Paul Jolly Photography Chapter 19

Infra-red photography

This form of photography produces unexpected results for most people. There are two main versions, monochrome or false colour. Film is one of the best mediums for this kind of photography for most people. Digital photography often precludes infra red (IR) as a filter is included in front of the sensor to prevent infra red from reaching it. This is done because the focus point for a lens is normally different from the white light focus point.

Prime lenses often have a red dot on the depth of field scale area. As well as the white focus line. In use we focus the image using the white line and after placing a IR pass filter on the front of the lens the focus point is rotated round to the red dot, this ensures the image is in focus for IR.
Infra red is becoming more common due to the availability of IR camera sensors in the consumer market.
IR has been in use for a long time for night photography and surveillance as the human eye, and most animals cannot detect the presence of IR illumination enabling night photographs without disturbing normal activities.
We use longer IR wavelengths to detect the temperature of objects but this requires the use of special sensors.
The use of special cameras can also give false colour images to look into the infra-red.

In these photos white is the hottest points and blue the coldest. As you can see the T-shirt is insulating the body, as when it is lifted the actual heat from the skin is shown.