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A gauge measures areal density variations as small as 0.1% in low density plastic foams and other materials. It has a spatial resolution of 0.5 mm. This system is safe, portable, easy to use, and is readily automated. The 6 keV x-ray flux from a /sup 55/Fe source is collimated to a 0.5 mm diameter beam and attenuated by the sample. Transmitted x-rays are detected with a proportional gas detector and counted with a scaler-timer. After measuring the x-ray attenuation in two areas of the sample, the areal density variation is calculated using an

We present measurements of the performance of a Pockels readout optical modulator (PROM) x‐ray imaging detector using a 30‐kV constant potential bremsstrahlung source. A nickel step wedge was used to measure the spatial resolution and noise of the PROM image. PROMs are unique high‐efficiency x‐ray detectors that image with good spatial resolution over a wide range of x‐ray energy. The PROM is constructed from an optically transparent, active crystal that is sensitive to x‐ray dose. This unique construction allows the use of a thick crystal to increase efficiency for x‐ray detection while avoiding the spatial resolution degradation that usually accompanies thick detectors.

Digital subtraction radiography serves angiography [1] and other medical applications successfully to extract minute x-ray signals from noisy backgrounds. Verhoeven [2] demonstrated that detecting surprisingly small changes in x-ray absorption makes this technique useful for industrial applications as well. I detect very small areal density differences and also wires as small as 2 µm in diameter although the spatial resolution of my x-ray apparatus is 1 mm.