The Threshold filter converts grayscale or color images into 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; and all pixels darker are converted to black. The Threshold command is useful for determining the lightest and darkest areas of an image.
Choose Filter > Adjustments > Threshold.
From the Layers palette or Layer menu, create a new Threshold adjustment layer, or open an existing Threshold adjustment layer.
The Threshold dialog box displays a histogram of the luminance levels of the pixels in the current selection.
To change the image to black and white, drag the slider below the histogram until the threshold level you want appears at the top of the dialog box, and click OK. As you drag, the image changes to reflect the new threshold setting.
To identify a representative highlight, drag the slider to the right until the image is pure black. Then drag the slider back until some solid white areas appear in the image.
To identify a representative shadow, drag the slider to the left until the image is pure white. Then drag the slider back until some solid black areas appear in the image.