Grayscale System

Definition

https://en.wikipedia.org/wiki/Grayscale#Converting_colour_to_grayscalehttps://en.wikipedia.org/wiki/Grayscale#Converting_colour_to_grayscale

In digital photography, computer-generated imagery, and colorimetry, a grayscale image is one in which the value of each pixel is a single sample representing only an amount of light; that is, it carries only intensity information. Grayscale images, a kind of black-and-white or gray monochrome, are composed exclusively of shades of gray. The contrast ranges from black at the weakest intensity to white at the strongest.[1]

Grayscale images are distinct from one-bit bi-tonal black-and-white images, which, in the context of computer imaging, are images with only two colors: black and white (also called bilevel or binary images). Grayscale images have many shades of gray in between.

Grayscale images can be the result of measuring the intensity of light at each pixel according to a particular weighted combination of frequencies (or wavelengths), and in such cases they are monochromatic proper when only a single frequency (in practice, a narrow band of frequencies) is captured. The frequencies can in principle be from anywhere in the electromagnetic spectrum (e.g. infrared, visible light, ultraviolet, etc.).

A colorimetric (or more specifically photometric) grayscale image is an image that has a defined grayscale colorspace, which maps the stored numeric sample values to the achromatic channel (==>say R of RGB, achromatic means "singularly 'colored'" ) of a standard colorspace, which itself is based on measured properties of human vision.

If the original color image has no defined colorspace, or if the grayscale image is not intended to have the same human-perceived achromatic intensity as the color image, then there is no unique mapping from such a color image to a grayscale image.

RGB Conversion

Depends on the applicable situation, say for human, we assign weights to each of the RGB channel per our retina's sensitivity to them ==> recall that grayscale is an intensity measure, hence we want to represent the sensation in light intensity when we see the colored image, with gray ones.  

A great online tool to play with:

Convert JPG to Grayscale - Online JPG Tools

This website allows online tweaks of RGB achromatic channel weights and also introduces you to the popular conversion formulae.

Digital Capture Option Comparison

Grayscale or Bitonal: Which is a better method for scanning my records? — Revolution Data Systemshere are several ways to capture digital images. Depending on the project you might use a variation of each of them. https://www.revolutiondatasystems.com/blog/grayscale-or-bitonal-which-is-a-better-method-for-scanning-my-records

Bitonal: black and white, two-tone scan. Most often used for printed or text documents. Handwriting can be a bit harder to pick up.

==> if you've ever taken a test where your hand written answers are scanned for scoring, then you know how horrible these binary scans are.

Grayscale: palette of up to 256 gray tones that works great for black and white photographs. Also best for handwriting because details display better than a bitonal scan. Also a great choice for older records where paper may be discolored. 

Color: palette of 256 to thousands of colors is used for photographs and documents where color must remain visible. Maps, diagrams and illustrations are great examples of records that should be captured in color. To capture the natural appearance of an item, color scans may be best.