Superinsulating window developers have shifted their attention to the edges and frames of these windows because these areas are now the biggest dissipaters of heat in the window system. According to Griffith, "IR thermography offers a fast, quantitative way of identifying the best frame and edge designs to optimize a window's performance."
The technique is also used to validate finite-element computer models of the components' thermal performance. Center researchers have also studied the use of IR thermography to develop standard tests of windows' ability to resist condensation, an important feature for someone thinking about buying superinsulating windows.
Brent Griffith, left, and Radim Jasek prepare a sample window for testing in the infrared thermography facility. Paul Donohoe, far right, adjusts the infrared camera. The structure attached to the bellows is the newly built room temperature chamber that maintains environmental conditions as constant as those in the refrigerated chamber to its left.
The thermography lab's facilities consist of a refrigerated and a room-temperature chamber, a sample mounting frame that fits between the two chambers, and a high-resolution IR scanning radiometer. This instrument plugs into a PC that stores and processes the images. Scanning at roughly the same rate as a television (20 to 50 frames/second), the radiometer measures the relative temperatures of the sample's surface to within 0.1 degree C. Since the system doesn't measure absolute temperatures as accurately, users take several images of the sample and the PC's post-processing software creates a composite, higher-resolution image. A new addition to the lab is an extended area reference emitter, a four-inch- (10-centimeter-) square sample that emits a known flux of IR radiation. Calibrating test samples against this reference offers more accurate absolute temperature maps.
The freezing chamber maintains steady temperatures between -40 and 10 degrees C. "In 1993," says Griffith, "the room-temperature chamber was upgraded so that its airflow and temperature regime could be controlled and repeated just as accurately as the refrigerated side is." With a standard sample size of about four square feet (0.4 square meters), the scanner can zoom from a view of the whole sample to a close-up of interesting areas as small as an eight-inch (20-centimeter) square. The temperature scans can be color-coded or converted to grayscale. The post-processing software can assign any desired color to contour zones, convert raw data into histograms, measure temperature gradients along the surface, and dress up the images for presentation.
Arasteh and his colleagues use the IR thermography facility primarily for analyzing the thermal performance of windows and insulating gas-filled panels (see CBS News, Winter 1993 page 9) Among their current work is a project aimed at understanding the two-dimensional thermal effects of refrigerator/freezer shell design. They have also analyzed lighting fixtures to detect overheating (see CBS News, Winter 1993 page 4) and selective glazings for automobile glass. The IR thermography lab is available to researchers outside LBL to solve scientific problems consistent with the facility's purpose. It is also available without charge to manufacturers developing or proving major new products and design approaches; results measured in the lab must be for internal use only.
--Allan Chen
Brent Griffith
Building Technologies Program
(510) 486-5827; (510) 486-4089 fax