One of the many enhancements in the iPhone 4S camera is reportedly an embedded infrared (IR) filter. Sounds good you say, but what good is an IR filter and how can we even tell if it’s really in there? One of the easiest IR filter testers may be located right beside you. If you have a TV remote with a bulbous knob on one end, that bulb is likely an infrared emitter. Just point it at your camera, press one of the buttons and take a picture. If your camera is missing an IR filter, you’ll see the invisible IR light rays recorded on your camera’s sensor. If the filter is working, you’ll see nothing but a darkened bulb. Note that in this case, the goal of the filter is to block IR light. This is not be be confused with filters used for IR photography which are designed to pass only IR light.
The TV remote is not the only IR source beaming its way around your house. The video below shows the iPhone 4 camera (without IR filter) pitted against the iPhone 4S (with IR filter) in various feats of IR imaging. For a grand finale, we imaged a calibrated IR source (950 nm) with both cameras.
As shown, the embedded 4S filter does a pretty good job of blocking IR light. But assuming we’re not interested in long term careers as TV remote photographers, what is the impact of an IR filter on an actual image? When IR light is allowed to pass through to the sensor, the IR light contaminates the channels (mostly the red channel) with information that was not visible in the original scene. The result is an image with a color cast. The images below show the impact of IR contamination on a scene with a dark background. The iPhone 4 image shows a reddish cast due to the extra IR light recorded on the red channel. The 4S image shows a background which much more closely resembles the original scene.
There are (ironically) a growing contingency of photographers who prefer the lack of an IR blocking filter so they can take images exclusively with IR light. Firms such as Life Pixel specialize in ripping embedded IR filters off of DSLR cameras so they can be used for IR photography. But for the purposes of taking color correct images with a camera phone, the IR filter is probably a worthy addition.
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