# Re: Pixel Density: when Moore is less

Started Jul 8, 2008 | Discussions thread
pixel density equally uncertain: how about linear pixel density, per mm?

... many of the things people have been 'asking for' ... are simply unavailable (such as exact pixel pitch). We do provide sensor dimensions where available (as you will note from the camera database).

For compatibility with the long-standing measurement of resolution in linear terms like line pairs per mm, I would suggest a middle way: linear pixel density, in pixels per mm: the reciprocal of pixel pitch. This could also avoid your concern that pixel pitch should be stated exactly or not at all, while accepting some margin of error in your published pixel density calculations. The inaccuracy in each case comes from the same source: you do not always know the exact area of the part of the sensor that contains the stated pixel counts, since you often have only sensor dimensions for a region that includes additional photosites not corresponding to output pixels.

As one example, you seem to compute pixel density for 4/3 sensors based on dividing the stated effective pixel count by the area given by the nominal 13.5x18mm dimensions of 4/3" format, whereas in fact the output pixels in fact all come from a region of about 13x17.3mm in Four Thirds sensors (except for the wider format GH1 sensor).

However linear density in "pixels per mm" might emphasize too much the advantages of higher density (higher resolution) rather than its alleged disadvantages (worse per pixel noise, at least when viewed at huge magnification of 100% on screen), which seems to what pixel density was invented to measure.

Aside: pixel pitch is by definition the spacing from one photosite (cell) to the next, including the space taken by any wiring between the electron wells and such, so there are indeed exact formulas for converting between "pixel density" (pixels per unit area), "linear pixel density" (pixels per mm in either direction), and "pixel pitch"; for square photosites anyway:

• pixels per mm = sqrt(pixels per square cm) / 10

• pixel pitch in microns = 1000/(pixels per mm)

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