What’s the Secret behind The Colored Bars Shown on TV ?

wikipedia :  Rendition of SD SMPTE color bars / https://commons.wikimedia.org/wiki/User:Denelson83

We used to enjoy watching TV when we were kids and it was really often when the signal cuts so we see colored squares and bars that became a usual decoration of our living room that time .
 We did not understand why  those bars appear because we were disappointed about not being able to watch our favorite  TV show  in this article we will reveal the reason behind those colored bars .
Actually these colored bars are called ‘’ Test Cards’’ and it is a paper card or carton the cameras focus on it all day as a prove of the signal existence all day
It shows to the audience that the channel is working in their receivers even though the shows  are not being displayed
And maybe you had noticed hearing some voices of radio or music in purpose to make sure the voice signals are good
These bars are still being used now in backstage for  the Color Test Calibration of cameras and adjusting the image colors . 
That’s useful for the audience too to adjust the screen colors .
We rarely see that now because the cameras now adjust colors automatically and also some new devices for the monitor calibration high quality TVs so we no longer need manual adjustment .
A lot of common things people do not notice and that’s the case for me I never thought about that before so I felt excited about knowing the story about those mysterious colored bars .