Color Temperature Thermometer using Black Body Radiation

A solution for expensive high-temperature IR Thermometers

Thermal Light determines the temperature of hot metal (iron, steel, copper, aluminum) using the black body radiation principle instead of infrared radiation (IR). A black body is an idealized physical body that absorbs all incident electromagnetic radiation. It is also the best emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body's temperature. If we know the color spectrum associated with each temperature of the metal, we can determine the temperature of the body. Below is the spectrum.

See it in the App Store here.

Thermal Light uses the camera's lens to capture the colors emitted by a black piece of metal. It is important that the object is as close to black as possible because the app uses color, and not IR, to determine the temperature. A white object will register as having a temperature close to 2700F. The app uses the visible light that is emitted to determine the temperature.

In order to provide the best accuracy, the app has two temperature ranges: 1000-1900F and 1500F-2700F. You determine which setting you want by selecting LO or HI. This allows the app to guide you to use more or less light for the calibration process. As you move the phone closer to a light source the calibration diamond will move more towards the right; as you move the phone closer to a shaded area the diamond will move towards the left. Hit the Calibrate button in rapid succession as you move the phone until the diamond is between the brackets of the calibation setting you want, LO or HI. For the high setting you will have to get very close to a 100-150W light source. The calibration will require so fine tuning for the LO setting once you are within the brackets. Continue to hit Calibrate, and follow the message to use more or less light until the box turns green. A message will state that the calibration is correct once completed.

Hold the iPhone within a foot of the object so the image fills the green square on the screen. It should exceed the boundaries by at least twice the length of the square's side for a more accurate reading because the values are sampled within the square. Any non-radiating images withing the green square's borders will introduce a calculation error.

Make sure you do not hold the iPhone too close or to the hot object, or directly over it in the path of the rising heat. The heat can damage the phone if you are too close. Acquiring the image from the side or at an angle from the top works well, and keeps the rising heat from directly hitting the phone.

Just remember that a cold piece of metal that has a color will register as having a hot temperature. Copper, for example, has a reddish luster, so this will show as having a temperature when it is cold. Once the copper starts to give off a glow the readings will become accurate.


IR reading of temperature at fireplace and Thermal Light reading