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Customary Method- Lab

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.

Lab conversion Figure 1.  Lab Conversion

Measured luminance Figure 2.  Preserve Measured Luminance