Ambient light vs Incident Light

[jeeperman]

Thanks Frank....you made me laugh out loud.

[pnodrog]

Ooops, did I give it away?

No - only if you are totally in the dark.

[Andrew76]

If I'm wrong, then so is Sekonic's Instruction Manual on how their meters work:

Incident Light

Incident light is the light that falls on a subject, either directly or indirectly. Direct sources of incident illumination are sources that emit light, like the sun or a strobe unit aimed at the subject. Indirect sources of incident illumination redirect light onto the subject. These could include a wall from which sunlight bounces back onto the subject or a reflector that redirects light from a strobe onto the subject. In both cases, the light is reaching the subject and can be metered and controlled.

I don't want to argue, but this is how I understand light to work, and I'm not a physicist, but I do, and have used a light meter.

[pnodrog]

Your not arguing with me I was just obscurely making the point that unless you are in the dark (provided you are not doing Infra-red photography) the flash is only providing some of the incident light - total agreement.

[Andrew76]

Sorry Paul! That one went right over my head!!

[Bobobird]

Ambient light is light that surrounds the entire scene and one has no control over its spread.

Incident light are the effects of how the ambient light lights up the individual objects within an entire scene.

[pnodrog]

Good question -- looking forward to reading the answers!

Can we request that our lawyers be present?

[Glenn NK]

Yes it could get ugly !

Dave

It's more fun to watch the confusion.

Pass the popcorn.

G

[Colin Southern]

That's downright scary.

Not really - we have a Google bot with us 24x7 which pushes content to Google -- it doesn't wait for Google to crawl us, so Google knows about us pretty much in real time.

[Colin Southern]

Can we request that our lawyers be present?

I think we're going to need them shortly

Malteser anyone?

[Inkanyezi]

I wonder whether there is less confusion now? Would Ed, Abitconfused, be less confused by the ramblings in the thread?

I think, assuming that the confusion is real, and that Ed really wants a clear answer, that we should not add to confusion even if we find the subject somewhat amusing. Like Colin, I might have leaned back to watch if I had seen the question before, but now, that so many posts are given that rather muddy the waters than clear things up, maybe we should explain a little about photographer's lingo?

I think the confusion stems primarily from the multitude of meanings of the word incident, whereas in our lingo it means only one thing regarding light, and likewise the word ambient may be interpreted in several ways, while we use it mainly in one context when talking light for photography.

It has been said in earlier posts, that incident is the light striking a surface. It must be understood, that this is not really physics, but photographer's lingo. If we were into physics, we might prefer not to call it light at all, but radiation. A photographer's concept of incident light is the electromagnetic radiation in the visible spectrum from all sources that reaches a subject. This radiation may be measured with an incident light meter, an instrument that receives the same light (radiation) as the subject. The sources of incident light can be any light sources, including flash.

The pair of ambient versus incident are not interrelated in a way that you should contrast them against each other. The antithesis of incident light in photographer's lingo is reflected light, which is the light that reaches the camera from the subject. Reflected light is the light that we perceive when we look at the scene. A light meter calibrated to measure the light that reaches the camera is a "reflected light meter", and we would call it so, even when we measure an actual light source, whereas in fact that light is direct light.

Ambient light is a bit easier to understand I think, as the word does not have so many meanings. In photographer's lingo, ambient light means any light that is present at a given time, except light that is added by light sources that you control and direct toward the scene. Hence we would not consider any studio lighting ambient, while normal room lighting and any outdoors lighting would be ambient, as long as we do not add any light source. Again, it is not physics, but rather our lingo.

So it is two different concepts in our lingo. Ambient light is often called available light, but a flash does not count even if it is available. In our lingo, both those terms would indicate that we did not add any light to the scene. The term is not related to incident light. The ambient light may be measured as incident light, and it may be measured as reflected light.

So the pair that may be analysed from a physicists point of view are incident vs reflected, and the physicist would use different units for evaluating them. Incident light is illuminance and measured in Lux (lx), while reflected light is luminance from the physicists view and measured in candela per square metre.

[Colin Southern]

I wonder whether there is less confusion now? Would Ed, Abitconfused, be less confused by the ramblings in the thread?

OK - I'll stop toasting my marshmellows and give a real-world answer ...

Ambient light is what a camera sees when not using a flash. When using a flash the ambient lighting is augmented by the flash light, which is typically used to fill in darker areas or to impart a direction of light.

Incident light is light that illuminates the objects we photograph (or enters the camera directly) -- and as such, we meter it differently from reflected light (because it's stronger).

In terms of the relationships between them ...

... don't worry about it!

Hope that helps

[Dave Humphries]

Hi Ed,

While Colin's last, and Urban's, answers do much to clarify the discussion for you and provide much necessary additional information, I'll try to answer your questions directly.

Ambient light vs Incident Light - Is there a difference?

Yes (see other answers above)

Is a flash incident light?

Yes - when it illuminates something - e.g. an incident (flash) light meter.

As both Colin and Urban have said, these two ("Ambient" and "Incident" light) are not usually related in a way that makes any sense to consider.

I don't know if this will help, but you might try to apply the terms to liquid in a container.
Consider a glass of water;
What the glass contains is its "ambient" water.
If we add to that, we do so with "incident" water, it doesn't matter whether that is from a tap, whether the tap water is 'reflected' off our hand, because it is left out in the rain, etc. - all the water that is added to the glass is "incident" water.
Don't over-think this, I am not concerned with volumes, flow rates, water that misses the glass or splashes back out.

If we return to the glass the next day, its now (probably increased) contents are the new "ambient" and anything we add will still be "incident", so (I hope) you can see the two aren't really related in a way that is relevant.

I apologise in advance if this analogy was unnecessary (or unhelpful).

I do hope this doesn't encourage people to take my simplified analogy to (silly) extremes, as that won't help Ed.

Cheers,

[davidedric]

Well, everyone seems to have added their bit, so now I can (quite possibly) create more confusion again. I would use

- Ambient Light to mean the light that just happens to be around before we start messing. It may be from the sun, moon, light bulbs, or whatever. It does not of course mean everything is evenly illuminated in any particular scene, which brings us to:

- Incident Light which is the light falling on any surface at any given time. It will include a contribution from ambient light, and maybe floods, flash or whatever. It's always there, but normally we are most interested when we press the shutter release, because then

- Reflected Light (some part of the Incident Light) from the scene being photographed enters the camera.

Simples. eeek.

Dave

[Manfred M]

Ambient light is just the light that normally occurs in a scene, prior to the photographer adding light using an external source, like a small flash, studio light or even continuous light. Added light could be daylight (via a reflector or opening a closed-over window) or via an artificial light source.

Incident light is a physics term - it refers to the incoming light before it is modified by an optical surface, either reflected (mirror or any other surface that does not absorb the light) or refracted (travels through a transparent optical surface, like a lens or a glass bottle).

[drjuice]

As "a trained geographer", according to one of my friends, I'm bound to confess that while I was teaching, most of the people I worked with on committees (our department chairs generally assigned us to interdepartmental committees) and got to know were physicists. One is a Presbyterian elder and another is a Danophile, neither of which avocations strikes me as something in which I would expect physicists to maintain an abiding interest.

IJS....

virginia

[John Morton]

Yes Paul, the question of incident light also comes into arguments against daylight saving (which we dont have here). The opponents to daylight saving, particularly those with dangling cork hats, get worried about things like fading curtains etc

I've been looking for one of those dangling cork hats for years, ever since seeing them in the Monty Python "Bruces" sketch!

[PengukirCahaya]

AMBIENT simply means surrounding, immediately arround you. Thus AMBIENT LIGHT can be understood as any available light at any particular moment arround you.

INCIDENT means striking or fall on. So to me, INCIDENT LIGHT is any light that strike or fall upon any surface or subject. And you can not see that light. What you can see is the light that bounce or reflected from the surface or subject.

So....
AMBIENT LIGHT (<--this is the source)----(incident light)-----> SUBJECT/SURFACE -(<---bounce from)---> REFLECTED LIGHT

[Abitconfused]

Thank you all. You have contributed to my understanding.

[jcuknz]

I know when I Google my name, first then last - Cambridge in Colour pops up number 2, right behind Face Book. Not that I care - it's the age that I live in, and the age that I'm raising my children in, so I will deal with it, and it truly doesn't bother me, but It's crazy since my last name is quite unique, and I'm the only 'Andrew' with that last name in the world, and the only place it occurs on this site is in my Project 52 thread - not even in my personal profile, and yet Google picks up on that. Crazy.

Anyhow - how about that Incident Flash Light??

That is one of the advantages of a pen-name ... mine comes up with at least ten pages of references while my real name was buried back on page four or five last time I checked

I would have said that in general usage ambient light is what we live and see by and incident light is the light coming from a light source. Both are reflected by our subjects for us and our cameras to see by. Way back I had an Invercone for my Weston Master meter which measured the incident light falling on it which in those day was also ambient light since I didn't have any flash gear then