Several previous papers have discussed imaging the infrared emission of LEDs in remote control devices.1–3 Most of these LEDs appear purple in digital images. Even the Paschen lines of hydrogen and infrared lines of helium appear purple when imaged with a digital camera.4 However, there is a missed opportunity in simply imaging an infrared LED (or spectral lines) with a digital camera. We can have students dig deeper by asking several questions regarding the nature of the observed infrared emission: “Why does the infrared LED appear purple?” “What might this tell us about the RGB filters on the imaging chip?” and finally “Does all infrared light appear purple when imaged with a digital camera?”

The answer to the first question allows us to introduce (or reinforce) color addition. Digital cameras are actually monochrome CMOS chips fitted with an RGB color filter grid.5 All colors in an image...

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