The Threshold command converts grayscale or color images to high-contrast, black-and-white images. You can specify a certain level as a threshold. All pixels lighter than the threshold are converted to white; all pixels darker are converted to black.
Choose Image > Adjustments > Threshold.
Choose Layer > New Adjustment Layer > Threshold. Click OK in the New Layer dialog box. The Threshold dialog box displays a histogram of the luminance levels of the pixels in the current selection.