(He notes that you might need to pull the IR filter from your camera to get a decent signal.) The pen is the usual IR LED with a power supply. To do the actual work, he wrote a custom application ...

[Makers Mashup] shows in a video how to use an adjustable IR filter to cut out all the light bleed to get a sharp image to make focusing simpler. The filter he shows adjusts from 530nm to 750nm ...