Click Adjust in the toolbar.
In the Adjust pane, click the arrow
Change the histogram to display luminance values (instead of RGB values): Click the pop-up menu below Levels, then choose Luminance. For example, you might want to change the histogram to luminance values when working with black-and-white photos. The histogram handles perform the same functions as when the RGB histogram appears.
Note: You can also drag the top handles of the Levels controls to change the range of adjustment. For example, to adjust only the lightest of highlights, move the top handle further to the right. To fine-tune an adjustment, press and hold the Option key and then drag the bottom handle. This moves both bottom and top handles in unison until you get the results you want.