Luminance vs. Luminosity

[Abitconfused]

What is the difference?

[Glenn NK]

http://en.wikipedia.org/wiki/Luminance

http://en.wikipedia.org/wiki/Luminosities

I was confused and still am a bit. Reading the two blurbs, it seems that:

Luminosity is the total amount of energy emitted . . . . in joules per second. This is the total mount of light emitted.

Luminance is a measure of the luminous intensity per unit area of light traveling in a given direction. This is the amount of light emitted per unit area.

This seem to be analogous to pounds and pounds per square inch (or Pascals and Pascals per square metre). The first is a force, the second is a force per unit area.

Hope this helps and that it isn't totally wrong.

However I would like to add that it may not be all that important unless you are doing astrophotography.

Glenn

[John Morton]

I think the key here is that luminosity can be used to measure light sources, whereas luminance would be used to measure things that are lit, rather than emitting light. So a flash unit, for instance, could be described in terms of luminosity; but its effects upon objects would be described in terms of luminance - kind of like a technical distinction holding between 'illuminating' and 'illuminated.'

But, don't quote me on that!

[xpatUSA]

As a watch collector, "Luminosity" caught my eye. However, good luck to anyone trying to understand the Wikipedia link, a miracle of obfuscation, IMHO.

Allow me to add:

Energy per unit time is just plain old power: 1 Joule/sec = 1 Watt. In the mechanical world, power is the rate of doing work. For example, so many foot-lbf per unit time = BHP, or, so many Newton-meters per sec = Watts.

As far as light of all wavelengths (visible or not) is concerned, the term radiance is preferable to this nebulous luminosity thing, I reckon. And photons/sec is one way of measuring radiance **, and when radiance arrives at our sensor it becomes irradiance if we stay in the power "domain".

[** it's not really that simple: The SI unit of radiance is watts per steradian per square metre (W·sr⁻¹·m⁻²) but just plain "Watts" keeps it simple for our understanding].

If, on the other hand, we're restricting our view to visible light then we would use luminance to describe visible light coming from a scene, and illuminance to describe the part of that visible light hitting our sensor.

One way of thinking about the difference is that you can't see the sun's IR that's hitting your matt black car but you sure can feel it, especially here in Texas! That's power for you. On the other hand, visible light from a bright speed light isn't going to heat your shiny white car up much - but it could easily blow your image highlights.

[Brownbear]

A little off track but reading your post brought a question to mind on luminosity...

When you are in a photo editing program - in noise reduction... reduce colour noise... reduce luminosity noise... if you reduce luminosity this will effect the lighting conditions in the photo?

Thank you.

[John Morton]

A little off track but reading your post brought a question to mind on luminosity...

When you are in a photo editing program - in noise reduction... reduce colour noise... reduce luminosity noise... if you reduce luminosity this will effect the lighting conditions in the photo?

Thank you.

Hi, Christina;

"Luminosity" is a term used quite loosely by those who design photo editing software. Once a word has acquired an ending of "-ity" it also tends to take on a meaning of "sorta kinda like that."

In noise reduction, 'luminosity' is used to distinguish from color noise reduction - noise that has a chroma component of distinctly red, green, or blue color (little colored specks).

Such noise filters often work by defining two small circular regions, and inner circle and an outer ring. If the pixels) in the inner circle are too different than the average of the ones in the outer ring, then the inner circle pixel(s) is considered to be 'noise and is converted into the average value for the outer ring pixels. There are many variations on the way that the value of the inner circle and outer ring are calculated, and the size of each circle/ring; but the end result is that small pixel groups which are very different than the pixels which surround them are changed to match their surroundings.

This form of filtering can lead to a loss of image detail in some cases, a blurring of the image in others, and the softening of edges (corners of angular objects, for instance) if applied too aggressively - which brings us back to your question. Luminosity noise reduction won't affect the lighting conditions of an image per say, but, it can 'blend together' or 'average out' details which in effect changes how light appears to interact with whatever is in an image; but, on a 'pixel by pixel' level, not in a global 'everything at once' kind of way.

[Brownbear]

Thank you for your detailed and very informative reply.

[Abitconfused]

Is this true?
Brightness and luminosity are our perceptions of luminance. Brightness is luminance but not quite. Both of these terms are subjective. Luminosity is our perception of the distribution of light. The distribution of light in a photograph greatly influences our understanding or appreciation of the subject of interest. Brightness in a scene or photograph is a component of luminosity. A rainbow has considerable luminosity when set against the background of a dark cloud but once the cloud passes the rainbow’s luminosity diminishes. But did the rainbow’s brightness ever really change? Luminosity is also related to contrast. Can a photograph without acceptable contrast have a pleasing luminosity? I think not. Image brightness is what first grabs our attention, physiologically then perceptually. Brightness may be perceived to be intolerable, perfect, adequate, disappointing or sadly lacking. But when we use the word brightness we are also making a judgment of luminosity. This must be true because, realistically, people hardly ever use the word luminosity. Brightness is a more human expression. “Aren’t those bright colors beautiful?” she asked, her eyes twinkling. “Well,” her dull date replied, “given the arrangement of hue and the abundant illumination, the perceived luminance does present an agreeable luminosity.” luminance may be used when a measurement of reflected light is suggested and brightness whenever a judgment regarding the adequacy or esthetics of light is required however individualized that may be.