance from IDS to the beam splitter and VLS to the beam splitter is  fir - nvis.  The longer distance will correspond to the smaller “n.”  A final positioning requirement of the integrating optics is that the orthogonal axis of the beam splitter (the surface axis perpendicular to the page) must also go through the cardinal point where the axes intercept. 

As the VLS and the IDS beams leave the beam splitter surface, their centerline axes are coincident in all respects and can be considered geometrically as a single beam. 

The single beam is shaped by baffle B1 and directed to baffle B2, which is located very close to the objective lens when it is positioned in the infinity focus position.  The beam diameter is shaped by B2 to approximately 50% of the lens’ clear aperture.  

Variable Focus

A primary requirement for a variable-focus infrared thermometer is that the calibration factor, or temperature reading, does not vary as the objective focal distance changes through its design range, for targets of constant temperature.

This is assured by the fact that the beam from B2, defined by B2, passes through the achromatic objective lens (AOL) unobstructed for a wide range of final foci as determined by the axial position of the AOL.

The beam from B2 continues to expand at the same solid angle as it had when leaving B2 until it passes through AOL, which focuses it onto the target spot.

Since AOL does not constrict the beam, it carries the same optical power through B2 and on to the infrared detector for any focus, as long as the beam diameter does not exceed the clear aperture (CA) of AOL.  This provides variable focal distances for the infrared thermometer from infinity where the beam diameter is defined by B2, to twice the simple focal length of AOL, defined by the CA of AOL.