About luminous flux density

[Doug Kerr]

In technical articles, we commonly see the “potency” of a light beam at a certain place in its travels quantified as the illuminance of the beam there.
But that is not apt. Illuminance is not a property of a beam of light at some place in its travels. It is a measure of the illumination afforded by a beam of light on some surface, having a certain orientation, at such a place.

What does properly characterize the “potency” of a beam of light at a certain place is its luminous flux density. Yet we almost never see that quantity mentioned. Luminous flux density and illuminance have very similar definitions, and are closely related, so it is perhaps understandable that authors get them confused.

I sort this out (hopefully) in a new technical article on The Pumpkin, "Luminous flux density: the rarely mentioned photometric quantity".

This is a link to its listing on The Pumpkin index page:

The Pumpkin

Index page of Doug Kerr's The Pumpkin

dougkerr.net

Best regards,

Doug

 

[Jerome Marot]

Personally, I find all these definitions much simpler to understand when talking about energy. There is a light or, generally, electromagnetic source. It emits some energy in some directions of space. "Flux" is that energy in a particular direction. When talking about "illuminance" we simply need to multiply by the particular spectral sensitivity of the human eye.

But that is just me. Thanks for the paper.

 

[Ted Cousins]

Personally, I find all these definitions much simpler to understand when talking about energy. There is a light or, generally, electromagnetic source. It emits some energy in some directions of space. "Flux" is that energy in a particular direction. When talking about "illuminance" we simply need to multiply by the particular spectral sensitivity of the human eye.

But that is just me. Thanks for the paper.

Found this on-line, quite helpful:

https://luminusdevices.zendesk.com/...lux-Illuminance-Luminous-Intensity-Lux-Lumens

Does not illustrate flux density ... but one can see the inverse square law at work in the beam from the candlewick ...