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Hemispherical total reflectivity of copper, nickel, and tungsten in ablation by nanosecond Nd:YAG laser pulses in air of atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1-100 J/cm(2). Our experiment shows that at laser fluences below the plasma formation threshold the reflectivity of mechanically polished metals remains virtually equal to the table room-temperature reflectivity values. The hemispherical total reflectivity of the studied metals begins to drop at a laser fluence of the plasma formation threshold. With increasing laser fluence above the plasma formation threshold the reflectivity sharply decreases to a low value and then remains unchanged with further increasing laser fluence. Computation of the surface temperature at the plasma formation threshold fluence reveals that its value is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on a real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.

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We design and assess an adaptive scheme to detect a subpixel target in a sequence of images in the presence of an additive correlated Gaussian background. The presence of the subpixel target decreases the background power that hence may be different under the null and alternative hypotheses. We use the generalized likelihood ratio test (GLRT) to adapt the recently proposed modified matched subspace detector (MMSD) to unknown background variances under the null and alternative hypotheses using the secondary and primary data, respectively. We derive a modified adaptive subspace detector (MASD) that is sensitive to both energy in the target subspace and reduced energy in the orthogonal subspace. We contrast it with the MMSD and the well-known adaptive cosine estimator (ACE). Numerical simulations attest to the validity of the theoretical analysis and show that the proposed detector performance outperforms the ACE, especially in the case of dark subpixel targets. The performance-degrading effects of limited secondary data are presented for the proposed detector.Index Terms-Adaptive detector, hypothesis dependent background power, subpixel subspace detection.

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