How can you have a scene linear reflectance greater than 100%?

Started 3 months ago | Questions
OP Mandem Junior Member • Posts: 28
Re: Words - why white is white except when it ain’t
1

alanr0 wrote:

Mandem wrote:

Mark Scott Abeln wrote:

Mandem wrote:

Does this not mean that there is some standard against which it is being measured? Perhaps the average illuminance of the scene? Say we have 2 areas in a frame, one significantly more illuminated than the other and both of these areas have an 18% middle gray card in them. Of course the 18% gray card in the strongly illuminated area will reflect in ABSOLUTE terms much more relative to the 18% gray card in the less illuminated area but proportionally they're both reflecting equal amounts. So how would we go about deciding which 1 is actually an 18% reflectance. This is getting quite messy and confusing the more I think about it.

There is a human physiological or psychological phenomenon called “lightness constancy”, which assigns an unvarying tone to our mental models of objects. A white object, in our mind’s eye, is always white and we always expect it to look white, and it almost always appears to be white, and are very surprised when it doesn’t.

An 18% gray card reflects 18% of the visible light falling on it, but how much light is falling on it? That varies, wildly. Direct sunlight might be a thousand times brighter than dim living room illumination at night.

Exposure and metering theory and standards typically *assumes* that the light falling on the scene is completely uniform, even if it isn’t.

If exposure and metering theory assumes uniform light throughout the scene then the only thing I can understand causing more than a 100% reflectance is like you said previously the conversion of non-visible light into visible light through whatever subtle process. Where can I learn more specifically about this? As most sources online barely ever even mention scene linear reflectance when discussing about dynamic range.

Uniform illumination throughout the scene is a sometimes convenient fiction, which generally will not apply to log gamma encoding and wide dynamic range displays.

In the real world, surfaces with greater than 100% reflectance are uncommon. What does occur is non-Lambertian directional scattering (in particular specular reflection) and especially non-uniform illumination.

Whiter than white objects are possible, but rarely amplify by a factor of 5 or more.

The various video tone curve standards specify how changes in recorded signal are converted to intensity by a display device. The numbers correspond to relative changes in luminance referred to the maximum output of the display. For reasons best known to themselves, the video community describe these brightness changes in terms of reflectance, even though most variation will arise from changes in subject illumination.

There is no mysterious physical process. It is simply a way to describe how the video signal represents different brightness levels.

So i'm guessing it is indeed "sloppy terminology" and a misnomer when they say "reflectance" . That would make sense as the formula for Reflectance=Reflected Light /Incident Light just doesn't work in any scenario as, like I said previously, it can't go past a 100% unless brighteners are involved.

The talk about diffuse reflectance and specular reflectance is starting to make more sense in the context of the intensity of light(what we'd call candelas/m2 or nits) hitting the sensor and that THIS IS what the "reflectance"(confusing misnomer) of the x-axis is actually referring to that gets converted into a digital code value(The ADC conversion process )

To make an example(and please correct me if I'm wrong. God I hope I'm not wrong... ), say we have a camera outside in an open field, at noon, facing head on a 100% diffuse reflective white paper and a mirror right next to it. Both the paper and mirror have incident light from the sun hitting them. Consequently, both are reflecting a 100% of the light incident on them. The fundamental difference arises in the fact that the diffuse reflective paper is scattering the light everywhere hence resulting in significantly less light intensity hitting the sensor as opposed to the mirror where the light rays are all directional towards the sensor thus a much stronger intensity of light is landing on the sensor. Which finally brings me to the formula:

"Reflectance" = Object Reflected or Incident Light Intensity / 100% diffuse reflected white paper Light Intensity.

Edit:perhaps instead of saying "Light Intensity" it would be more clarifying just to say Luminance (Candela/m2)

Thus, in cases where the light intensity is greater than 100% diffuse white reflector we get above 100% "reflectance" (again awful misnomer) values.

Am I correct? Even if I'm not. I'd still like to thank all of you for taking the time and effort for trying to teach me. It just means something isn't really clicking for me. But I'll still try get to the end of this. If anyone has some other way of looking at it or explaining it. I'm all ears. Thanks

Jack Hogan Veteran Member • Posts: 7,873
Clouds, Snow and Mixed Reflectance
1

alanr0 wrote:

Mandem wrote:

I' m gonna try and explain this conundrum with an example. Imagine we have a frame where we are OUTSIDE and we can see:

-The sun at noon(Incident)

-Clouds(reflects)

-A caucasian person(reflects)

-White Paper(reflects)

-18% middle gray card(reflects)

Now in many measurements the clouds(especially ones close to the sun) will have a reflectance % significantly above 100%, whereas white paper will be somewhere 90-100%

What makes you say that the clouds will have reflectance greater than 100%? As you point out, the total light scattered cannot exceed the total light incident.

In practice reflectance comes in three flavors: diffuse, specular and mixed - mixed being a combination of the other two.

'Properly' capturing mixed reflectance is one of the reasons why I think Ansel Adams put mid-gray in the middle of Zone V (L*50 in L*ab, out of L*100 with 100 representing maximum diffuse white), leaving 4 full zones above that before saturation in Zone X. Otherwise 2 Zones of headroom to the top of Zone VII would have sufficed to capture all diffuse reflections (L*50 is centered around 18% of max diffuse white linearly, 2.5 stops below L*100).

I like clouds, I think those with texture often add value to a composition and I strive to capture the detail in them (within reason). I often find that clouds in my alpine landscapes contain some mixed reflectance above 100% diffuse white, as do distant fields of snow for instance.

The reason is what Alan alludes to: more or less crystallized droplets of water which intersperse specular reflectance in an otherwise diffuse surface, interacting with it. In those situations I typically have to dial in about a stop of negative Exposure Compensation to capture the amount of detail I like. Adams was generous adding a margin of 2 stops beyond diffuse white to get to full saturation in Zone X. Then again film's non-linear response was inexpensive in the highlights.

Jack

alanr0 Senior Member • Posts: 2,483
Reflectance & scatter - why solid angles matter
3

Mandem wrote:

alanr0 wrote:

<snip>

There is no mysterious physical process. It is simply a way to describe how the video signal represents different brightness levels.

So i'm guessing it is indeed "sloppy terminology" and a misnomer when they say "reflectance" . That would make sense as the formula for Reflectance=Reflected Light /Incident Light just doesn't work in any scenario as, like I said previously, it can't go past a 100% unless brighteners are involved.

It can make sense if you clarify what we mean by "reflected light" and "incident light". One source of confusion is failing to account for the directional properties of the incident and reflected light.

The SI system defines a whole zoo of different units used in photometry. The trick is to choose appropriate metrics for the situation you are analysing.

Instead of luminance, or illuminance, think in terms of the luminous flux (measured in lumen) reaching a defined area of the surface. If all of the incident photons are reflected in whatever direction, the ratio reflected flux / incident flux is 100%. If half the light is absorbed, and half reflected we have 50% reflectance.

Ignoring oddities such as fluorescence, this works for any surface.

The talk about diffuse reflectance and specular reflectance is starting to make more sense in the context of the intensity of light(what we'd call candelas/m2 or nits) hitting the sensor and that THIS IS what the "reflectance"(confusing misnomer) of the x-axis is actually referring to that gets converted into a digital code value(The ADC conversion process )

To make an example(and please correct me if I'm wrong. God I hope I'm not wrong... ), say we have a camera outside in an open field, at noon, facing head on a 100% diffuse reflective white paper and a mirror right next to it. Both the paper and mirror have incident light from the sun hitting them. Consequently, both are reflecting a 100% of the light incident on them. The fundamental difference arises in the fact that the diffuse reflective paper is scattering the light everywhere hence resulting in significantly less light intensity hitting the sensor as opposed to the mirror where the light rays are all directional towards the sensor thus a much stronger intensity of light is landing on the sensor.

OK, let's plug in some numbers:

Seen through the atmosphere at noon, the luminance of the sun is around 1.6·10⁹ cd/m².

The angle subtended by the sun is roughly 0.53 degrees. For a circular disk, this corresponds to a solid angle of 0.000067 steradian (67 µsr).

The illuminance reaching our surface is: Ev = Lv Ω = 108000 lm/m²

For an ideal diffuse reflector (reflectance R=1), the incident light is scattered into solid angle 2π steradian. When we take into account the cosine inclination factor, the surface luminance, Ls = R Ev / π = 34000 cd/m² (lm/sr/m²).

In contrast, with 100% specular reflection, if the camera is in the reflected beam, it sees the full 1.6·10⁹ cd/m² of the sun's surface, which is around 47000 times brighter.

In practice, it can be useful to measure specular and diffuse reflections from the same surface independently. Suppose we replace our diffuse reflector with white gloss paint.

We have specular reflection of 4% from the glossy surface.

The remaining 96% of incident light is transmitted and suffers 90% diffuse reflection by the white pigment.

Of the incident light, 9.6% is absorbed, net diffuse reflectance is 86.4%, with 4% specular reflection.

In noon sunlight, we have 29400 cd/m² diffuse emittance, and 65000000 cd/m² in a narrow specular reflection beam 0.53 degrees wide - now only 2200 times brighter.

Which finally brings me to the formula:

"Reflectance" = Object Reflected or Incident Light Intensity / 100% diffuse reflected white paper Light Intensity.

Edit:perhaps instead of saying "Light Intensity" it would be more clarifying just to say Luminance (Candela/m2)

Thus, in cases where the light intensity is greater than 100% diffuse white reflector we get above 100% "reflectance" (again awful misnomer) values.

Calculate using any of

  • reflected / incident luminous flux (lm)
  • reflected / incident luminous energy (lm.s)
  • ratio of reflected luminous excitance / incident irradiance (lm/m²).

and you are good to go.

Luminous intensity (cd = lm/sr) and luminance (cd/m² = lm/sr/m²) can be used for specular reflections, but not when the angular spread of the incident light is different for the reflected light.

Am I correct? Even if I'm not. I'd still like to thank all of you for taking the time and effort for trying to teach me. It just means something isn't really clicking for me. But I'll still try get to the end of this. If anyone has some other way of looking at it or explaining it. I'm all ears. Thanks

Hope this helps.

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Alan Robinson

OP Mandem Junior Member • Posts: 28
Re: Reflectance & scatter - why solid angles matter

alanr0 wrote:

Mandem wrote:

alanr0 wrote:

<snip>

There is no mysterious physical process. It is simply a way to describe how the video signal represents different brightness levels.

So i'm guessing it is indeed "sloppy terminology" and a misnomer when they say "reflectance" . That would make sense as the formula for Reflectance=Reflected Light /Incident Light just doesn't work in any scenario as, like I said previously, it can't go past a 100% unless brighteners are involved.

It can make sense if you clarify what we mean by "reflected light" and "incident light". One source of confusion is failing to account for the directional properties of the incident and reflected light.

The SI system defines a whole zoo of different units used in photometry. The trick is to choose appropriate metrics for the situation you are analysing.

Instead of luminance, or illuminance, think in terms of the luminous flux (measured in lumen) reaching a defined area of the surface. If all of the incident photons are reflected in whatever direction, the ratio reflected flux / incident flux is 100%. If half the light is absorbed, and half reflected we have 50% reflectance.

Ignoring oddities such as fluorescence, this works for any surface.

The talk about diffuse reflectance and specular reflectance is starting to make more sense in the context of the intensity of light(what we'd call candelas/m2 or nits) hitting the sensor and that THIS IS what the "reflectance"(confusing misnomer) of the x-axis is actually referring to that gets converted into a digital code value(The ADC conversion process )

To make an example(and please correct me if I'm wrong. God I hope I'm not wrong... ), say we have a camera outside in an open field, at noon, facing head on a 100% diffuse reflective white paper and a mirror right next to it. Both the paper and mirror have incident light from the sun hitting them. Consequently, both are reflecting a 100% of the light incident on them. The fundamental difference arises in the fact that the diffuse reflective paper is scattering the light everywhere hence resulting in significantly less light intensity hitting the sensor as opposed to the mirror where the light rays are all directional towards the sensor thus a much stronger intensity of light is landing on the sensor.

OK, let's plug in some numbers:

Seen through the atmosphere at noon, the luminance of the sun is around 1.6·10⁹ cd/m².

The angle subtended by the sun is roughly 0.53 degrees. For a circular disk, this corresponds to a solid angle of 0.000067 steradian (67 µsr).

The illuminance reaching our surface is: Ev = Lv Ω = 108000 lm/m²

For an ideal diffuse reflector (reflectance R=1), the incident light is scattered into solid angle 2π steradian. When we take into account the cosine inclination factor, the surface luminance, Ls = R Ev / π = 34000 cd/m² (lm/sr/m²).

In contrast, with 100% specular reflection, if the camera is in the reflected beam, it sees the full 1.6·10⁹ cd/m² of the sun's surface, which is around 47000 times brighter.

In practice, it can be useful to measure specular and diffuse reflections from the same surface independently. Suppose we replace our diffuse reflector with white gloss paint.

We have specular reflection of 4% from the glossy surface.

The remaining 96% of incident light is transmitted and suffers 90% diffuse reflection by the white pigment.

Of the incident light, 9.6% is absorbed, net diffuse reflectance is 86.4%, with 4% specular reflection.

In noon sunlight, we have 29400 cd/m² diffuse emittance, and 65000000 cd/m² in a narrow specular reflection beam 0.53 degrees wide - now only 2200 times brighter.

Which finally brings me to the formula:

"Reflectance" = Object Reflected or Incident Light Intensity / 100% diffuse reflected white paper Light Intensity.

Edit:perhaps instead of saying "Light Intensity" it would be more clarifying just to say Luminance (Candela/m2)

Thus, in cases where the light intensity is greater than 100% diffuse white reflector we get above 100% "reflectance" (again awful misnomer) values.

Calculate using any of

  • reflected / incident luminous flux (lm)
  • reflected / incident luminous energy (lm.s)
  • ratio of reflected luminous excitance / incident irradiance (lm/m²).

But using the above 3 formulas we can never get a value higher than 1(100%) for reflectance.

Or are you saying calculate the Reflectance value for the diffuse reflective 100% paper and the reflectance value for whatever object and divide the 2 so:

Reflectance of Object / Reflectance of 100% diffuse reflective white paper

and if it's higher, like a mirror for example, we get supra 100% values? In the case of the mirror 47000(4700000%)

alanr0 Senior Member • Posts: 2,483
Definitions, jargon and sloppy terminology
1

Mandem wrote:

alanr0 wrote:

Calculate using any of

  • reflected / incident luminous flux (lm)
  • reflected / incident luminous energy (lm.s)
  • ratio of reflected luminous excitance / incident irradiance (lm/m²).

But using the above 3 formulas we can never get a value higher than 1(100%) for reflectance.

Correct. That is how reflectance works in the real world.

Or are you saying calculate the Reflectance value for the diffuse reflective 100% paper and the reflectance value for whatever object and divide the 2 so:

Reflectance of Object / Reflectance of 100% diffuse reflective white paper

and if it's higher, like a mirror for example, we get supra 100% values? In the case of the mirror 47000(4700000%)

No.

That is not reflectance in the accepted metrological sense. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/fundamentals-uv/reflectance_measurements.html

If you wish to compare luminance values, divide by some reference level, and call that reflectance, then I can't stop you. However it is not what most scientists, engineers and architects understand by reflectance.

Conventions do vary between fields of endeavour. For instance throughout my career in telecommunications and photonics, it was accepted for "intensity" to refer to power density. It took me a while to appreciate that in radiometry this is called "irradiance", and radiometric intensity is measured in Watts per steradian, not Watts per square metre. The corresponding photometric terms are illuminance and luminous intensity.

In the case of your screen shot, I suspect it is sloppy usage, rather than accepted video terminology, though I could be wrong here. Perhaps spider-mario can comment.

It is worth noting that ITU report BT.2408-4 has lots of percentages, but a quick search found no reference to reflectance greater than 100%. The closest I encountered was a "superwhite" level of 109%, referenced to a 90% reflectance reference.

As far as I can tell, ITU prefer either absolute luminance values (cd/m²) or relative camera exposure, expressed as a percentage.

"Guidance for operational practices in HDR television production" ITU report BT.2408-4 (03-2021), Appendix 1, page 39.

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Alan Robinson

Jack Hogan Veteran Member • Posts: 7,873
Re: Reflectance & scatter - why solid angles matter
1

Mandem wrote:

Or are you saying calculate the Reflectance value for the diffuse reflective 100% paper and the reflectance value for whatever object and divide the 2 so:

Reflectance of Object / Reflectance of 100% diffuse reflective white paper

Paper is typically not a good material if one is after a reference for white diffuse reflectance. Perfection is never possible in nature but decent approximations can be obtained from dry powders made from solid particles of the right size with a high index of refraction. Diamond powder is apparently tops, if a bit expensive compared to alternative powders like MgCO3, TiO2 or PTFE (Teflon).

Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 18,119
Re: White is white except when it ain’t
5

Mandem wrote:

If exposure and metering theory assumes uniform light throughout the scene then the only thing I can understand causing more than a 100% reflectance is like you said previously the conversion of non-visible light into visible light through whatever subtle process. Where can I learn more specifically about this?

No, that's not what they are talking about.

As most sources online barely ever even mention scene linear reflectance when discussing about dynamic range.

A diffuse reflector that's close to 100% reflectance is practically by definition what everyone calls "white".

Now take a 100% pure specular reflector and put it in your scene:  that kind of reflector is what everyone calls a "mirror", and if you aim it just right, you'll see a very bright, perhaps blinding reflection of the light source in the mirror. That's going to be brighter than white.

That something happens to be brighter than a 100% white diffuse reflector doesn't mean that there is any fluorescence going on, it just means that there is a difference between a diffuse and a specular reflector. A diffuse 100% reflector has some good properties as a standard, not the least because everyone can immediately identify it as being white, and diffuse reflectors cover a good percentage of photographic subjects.

But you may want to reserve some of the camera's dynamic range to stuff that's brighter than such a diffuse reflector, because of specular highlights, and any light sources that happen to be within the camera frame, or lens flare, etc.

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MrBrightSide
MrBrightSide Contributing Member • Posts: 996
Re: How can you have a scene linear reflectance greater than 100%?

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Mandem wrote:

I was looking at the curve of SLOG3 but didn't understand how light can be reflected more than a 100%? This indicates to me that 100% reflectance isn't really 100% and that instead it's in relation to some standard. Consequently, this would indicate that 18% gray reflectance isn't reflecting 18% of all light either. Could someone please explain what exactly is happening here? Thanks.

alanr0 Senior Member • Posts: 2,483
Re: How can you have a scene linear reflectance greater than 100%?

MrBrightSide wrote:

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Anything is possible, but the OP is convinced they represent percentage reflectance values - possibly because that is how they are labelled on the screen shot.

The choice of 18% and 90% reflectance values seems significant. These numbers crop up in the ITU documents that spider-mario linked, including ITU report BT.2408-4, where they appear as widely used grey scale reflectance values. See for instance the figure here.

You could check out the gitbub repository for LUTCALC which I believe is the source of the graphic.  My guess is careless terminology for relative exposure values, but I have not dug deeper to find out.

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Alan Robinson

OP Mandem Junior Member • Posts: 28
Re: How can you have a scene linear reflectance greater than 100%?

alanr0 wrote:

MrBrightSide wrote:

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Anything is possible, but the OP is convinced they represent percentage reflectance values - possibly because that is how they are labelled on the screen shot.

The choice of 18% and 90% reflectance values seems significant. These numbers crop up in the ITU documents that spider-mario linked, including ITU report BT.2408-4, where they appear as widely used grey scale reflectance values. See for instance the figure here.

You could check out the gitbub repository for LUTCALC which I believe is the source of the graphic. My guess is careless terminology for relative exposure values, but I have not dug deeper to find out.

Quite the contrary. After our discussion, I am fully convinced the "reflectance" values on Lutcalc are indeed luminance values. The idea of an 18% or 90% is in reference to the luminance of an 18% reflectance gray card or 90% reflectance card. Like you yourself said, you can't have a reflectance greater than 100%.

Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 18,119
Re: How can you have a scene linear reflectance greater than 100%?

Right, diffuse vs specular reflections.

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Jack Hogan Veteran Member • Posts: 7,873
Diffuse = max 100%, Mixed and Specular > 100%, Linearly
2

Mandem wrote:

alanr0 wrote:

MrBrightSide wrote:

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Anything is possible, but the OP is convinced they represent percentage reflectance values - possibly because that is how they are labelled on the screen shot.

The choice of 18% and 90% reflectance values seems significant. These numbers crop up in the ITU documents that spider-mario linked, including ITU report BT.2408-4, where they appear as widely used grey scale reflectance values. See for instance the figure here.

You could check out the gitbub repository for LUTCALC which I believe is the source of the graphic. My guess is careless terminology for relative exposure values, but I have not dug deeper to find out.

Quite the contrary. After our discussion, I am fully convinced the "reflectance" values on Lutcalc are indeed luminance values. The idea of an 18% or 90% is in reference to the luminance of an 18% reflectance gray card or 90% reflectance card. Like you yourself said, you can't have a reflectance greater than 100%.

If you are a practitioner and you've followed posts in this thread, you should by now understand that reflectance comes in three flavors: diffuse, specular and mixed diffuse and specular.

Maximum diffuse white reflection in photography is considered to be 100%. From the ITU link above:

"Reference Level: HDR Reference White (100%) also diffuse white and Graphics White"

However, there is often detail above 100% diffuse white which one may want to capture, as explained in the earlier post on Ansel Adams, clouds and snow. Such detail is typically the result of mixed reflections which live above 100%. In some extreme cases one may actually want to capture the illuminant source itself (say the sun), in which case we would be capturing a signal several thousand % above maximum diffuse white. All this linearly, thanks to the physics of radiometry.

Photography normally works in relative units, that's why many people don't know at what absolute exposure their cameras clip in lx-s - but are interested in where an 18% gray card fits on the histogram in order to choose the appropriate relative aperture and exposure time to capture all the detail they are interested in.

Absolute values of reflected Radiance or Luminance in photography are mainly relevant when perceptual phenomena need to be taken into consideration, say for instance in for adaptation and HDR applications.

Jack

OP Mandem Junior Member • Posts: 28
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly
1

Jack Hogan wrote:

Mandem wrote:

alanr0 wrote:

MrBrightSide wrote:

Those numbers from 0 to 720 look like mV values for the luminance channel. Is that what you’re asking about?

Anything is possible, but the OP is convinced they represent percentage reflectance values - possibly because that is how they are labelled on the screen shot.

The choice of 18% and 90% reflectance values seems significant. These numbers crop up in the ITU documents that spider-mario linked, including ITU report BT.2408-4, where they appear as widely used grey scale reflectance values. See for instance the figure here.

You could check out the gitbub repository for LUTCALC which I believe is the source of the graphic. My guess is careless terminology for relative exposure values, but I have not dug deeper to find out.

Quite the contrary. After our discussion, I am fully convinced the "reflectance" values on Lutcalc are indeed luminance values. The idea of an 18% or 90% is in reference to the luminance of an 18% reflectance gray card or 90% reflectance card. Like you yourself said, you can't have a reflectance greater than 100%.

If you are a practitioner and you've followed posts in this thread, you should by now understand that reflectance comes in three flavors: diffuse, specular and mixed diffuse and specular.

Maximum diffuse white reflection in photography is considered to be 100%. From the ITU link above:

"Reference Level: HDR Reference White (100%) also diffuse white and Graphics White"

However, there is often detail above 100% diffuse white which one may want to capture, as explained in the earlier post on Ansel Adams, clouds and snow. Such detail is typically the result of mixed reflections which live above 100%. In some extreme cases one may actually want to capture the illuminant source itself (say the sun), in which case we would be capturing a signal several thousand % above maximum diffuse white. All this linearly, thanks to the physics of radiometry.

Photography normally works in relative units, that's why many people don't know at what absolute exposure their cameras clip in lx-s - but are interested in where an 18% gray card fits on the histogram in order to choose the appropriate relative aperture and exposure time to capture all the detail they are interested in.

Absolute values of reflected Radiance or Luminance in photography are mainly relevant when perceptual phenomena need to be taken into consideration, say for instance in for adaptation and HDR applications.

Jack

I'm not a practitioner. Just someone who is genuinely interested. But yes, I have grasped the concept of Diffuse, Specular, and Mixed. If you look at my example above with the camera out in a field at noon directly facing a 100% diffuse reflector and a mirror, you'll see that's exactly where I take into consideration the nature of diffuse and specular reflectance. Both of the objects are reflecting a 100% of the light(Theoretically a perfect mirror and a perfect diffuse reflector). You just can't get more than a 100% reflectance according to the formulas Alan put up(exception being Brighteners but they're a rarity in the natural world):

Reflected/Incident Luminous Flux

Reflected/Incident Luminous Energy.

The fundamental difference lies within the DIRECTIONALITY of the light rays being reflected (Hence what makes something Diffuse or Specular or Mixed). Of course, a mirror reflecting will have much stronger Luminance Values than the 100% diffuse reflector purely due to there being no scattering of the rays, even though they both reflect a 100% of the light

Again this all comes back to trying to understand why there were values greater than a 100% on an x-axis called "reflectance" In that LutCALC Graph. It just didn't make any sense since, again, you can't get a value bigger than 1 on the formulas Alan posted above. Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

Jack Hogan Veteran Member • Posts: 7,873
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly
1

Mandem wrote:

Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

I am having difficulty understanding the issue you are exposing.

Say that in certain lighting conditions, independently of absolute radio/photometric levels, the brightest diffuse white possible in nature shows up at 10000 DN in the raw data with the given camera settings. If an 18% mid-gray card is inserted into the scene it will be around 1800 DN. Mixed clouds perhaps at 15000 DN.

So if max diffuse white is considered 100% diffuse reflectance at 10000 DN, it follows linearly that the clouds can be said to provide a mixed response equivalent to 150% of max diffuse reflectance.

Most subjects that photographers wish to retain detail in are diffuse reflectors, so below 100% reflectance.  Perhaps it would be a good idea then to choose Exposure and camera settings that place max diffuse white at 100% of full scale

Yet Ansel Adam's teaches us that it is wise to maintain some headroom for non-idealities and subjects with mixed reflectance. Which is why no camera I know has metering that puts middle gray at 18% of full scale in the raw data, most put it below the teens. And even then.

Jack

OP Mandem Junior Member • Posts: 28
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly

Jack Hogan wrote:

Mandem wrote:

Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

In certain lighting conditions, independently of absolute radio/photometric levels, the whitest diffuse white possible shows up at 10000 DN in the raw data with the given camera settings. If an 18% gray card is inserted in the scene it will be around 1800 DN. Mixed clouds perhaps at 15000 DN.

So if max diffuse white is considered 100% diffuse reflectance at 10000 DN, it follows linearly that the clouds can be said to provide a mixed response equivalent to 150% of max diffuse reflectance.

I'll try my best to explain it from my pov. Bear with me.

If both the Max Diffuse White, Clouds and the Mirror are lighted by the same light source, say the sun, which is providing an X amount of light, how can their reflectance ever be greater than a 100% as

Reflectance= Reflected Light / Incident Light.

If it was greater than a 100% that would mean somehow, magically, more Lumens are coming out of the objects than are incident on them. Since these are only reflective objects and not light sources, they can only give out at a max amount of light of as much as they get hit with. So they can only reflect a 100% of light.

And yet looking at these 3 objects each of them have different intensities of light hitting the viewers eye. Despite the fact that they're all reflecting all the light that's hitting them. The 100% Max Diffuse is visibly white, the clouds white but certain spots even brighter than the Diffuse reflector, and the mirror blindlingly white and bright still. And yet they can all reflect only the maximum of what's incident on them otherwise that would mean they're magically producing extra light out of nowhere(we're not considering optical brighteners here and remember these are only reflective objects).

So what's happening? They can all reflect a maximum of a 100% light, yet the mirror is blindingly white but the 100% diffuse reflector is much less brighter but still white.Answer is Directionality of the light rays. While the 100% diffuse reflector is indeed reflecting all the light, it's all scattered everywhere, so not all the light rays incident on it are ACTUALLY reaching the sensor. Moving onto the clouds. A huge amount of it is diffusely reflective but a small portion of it due to the arrangement of water molecules is specularly reflecting much more of the light rays incident on it back at the camera sensor(which is why it looks more brighter than the diffuse reflector and the surrounding diffusely reflective clouds and yet any given portion of the cloud can only reflect a 100% of the light incident on it) Finally, the mirror. Positioned correctly, the camera will see the brightest light it will ever see in its lifetime in the mirror,THE SUN. And yet again, the mirror can only reflect a 100% of what is incident on it(just like the diffuse reflector) but this time the light doesn't get scattered everywhere, every single ray that hit the mirror reflected right to the camera. This is why the term luminance makes more sense instead of reflectance as you can have multiple things with a 100% reflectance(never over according to Alans formulas and intuition in general) yet some of them will be a visibly more intense/bright white.

J A C S
J A C S Forum Pro • Posts: 19,800
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly
1

I am following this casually only - my understanding is that 100 reflectance (for a diffuse surface) means that all falling light is reflected but in all directions. You are looking from some specific one. You see a small portion of all that light coming at you only. A mirror-like surface can direct it all to you.

OP Mandem Junior Member • Posts: 28
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly

J A C S wrote:

I am following this casually only - my understanding is that 100 reflectance (for a diffuse surface) means that all falling light is reflected but in all directions. You are looking from some specific one. You see a small portion of all that light coming at you only. A mirror-like surface can direct it all to you.

This is exactly the conclusion I've come to.

alanr0 Senior Member • Posts: 2,483
Luminance rather than reflectance
1

Mandem wrote:

Again this all comes back to trying to understand why there were values greater than a 100% on an x-axis called "reflectance" In that LutCALC Graph. It just didn't make any sense since, again, you can't get a value bigger than 1 on the formulas Alan posted above. Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

Once you accept that the percentage scale represents luminance (or relative exposure), rather than reflectance, you no longer need assume that the entire scene is lit by the same source.

The reason more-or-less logarithmic tone curves are used is because the real world very often diverges from this ideal, and human vision is pretty good at coping with this.

The most likely cause of wide dynamic range in an image is non-uniform illumination.  Either there are light sources in or near to the frame, or parts of the scene are in deep shadow.  A common example is where one wishes to show both the interior of a building and capture an exterior sunlit view through a window or doorway.

Specular reflections contribute too, but in practice we rarely need to reproduce their relative luminance with high accuracy, provided they appear sufficiently brighter than their surroundings.

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Alan Robinson

Jack Hogan Veteran Member • Posts: 7,873
Re: Diffuse = max 100%, Mixed and Specular > 100%, Linearly
1

Mandem wrote:

Jack Hogan wrote:

Mandem wrote:

Which is why the idea that "reflectance" actually refers to LUMINANCE is more fitting. When it says 18% reflectance or 90% reflectance or 100% reflectance in that LutCALC graph I believe it's referring to the Luminance of an 18%,90% or 100% diffuse reflector. The fact that it is LUMINANCE is also the reason why, I believe, the values can go above a 100% because certain reflectors(specular or mixed) or even incident light sources due to their directionality can be significantly more intense than a 100% diffuse reflector. Hope this made sense.

In certain lighting conditions, independently of absolute radio/photometric levels, the whitest diffuse white possible shows up at 10000 DN in the raw data with the given camera settings. If an 18% gray card is inserted in the scene it will be around 1800 DN. Mixed clouds perhaps at 15000 DN.

So if max diffuse white is considered 100% diffuse reflectance at 10000 DN, it follows linearly that the clouds can be said to provide a mixed response equivalent to 150% of max diffuse reflectance.

I'll try my best to explain it from my pov. Bear with me.

If both the Max Diffuse White, Clouds and the Mirror are lighted by the same light source, say the sun, which is providing an X amount of light, how can their reflectance ever be greater than a 100% as

Reflectance= Reflected Light / Incident Light.

If it was greater than a 100% that would mean somehow, magically, more Lumens are coming out of the objects than are incident on them. Since these are only reflective objects and not light sources, they can only give out at a max amount of light of as much as they get hit with. So they can only reflect a 100% of light.

And yet looking at these 3 objects each of them have different intensities of light hitting the viewers eye. Despite the fact that they're all reflecting all the light that's hitting them. The 100% Max Diffuse is visibly white, the clouds white but certain spots even brighter than the Diffuse reflector, and the mirror blindlingly white and bright still. And yet they can all reflect only the maximum of what's incident on them otherwise that would mean they're magically producing extra light out of nowhere(we're not considering optical brighteners here and remember these are only reflective objects).

So what's happening? They can all reflect a maximum of a 100% light, yet the mirror is blindingly white but the 100% diffuse reflector is much less brighter but still white.Answer is Directionality of the light rays. While the 100% diffuse reflector is indeed reflecting all the light, it's all scattered everywhere, so not all the light rays incident on it are ACTUALLY reaching the sensor. Moving onto the clouds. A huge amount of it is diffusely reflective but a small portion of it due to the arrangement of water molecules is specularly reflecting much more of the light rays incident on it back at the camera sensor(which is why it looks more brighter than the diffuse reflector and the surrounding diffusely reflective clouds and yet any given portion of the cloud can only reflect a 100% of the light incident on it) Finally, the mirror. Positioned correctly, the camera will see the brightest light it will ever see in its lifetime in the mirror,THE SUN. And yet again, the mirror can only reflect a 100% of what is incident on it(just like the diffuse reflector) but this time the light doesn't get scattered everywhere, every single ray that hit the mirror reflected right to the camera. This is why the term luminance makes more sense instead of reflectance as you can have multiple things with a 100% reflectance(never over according to Alans formulas and intuition in general) yet some of them will be a visibly more intense/bright white.

Got it, I should have read your exchange with Alan more carefully.

Yes, of course, in photography there is an explicit assumption that the terms refer to what the eye or the camera see, that's where the steradians come into radio/photometric units.

That's why for instance we never consider the overall amount of energy that the sun generates at any given time to estimate Exposure Values - but only the infinitesimal proportion that reaches our scene.

And the much tinier amount that is reflected into our eyes and cameras - everything is relative to that. Hence the 100% and 150% example above.

Jack

MrBrightSide
MrBrightSide Contributing Member • Posts: 996
You Need To Ask Video Experts

I would suggest that you are making a huge mistake by not asking this question in the video form or at cinematography.com where people who are much more familiar with video could weigh in.

I am following this casually only - my understanding is that 100 reflectance (for a diffuse surface) means that all falling light is reflected but in all directions. You are looking from some specific one. You see a small portion of all that light coming at you only. A mirror-like surface can direct it all to you.

This is exactly the conclusion I've come to.

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Sit!

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