This method is also easy. And even though we are converting a RGB image
to Lab, I still consider it
non-destructive because we can convert the Lab image back to RGB without
losing color information. This is because Lab is a device-independent
color space. In simple terms, this means the Lab definition of color is
not specifically related to, or dependent on, a particular device, such as a
printer or monitor. Lab is commonly used as an intermediate step when
converting one color space to another. For example, if we use Adobe
Photoshop to convert a RGB image to CMYK, Photoshop first converts it to Lab and
then the Lab version is converted to CMYK. This two step conversion is
performed by Photoshop seamlessly.
In this method, click Image > Mode > Lab Color. Photoshop will convert the
image to the Lab color space. Go to the Channels panel and turn off the
a and b channels. The a and b channels are the color channels. This
should leave only the Lightness channel active. The Lightness channel is
the luminance channel. Switch back to the Layers panel and we have our
black and white image.
The disadvantage of the Lab method is we lose the ability to manage tone by
color. Also, my experience is the luminance of the Lab version of an image
is lighter than the other methods shown here.
Figure 1. Lab Conversion
Figure 2. Preserve Measured Luminance