Luminance or Illuminance ?

Started Mar 28, 2013 | Discussions
GeorgianBay1939
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Luminance or Illuminance ?
Mar 28, 2013

For my personal use I am writing a note on how I use my GH2 to ETTR (sometimes whilst bracketing) a scene using the camera's spot meter.
 My hands and eyes know what I am doing but my maturing brain is well challenged.
Last fall I decided to replace the potentially misleading terms in popular photography with more accurate ones.
See why:  http://www.dpreview.com/forums/thread/3453097
A while ago I started using the term Luminance Range of a scene to replace DR in my own thinking.   I thought that I had the correct term.  But now I am not so sure.
I use the camera's spot (exposure) meter to "look" at a scene with the histogram showing in the EVF and play with the half-press.   I think that I am surveying the Range of Luminance in the scene.  But I might be surveying the Range of Illuminance.  Both are (light) energy per area.  
Obviously what I am measuring (off of the sensor in the GH2) is also affected by the f/ (more accurately the T) of the lens.  I am NOT so worried about that.
I am trying to get at the notion that what "loads" (energizes) the sensor is determined by
the Luminance/Illuminance of various parts of the scene
the f/ (T) of the lens, and
the SS (the length of time that the shutter is open)
Luminance?  Illuminance?  ______________?
Thanks for your help.  
Tom

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s_grins
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

You could call it 'Nancy', and I would not mind

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to s_grins, Mar 28, 2013

s_grins wrote:

You could call it 'Nancy', and I would not mind

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Looking for equilibrium...

No, but my friend, Nancy, might.  (On the other hand she might be honoured to have an important physical variable named after her!)

Simon, you've been through all of this stuff before so it is not important to you.  But consistent terminology sure makes life a lot easier for beginning photogs like me.  So I have to use vocabulary which is internally consistent and (hopefully) is consistent with generally accepted SI terminology.

I prefer not to use Dynamic Range to describe the variation in "brightness" when looking through a viewfinder since DR is used for so many other variables -- including all the fluff that manufacturers use to describe their wonderful megapixel sensors (to uninformed consumers).

But I do like the levity that you add to a life/death matter!!!  

t

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Chas2
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

I have to take time to look this up, but I would just say be careful of the terminology.  I seem to remember from my wife's interior design lighting class that these are two separate definitions in photometry (think it was a test question she got wrong).  I appreciate the distinctions you are trying to make, but you may wish to make sure your usage is consistent with international standards...if you care.  So long as you know what you are using the term for, I guess it does not matter.

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Tom Axford
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

The definitions I have seen distinguish between the brightness of a surface (luminance) and the amount of light incident on a surface (illuminance). I'm not sure how widely recognised this distinction is, but using google produces some confirmation:

http://www.jimonlight.com/2008/12/18/lighting-101-luminance-vs-illuminance/

http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Chas2, Mar 28, 2013

Chas2 wrote:

I have to take time to look this up, but I would just say be careful of the terminology.  I seem to remember from my wife's interior design lighting class that these are two separate definitions in photometry (think it was a test question she got wrong).  I appreciate the distinctions you are trying to make, but you may wish to make sure your usage is consistent with international standards...if you care.  So long as you know what you are using the term for, I guess it does not matter.

Good advice. Yes, I care.  I want to keep it consistent with SI but understandable by my photog friends who don't have a lot of science/engineering background.  This is pretty esoteric (useless?) stuff for some of them.

Thanks,

t

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

The definitions I have seen distinguish between the brightness of a surface (luminance) and the amount of light incident on a surface (illuminance). I'm not sure how widely recognised this distinction is, but using google produces some confirmation:

http://www.jimonlight.com/2008/12/18/lighting-101-luminance-vs-illuminance/

http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml

Both very useful.   luminance is a measure of the "brightness" of light that you "see" versus illuminance which is a measure of the intensity of light falling on your retina.  Thanks Tom.

I also went here:  http://www.cambridgeincolour.com/tutorials/camera-metering.htm   and now realize that dependence on evaluative in-camera metering can lead to significant issues.

For my purposes, using spot metering, I think that the following is true:

Luminance is what my eyes "see".  It is a measure of apparent "brightness" (intensity) of the visual field.  It is what the front of the objective lens "sees".  Luminous intensity/area, measured in Cd/meter^2, energy per area.

Illuminance is what is falling on my retina. It is a measure of illumination on an area of a sensor. It is what causes electrons to be produced in photosites.  In general higher Illuminance leads to more electrons ... up to a point. Luminous flux/area, measured in lux/meter^2, also energy/area

So when I am spot metering, looking at a scene with an EVF, I am getting an impression of luminance of (parts of) the scene, both visually and with the histogram.  But since both the (jpeg) image and the histogram in the EVF are generated by illuminance of the sensor I am really measuring illuminance of (parts of) the sensor.

(I suppose that if I were to use an OVF, I would get a real, actual measure of luminance, affected only by the optics.)

Bottom Line (I think):

When spot metering with the live view EVF using the histogram I am surveying the illuminance on the sensor to give me an idea of the range of luminance of the scene.

Do I have it?

t

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Swamp Duck
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

It has been my thought that Luminance was the light reflected from a given subject, Illuminance was the light on the subject?

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Swamp Duck, Mar 28, 2013

Swamp Duck wrote:

It has been my thought that Luminance was the light reflected from a given subject, Illuminance was the light on the subject?

Yes, that is what I am concluding also. But, contrary to most photo essays, I see no reason to care about the light on the subject.  I really, really care, though, what my camera's sensor is sensing.

That was a significant reason why I decided on a MFT camera with a live view EVF instead of a DSLR with an OVF a couple of years ago.  Of course there were other reasons, especially the ability to use good small diameter, short lenses.

Thanks,

tom

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Tom Axford
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

GeorgianBay1939 wrote:

Swamp Duck wrote:

It has been my thought that Luminance was the light reflected from a given subject, Illuminance was the light on the subject?

Yes, that is what I am concluding also. But, contrary to most photo essays, I see no reason to care about the light on the subject.  I really, really care, though, what my camera's sensor is sensing.

That was a significant reason why I decided on a MFT camera with a live view EVF instead of a DSLR with an OVF a couple of years ago.  Of course there were other reasons, especially the ability to use good small diameter, short lenses.

Thanks,

tom

I think one of the historical reasons for caring about the light on the subject was in a method of metering that was more common in the days of film: that of using an incident light meter (measuring illuminance) instead of using a reflected light meter (measuring luminance). These days, metering is nearly always by the camera which is essentially a reflected light meter.

The advantage of using an incident light meter was that white snow came out white and a black cat came out black, whereas using a reflected light meter both would come out mid-grey (if they more-or-less filled the frame).

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

GeorgianBay1939 wrote:

Swamp Duck wrote:

It has been my thought that Luminance was the light reflected from a given subject, Illuminance was the light on the subject?

Yes, that is what I am concluding also. But, contrary to most photo essays, I see no reason to care about the light on the subject.  I really, really care, though, what my camera's sensor is sensing.

That was a significant reason why I decided on a MFT camera with a live view EVF instead of a DSLR with an OVF a couple of years ago.  Of course there were other reasons, especially the ability to use good small diameter, short lenses.

Thanks,

tom

I think one of the historical reasons for caring about the light on the subject was in a method of metering that was more common in the days of film: that of using an incident light meter (measuring illuminance) instead of using a reflected light meter (measuring luminance). These days, metering is nearly always by the camera which is essentially a reflected light meter.

The advantage of using an incident light meter was that white snow came out white and a black cat came out black, whereas using a reflected light meter both would come out mid-grey (if they more-or-less filled the frame).

Thank you, Tom.

That is very helpful.   I suppose if one is lighting a scene on a set or in a studio, an incident light meter would also be useful.... when changing the intensity of the various lights to change their luminance.

Would the white snow / black cat situation be an issue if you used spot metering to give the highest "exposure" of the sensor (ETTR)?

In my case, when shooting RAW I always load the sensor up to white clipping by surveying the scene with the spot meter, half pressing, recomposing and then releasing the shutter.  (with due regard to DOF, motion blur and art.)

The past couple of months I challenged myself with trying to capture "sparkles" in snow, like this scene:

301302 twig with snow crystals

This was also a delicate image to expose properly:

201302 birches Skerryvore

I suspect that the above images aren't large enough to show the delicate sparkles in the foreground snow.  In those cases I did get the little spike in histogram showing the specular reflections.  In both cases I used spot metering, moved it around to let the spike move to the right, and made the exposure.  I then checked the exposure in R,G,B and Y on the postview histogram and had no clipping (except for specular), although IIRC the blue was close in the birches against the sky shot.  Didn't have to bracket or re-expose.

I was having a devil of a time with this challenge until I decided to continue expose the scenes as much as possible and then to normalize them in post processing to what they would look like when using neutral gray sunglasses!!!

Sometimes cheating is helpful, eh?

Yes, I would appreciate your advice re the black cat/ white snow issue as I don't think that I properly understand it.

Many thanks,

t.

PS  Plse forgive my wordiness....  and the imagery.   I try to relate what I am learning here with how I use my GH2.   Old habit!

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Tom Axford
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Re: Luminance or Illuminance ?
In reply to GeorgianBay1939, Mar 28, 2013

Beautiful images and very expertly exposed.

I must confess it is not something I have really thought about before, but I am inclined to think that the sophistication of ETTR (coupled with PP) renders incident light metering obsolete (having said that, I feel sure that someone will say that they still use it and swear by it).

Yes, I entirely agree with the method you outlined for obtaining the 'best' exposure. Some years ago, I mainly used incident light metering with medium format film cameras, but I would have to search to find my old light meter - it hasn't been used for years. With colour transparency film there was effectively no opportunity for post-processing correction after the shot had been taken, so it was more important to get the exposure exactly right beforehand.

These days, it is more important to ensure that you don't clip the highlights but otherwise use the highest exposure you can (i.e. ETTR) so that you reduce the noise to the minimum. If the result looks either too light or too dark, you simply adjust in PP. That was not a possibility with colour transparency film (at least, not in most circumstances).

Concerning the black cat/white snow, all I meant was that if you set your camera on auto exposure and photograph a black cat (filling the frame), the result will look more like a grey cat; while if you do the same thing with white snow, the result will look like grey snow (your photo of the twig in the snow shows this effect, but in this case you deliberately did it to allow the sparkling twinkles in the snow to show up - if the twinkles hadn't been there, the snow would look rather dark and you would probably have brightened it as in your second image).

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GeorgianBay1939
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Re: Luminance or Illuminance ?
In reply to Tom Axford, Mar 28, 2013

Tom Axford wrote:

Beautiful images and very expertly exposed.

I must confess it is not something I have really thought about before, but I am inclined to think that the sophistication of ETTR (coupled with PP) renders incident light metering obsolete (having said that, I feel sure that someone will say that they still use it and swear by it).

Yes, I entirely agree with the method you outlined for obtaining the 'best' exposure. Some years ago, I mainly used incident light metering with medium format film cameras, but I would have to search to find my old light meter - it hasn't been used for years. With colour transparency film there was effectively no opportunity for post-processing correction after the shot had been taken, so it was more important to get the exposure exactly right beforehand.

These days, it is more important to ensure that you don't clip the highlights but otherwise use the highest exposure you can (i.e. ETTR) so that you reduce the noise to the minimum. If the result looks either too light or too dark, you simply adjust in PP. That was not a possibility with colour transparency film (at least, not in most circumstances).

Concerning the black cat/white snow, all I meant was that if you set your camera on auto exposure and photograph a black cat (filling the frame), the result will look more like a grey cat; while if you do the same thing with white snow, the result will look like grey snow (your photo of the twig in the snow shows this effect, but in this case you deliberately did it to allow the sparkling twinkles in the snow to show up - if the twinkles hadn't been there, the snow would look rather dark and you would probably have brightened it as in your second image).

Thank you for your very generous comments and explanations.  It really helps me put things in context.

I tend to narrow in on techniques that will work MY camera (and me!) to their limits in producing quality imagery.  That sometimes obscures a broader view of photography.

I have to be careful around some of my very experienced (and competent) photog friends who were expert with their cameras and darkrooms many years ago.  They still talk zones, sunny f/16s, grey cards, exposure triangles, etc and sometimes get their sekonics out.  I sometimes try to cut though that historical information to the physical optics and image processing of what is going on and they often clam up, or say that they don't need to know that stuff.     Some folk have all that good stuff as background AND also really understand the technicalities of modern digital imaging, including thereasons behind ETTR etc.   Some of them have moved from FF cameras to MFT and are realizing that EVF's aren't really evil,  but even have some advantages compared to OVFs.  Those are the folks who generally understand why and how to load their sensors in critical situations.

In the meantime I am thoroughly enjoying my GH2 (AND LR4 and PhotomatixPro) and hope to have it well explored before long.

Outgrowing it seems to be the only acceptable reason for considering getting another camera with more capability.

Thanks again for your help.

t

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