# dpi -dots per inch

We were having a discussion about printer resolution in class. We were trying to understand what is exactly is meant by dots per inch. In Visual Culture TW had said that he we was quite happy with 300dpi for printing and anything over this resolution in the original image was of no use because commercial printing presses use that technology.

So dots per inch is the dots put on a linear line 1″ long , so 300 x 300 per square inch. So roughly A4 size at about 21 x29 inches gives  21 x 29 x 900 = 548, 100 dots. So how does that equate to camera res. Well a 5472 x 3648 image is recorded by my Mac pro  as coming from the Canon 6D’s  full size sensor at 19,961,856 pixels. So that is a fair few px on the printing room floor i.e. 19M – .5M = 18.5 million redundant pixies!

The redundant pixels are used by algorithms to improve the final image by selecting one pixels in preference to another. For instance this happens in to smooth edges using the bi-cubic algorithm in Photoshop.

This article from the BBC http://www.bbc.co.uk/news/technology-35110483, Published on 16 Dec 2015 , accessed on 20 Dec 2015 tells the story of ultra high res “quantum” dots. Apparently quantum  dots reflect light at different wavelengths according to their size. They are producing 25k dpi. So for an A4 that would roughly be 2 416 907 dots or alternatively print 1 pixel = 1 dot with  a 0.178 ” X 0.178 (0.0319 are inch) output!

One caveat though is that they may be reporting the total number of dots laid down in their 25k figure  i.e. 1 red 1 green 1 blue. The two paragraphs below seem contradictory. If so the  actually the resolution would obviously be 25k/3.

The press release from ETH  is more  definitive.

“The photo of the fish – which are 3,333 times bigger in real life than in the picture – was printed at a resolution of 25,000 dots per inch (dpi)”.

“There is 500 nanometres (0.0005mm) between each dot on each of the three colour layers deposited – red, green and blue”(Zürich, 2014).

LP- Camera sensors will need to improve to get the most out of quantum printing technology. However, software may improve the quality of the final image even at 300dpi.