Greyscale and Color Image

In the previous blog, we learned how the digital image is formed but we didn’t discuss whether the image formed was greyscale or colored. In this blog, let’s go through this.

A greyscale image is the one where pixels contains only the intensity information and not wavelength. For example, a pixel value of 165 represents the amount of light captured by the sensor(pixel). It doesn’t tell how much of this light belongs to red wavelength or any other.

For an 8 bit image, there will be 28=256 intensity levels where 0 means no light(black) and 255 means White light and in between levels represents shades of grey as shown below

The above image is created using numpy and OpenCV (See here for code).

A color image, on the other hand, contains intensity information corresponding to three wavelengths red, green, and blue (RGB)collectively called primary colors or channels. The reason for choosing these 3 colors lies in the fact that cone cells in the human eye, that is responsible for color vision, are sensitive to red, green, and blue light.

Note: Combining the primary colors(their wavelength may vary) in various intensity proportions, we can produce all visible colors.

So, in color image, corresponding to every pixel, we have 3 intensity values Red, Green, Blue (for example 160,30,45) that produces different colors in the image.

How a Color image is formed?

To form a color image, we want something that absorbs light in red, green, and blue wavelengths. The advancements in color image formation can be summarized by the figure below

Source: Foveon

The drastic advancement in color image formation came with the introduction of digital sensors like CCD and CMOS. Most of the digital cameras today only contain one imaging sensor so they cannot collect red, green, and blue information simultaneously at each pixel location. One possible solution is we use 3 imaging sensors with RGB filters but that is expensive in terms of money and time.

So, in 1976 Bryce Bayer of Eastman Kodak, invented the Bayer filter that revolutionized color image formation. Bayer filter is a color filter array(CFA) in which RGB color filters are arranged in a pattern on the sensor. Below figure shows a Bayer filter.

Source: Wikipedia

In the bayer filter, there are twice as many green elements as red or blue to mimic the human eye’s greater resolving power with the green light. In order to construct an RGB picture, we calculate Green and Blue values for each Red pixel, Blue and Red values for each Green pixel and Red and Green values for each Blue pixel through interpolation or color demosaicing algorithm (For more details See here).

Foveon X3 direct image sensor is the most advanced color image sensor. This combines the power of digital sensor with the essence of the film. Like the film, this has three layers of pixels embedded in silicon. The layers are positioned to take advantage of the fact that silicon absorbs red, green, and blue light to different depths. The bottom layer records red, the middle layer records green and the top layer records blue. Pros are superior quality and less processing power.

Now, you might have got some feeling about the Color image and how it is formed. Hope you enjoy reading.

If you have any doubt/suggestion please feel free to ask and I will do my best to help or improve myself. Good-bye until next time.

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