Currently I'm using the GY-906 (off eBay) MLX90614 - non-contact IR temperature sensor breakout board. I chose this sensor mainly because it had a smaller field of view than Texas Instruments TMP006 sensor. Adafruit has tutorial's and libraries for both of these sensors (here's the link for the MLX90614 Adafruit Tutorial and Arduino Libraries).

My initial tests for this sensor worked flawlessly - no issues with communication through an Arduino and sensor reacted as anticipated to different temperature gradients.

While I can't find my bookmark for it, TI has a tutorial on limiting the field of view of their TMP006 sensor and I'm using that information to attempt to do the same with the MLX90614. Their approach is fairly simple and straight forward though it does require simple measurements and a little bit of trigonometry.

Below is an illustration of the basic concept:

To make it a bit easier to understand (conceptually) I rotated the view 90 degrees:

The field of view is circular with a radius of "r1" for my purposes I'm using a radius, for the field of view, of 5cm (so a circle with a diameter of 10cm - basically want to view the cheeks on the face of the subject). As a starting point I'm using 50cm as the distance from the sensor to the subjects face. This gives us:

Once we know the required angle we just need to figure the size of the hole for the aperture and how far away it needs to be from the sensor:

The needed angle (5.7 degrees) was calculated above now we only have to plug in the distance from the aperture that we want, then we'll have the radius hole we need (just double that to get the diameter).

For the aperture I plan to use some thin aluminum sheet and polish the side facing the sensor. I'll start testing this part tomorrow to see if it works in the way I'm hoping it will.