Effects > Keyer > 
Adjusting the Luminance of the Key
 
 
 

After creating a key, you can adjust luminance values to fine-tune the result. The histogram displays the distribution of luminance values within the matte. The horizontal axis of the histogram represents the range of luminance values in the matte and spans from 0 (black) to 255 (white). The vertical axis shows the number of pixels at each luminance value.