2. The disappearing filament principle used in Optical Pyrometers Note: Click to advance each slide. Artwork courtesy of Spectrodyne, Inc.Spectrodyne, Inc. Page URL: http://www.temperatures.com/Howopticals.htmlhttp://www.temperatures.com/Howopticals.html temperatures.com - 2001 PLEASE VIEW IN SLIDE SHOW MODE. Navigation-Links Home Optical Pyrometers Radiation ThermometersRadiation Thermometers The E-missivity Trail

3. Eye lens Field Stop Field Lens Exit Stop Red Filter Lamp Erecting Lens Range Filter Entrance Stop Objective Lens Figure - Optical system of DFP 2000 pyrometer The disappearing filament principle—optical system - Navigation-Links |Home| |Optical Pyrometers| |Radiation Thermometers| |The E-missivity Trail|HomeOptical PyrometersRadiation ThermometersThe E-missivity Trail

4. The disappearing filament principle-1. An operator sights onto a hot target, adjusts the range until its image is seen in red. The lamp filament is initially cooler than the target and its image appears as a darker red or black superimposed on the target’s image. Image of Filament (Cooler) Image of Hot Target What the operator sees when looking into the eyepiece; the target in red, its surroundings in black (cooler) or red (hot) and superimposed on the target, the filament. The view is circular because the optical system is made up of circular lenses, apertures etc. Navigation-Links Home Optical Pyrometers Radiation ThermometersRadiation Thermometers The E-missivity Trail

5. The disappearing filament principle-2. The lamp current is raised until the image of the filament becomes hotter than the target and it appears as brighter red than the target. Image of filament (Hotter) Image of hot target Pointer indicating the center of the filament. Navigation-Links Home Optical Pyrometers Radiation ThermometersRadiation Thermometers The E-missivity Trail

6. The disappearing filament principle-3. The lamp current is adjusted until the lamp filament’s brightness* temperature equals that of the target. The filament’s image blends into the image of the target. The filament “disappears”. * Brightness or radiance temperature is the temperature that a blackbody would have when it looks as bright as the target. It is almost always a lower temperature than the true temperature because of the effect of the target’s emissivity. However if the target is an object in a furnace or oven of about the same temperature, the true and brightness temperatures are very close to the same value. Also, if the target is in a cooler surroundings and has a relatively high emissivity, the difference between the true and brightness temperatures may be small. The difference for a wide range of conditions can be estimated from a table in ASTM Standard E1256. Navigation-Links Home Optical Pyrometers Radiation ThermometersRadiation Thermometers The E-missivity Trail