Reproduction From Digital Images

Here you will find some tips for digital images reproduction.

How large the image can be reproduced depends on the image size (in pixels) and the application, video display (web, multimedia) or print reproduction (offset press, laser printer, inkjet printer,…).

Video Display
To completely fill with an image a standard VGA screen a resolution equal to 640 x 480 pixels is enough.

Print Reproduction
The term "dots per inch" (dpi), here refers to the pixel frequency of the digital image.
The term "lines per inch" (lpi) refers to the frequency of halftone dots on the printed image (offset printing).
With a given digital image size (pixels) how large you can print it will vary on what your target lpi is, (what kind of hardware you are outputting it with). 

First step is you must know what lpi you need for your output device.
In general to print a digital image it is necessary to "over-sampling", that is for each halftone dot reproduced on the printed page a number of pixel of the digital image is necessary.
Depending on the quality of the output (sharpness, pixelization) an over-sampling varying from 2 pixels per printed dot (for maximum quality) to 1.5 pixels per printed dot (adequate quality) is required.
As an example you can consider that you can get best print quality with 300 dpi, although outstanding results can be obtained at only 200 dpi, depending though from the nature of the image and from the quality of the printer.

As reference this table shows the maximum printed image sizes that are possible from our digital image file sizes, using various halftone screen frequencies (lpi) and the relevant pixel frequency (dpi) you must set in Photoshop (based on the 1.5 to 1 pixel to halftone dot ratio).

Image size (Pixels)	MAXIMUM POSSIBLE PRINT SIZES (inches) at various lpi applications (dpi/lpi=1.5)
	55 lpi	65 lpi	85 lpi	100 lpi	133 lpi	150 lpi	175 lpi
	83 dpi	98 dpi	128 dpi	150 dpi	200 dpi	225 dpi	263 dpi
1800 ×1200	21×14	18×12	14×9	12×8	9×6	8×5	7×4.5
3600×2400	43×29	36×24	28×19	24×16	18×12	16×10	13×9
5400×3600	65×43	55×36	42×28	36×24	27×18	24×16	20×13

Some typical applications of various Halftone Screen frequencies are:

85 lpi -- used in 600 dpi laser printers, most newspapers and some medium quality newsletters.
133 lpi -- Halftone frequency used in most magazines, books, and better quality newsletters printed on 4-color offset presses; and short-run offset "web" presses used for flyers and brochures.
150 lpi -- Halftone frequency used in standard quality brochures and "high-gloss" newsletters printed on 4-color offset presses.
175 lpi -- Halftone frequency used for very high quality 4-color offset printing.

For different print sizes than the sizes shown in the above tables you can modify digital image size by means of the interpolation facility of your editing software (Adobe Photoshop, Altamira Genuine Fractals for example). In the attempt to increase the file size by interpolation (in order to obtain larger prints) you will decrease the sharpness of the printed image, although experimentation will often show that this loss is often minimal.

The above is only a guide. We cannot be held responsible for any mistakes in your final product. Check with your service bureau or printer to find what dot/screen frequency they are capable of reproducing. 
It is always best to seek the advice of the Printer, whose method of working will dictate what is ideal in the particular circumstances.

NOTICE
The digital image files you receive from Traveller Dreams are never interpolated