Grayscale on Stamp sheet..

 

   

Hi !

Here is a stamp sheet showing a scale  on the tab of the sheet. This is known as  Gray scale. I am sharing here detailed information about Grayscale provided by distinguished philatelist of Vadodara, Mr Prashant Pandya.

Grayscale (Ruler type strip) printed on Stamp Sheet

In modern offset printing, a colored image is reproduced by the halftone separation process. The areas of light and darkness on the original are represented by dots of different sizes and the tonal range is generally modified to meet the reduced capability of the printing process. To reproduce original color, the image is separated into its red,green, blue and black components and a halftone screen made for each.

These screens are then used to make plates which are used in turn to print the image using the four process inks, cyan, magenta, yellow and black. To ensure color control and to maintain a consistent printed product, the size and color strength of these dots must be monitored, but since the finished printed image consists of the overprint of the four halftone screens it is very difficult to isolate the various components affecting the reproduction of the original image.

To overcome this problem, a series of test elements can be printed along with the image, and each element can be designed to highlight a particular aspect of the printing process. While some of these test targets can be evaluated by eye, others require the use of measuring equipment. The usual form of these test elements is as a strip across the edge of the printed sheet. These test strips, called control strips or color bars, are available commercially from various vendors, and consist of strips of film containing the various test elements for each of the four colors. In some cases six color versions are available where special colors might be used.
The ruler type scale printed on one side of stamp sheet is known as Gray Scale (One type of calibration strip), which is used to control the percentage of ink density. The percentile notation is used in printing to denote how much ink is employed in half toning. 0% means the paper white (no ink) and 100% a solid black (full ink).

Courtesy : Prashant Pandya http://www.indianphilately.net/