These photos were taken with a video camera without and with an infrared filter, seconds apart. Note the change in shirt colour.
“Continue reading” for more details and photos.

Infrared filter

An infrared filter allows a video camera to take infrared photos as the light response of many cameras extends a bit into the near infrared. A video camera was used as it has a small enough lens to be covered by the small filter.
The filter coating achieves 90 % reflection of the visible, with the remaining energy attenuated by absorption, to a level of OD 5+ attenuation of the visible, while transmitting in excess of 85 % of the Infrared. Cut on wavelength; 810 nm +-5 nm so this is in “near” infrared. Infrared starts at around 700 nm nominally.

This is to be contrasted to “long” wave or thermal infrared which can pick up body heat and is typically 8000 – 1600 nm. (8 – 16 micron).

The filter is a small disc with a gold reflective coating.

Infrared light source

A 150 W globe is shown in visible light glowing at reduced power. Note the “bright” neon indicator lightsLooking through the filter at the 150 W incandescent globe at reduced power shows only a faint outline of the the filament to the eye.  Digital cameras light response often extends into the infrared. One digital video camera got a reputation for being able to “see through” some visibly opaque but infrared transparent clothing in this way. The LED indicator lights and the reflected fluorescent lighting don’t emit infrared .

Infrared from sunlight

The silicon laser meter here reads 10 mV per mW. Hence if you shine a 1mW laser pointer at it you get 10 mV out. It is reading 355 mV = 35.5 mW of sunlight on that surface in late afternoon sun.
The detector is 7 mm x 7 mm ie 47 mm². Hence 35.5 mW is equivalent to 710 W/m². Sunlight at noon is generally thought of as 1000 W/m² so that is pretty close.

The infrared filter in front of the light meter still allows 21 mW/mm² or 440 W/m². Hence this rough experiment suggests that 62% of sunlight radiated power that we receive is infrared. Wikipedia says 527 W/m² of 1004 W/m² (ie 52.5 %) is infrared but my sensor will have its own response curve and IR is defined as starting at 700 nm rather than 810 as with my filter.

To Do with infrared

I am hopefully getting a crowd sourced video IR camera attachment for a smartphone. This is the Mu thermal  camera and covers the range 8 – 14 micron and temp -66 to 90 °C with normal visual overlay and ability to do video 160 x 120 at 30 fps. Note that normal infrared starts at 0.7 micron = 700 nm so this camera sees over 10 times the wavelength.

I could also try this filter in front of my pinhole camera.

Related pages

  Ultraviolet

Try something else

  Plasma globe effect

External links

Infrared – Wikipedia

Photo Date: Sept 29, 2004