Get dimension of pixel in mm

[ldima]

Hello.
I have an image of size 640x480 pixels.
It's possible to obtain the dimension in mm of one pixel from that image given only that size?I do not have the size of the image in mm,however.
Thanks for helping.

[weaknessforcats]

Your question has no meaning.

You already said your image is 640x480 pixels.

Each pixel will have a size based on the resolution of the display.

The more pixels in your display, the smaller your picture.

[ldima]

Thanks for answering weaknessforcats.
I try to explain better:I have loaded an image taken from a camera,then i need pixel dimension in mm of that image, for example obtain that each pixel is wide 0.2 mm.
This is not possible?

[Rabbit]

It depends on the display. Every display has a different size for each pixel. Imagine drawing a 10 by 10 grid on a 10 inch monitor. What's the size of a single square? Now draw a 10 by 10 grid on a 20 inch monitor, what's the size of a single square? It depends on the size of the display and its native resolution.

[weaknessforcats]

Not it is not possible.

Suppose your computer screen was 640 x 480. Your picture would fill the screen.

Change the resolution to 1280 x 960 and the same picture would occupy only 1/4 of the screen.

[Stewart Ross]

If you are really meaning that you have data stored by a digital camera which recorded additional information about the image, including its nominal size, you would need to read the EXIF (Exchangeable Image File Format) data for that image. This is not a trivial task for JPG files, for example.

See the following Wikipedia articles for guidance:
Exchangeable image file format
JPEG

Otherwise, as my colleagues have already said, it s not possible to determine what the nominal 'size' of a pixel is without knowing anything at all about the size of the sensor in the camera and the focal length of the lens involved.

-Stewart