Each pixel of a CDD or CMOS sensor transform the amount of light received during exposure into an electrical level that is then quantized into a binary value. This means that those chips are only sensitive to the brightness component of light and not its chromatic value.
One way of recording color with such chips is to use 3 sensors and two dichroic prisms assembled back to back (sometimes called a trichroic prism).
As you can see on the diagram, after the light enters the prism a first dichroic mirror (F1) separates the blue component and send it to a first sensor. The second mirror (F2) separates the red component and sends it to a second sensor. What remains (green light) hits a third sensor.
A three-chip system is more expensive since you need three of them plus the prism. It also takes more space, especially if you want 35mm size sensors, and is heavier. Finally, you must be certain that the 3 sensors are perfectly aligned otherwise you will end up with some unwanted artifacts.
Yet, the light absorption of the prism is minimal.
The Bayer Pattern
There is another way of capturing a color image, by using a Bayer pattern.
The Bayer filter is applied right on top of the sensor so each ‘pixel’ records only the intensity value of one primary color. As you can see, the Bayer filter has 2 green filters for each blue and red one. This is to mimic the human eye in which the cones sensitive to (what is almost our) pure green are as twice more sensitive to the blue are red ones. We are, then, more sensitive to green information than any of the two other primary colors.
The image coming out of a Bayer sensor contains only one intensity value per pixel.
So, if your camera has a pixel count of 11,480,800 intensity values for each image, it records only 2,870,200 red and blue values as well as 5,740,400 green values.
To recreate a color image, it has to go through what is called a ‘de-Bayering’ process. That process interpolates the other color values of each pixel through an interpolation using its neighbor’s information.
There are different de-bayering processes with different processing loads and time. But you should always use the highest quality de-bayer process available.
Also, a typical Bayer filter absorbs at least two-thirds of the visible light before it hits the sensor. Because of that, the three-chip sensor has better low light capabilities.
People are often over-focused on resolution while forgetting about low-light sensitivity and color depth, which is, to me, far more important and will be discussed in another post.
Copyright © 2010. Jean-Dominique Vandenberghe. All rights reserved