What is meant by monochrome?

[TonyW]

Ted, with reference to your diagram above, at any point in a black and white image, the point is on the vertical diagonal of the cube which projects on to the centre of the hexagon below. When a tint is applied, the point moves off that line and you get a point like the point P in the hexagon. The formula for the chomaticity depends on the difference between two colour values and similarly the formula for the hue depends only on the difference. Hence, if you change the colour values by adding the same amount to each, the chromaticity and hue are unchanged, i.e. the colour is unchanged except for its brightness.

This is consistent with the result I got for the colour values for a monochrome image.

[barrydoig]

When I use photoshop to convert an image to black and white, at any point in the image the levels of red, green and blue channels on a scale from 0 to 255 are the same. This is still true when I convert from AdobeRGB to sRGB. But when I introduce a tint like a sepia, the relationship between the levels is not at all obvious.

Are the colours in a monochrome image adjusted according to some subjective idea of points of different luminosity being of the same colour or is there a well defined arithmetical relationship?

I have had a picture of mine called monochrome when I have done nothing to make it so and conversely I have looked at a print of a supposedly monochrome picture and seen some colorisation.

FIAP, the International Federation of Photographic Art, gives this definition of Monochrome:
This definition supersedes the Document 223 and the INFO 1991/12.
A black and white work fitting from the very dark grey (black) to the very clear grey
(white) is a monochrome work with the various shades of grey.
A black and white work toned entirely in a single colour will remain a monochrome
work able to stand in the black and white category; such a work can be reproduced in
black and white in the catalogue of a salon under FIAP Patronage.
On the other hand a black and white work modified by a partial toning or by the
addition of one colour becomes a colour work (polychrome) to stand in the colour
category; such a work requires colour reproduction in the catalogue of a salon under
FIAP Patronage.
This is the definition used by most Photographic Societies
Barry

[Manfred M]

FIAP, the International Federation of Photographic Art, gives this definition of Monochrome:
This definition supersedes the Document 223 and the INFO 1991/12.
A black and white work fitting from the very dark grey (black) to the very clear grey
(white) is a monochrome work with the various shades of grey.
A black and white work toned entirely in a single colour will remain a monochrome
work able to stand in the black and white category; such a work can be reproduced in
black and white in the catalogue of a salon under FIAP Patronage.
On the other hand a black and white work modified by a partial toning or by the
addition of one colour becomes a colour work (polychrome) to stand in the colour
category; such a work requires colour reproduction in the catalogue of a salon under
FIAP Patronage.
This is the definition used by most Photographic Societies
Barry

I really hate these definitions because they are so very misleading.

The B&W definition is fair enough for a digital image (but not technical enough to be correct), but totally falls apart when you print, as white is defined by the colour of the medium you print on. By this definition anything printed on an off-white paper is NOT B&W, even though you start with a B&W digital file.

Sticking with the 8-bit (jpeg) world, to me black, in a RGB colour space has the value of (0,0,0) and the corresponding value of white is (255,255, 255).