Adobe:
"When converting a color image to Bitmap mode, first convert it to Grayscale mode. This removes the hue and saturation information from the pixels and leaves just the brightness values. However, because only a few editing options are available for Bitmap mode images, it’s usually best to edit the image in Grayscale mode and then convert it to Bitmap mode. "
After this, there are several tools you MAY be able to use.
The one I found that may work for you is Threshold, which converts the image to black and white I believe (according to the threshold level you select certain pixels can be omitted)
Search for this plugin or check, it is in the adjustments menu.
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Other plugins may be available though.