Fire Science Show

203 - The lessons from repeating Jin's experiment on visibility in smoke

• Wojciech Węgrzyński

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I've finally done it. We've repeated Jin's experiment! I thought I knew-it-all about that experiment, but boy... knowing and doing it are two different things. I can say, I've finally cleared my mind on some thoughts after this, which I am finally happy to share with all of you!

First things first, massive thanks to my partner in crime Wai-Kit Wilson Cheung, from the group of prof. Xinyan Huang, who was the man on the ground doing the experiments with me. Together we went further into this model, than ever before. 

The revelations are far-reaching. We found that Jin used extraordinary lighting conditions—180 lux background brightness and impossibly bright signage—far from realistic building emergency conditions. Background brightness emerges as perhaps the most critical factor in determining what can be seen through smoke, with dramatic differences between light-emitting and light-reflecting signs. Most significantly, the experiment's careful constraint of sign size (using proportionally larger signs at greater distances) created elegant mathematics but removed a crucial real-world variable from the model.

These insights have profound implications. Engineers likely overestimate visibility in many scenarios, particularly with reflective signage. The widely used K-values (3 for reflective signs, 8 for light-emitting signs) appear reasonably conservative for typical building conditions, though higher values might be warranted in darker environments. Most provocatively, simply increasing sign size would almost certainly improve evacuation safety, yet our current models provide no mechanism to quantify this benefit.

Fire safety practitioners will find this episode transformative, offering both practical guidance and theoretical understanding. Should we stick with visibility distance or shift to smoke density as our primary metric? How can we balance lighting conditions to optimize visibility of both obstacles and signage? And most critically, how might next-generation visibility models better serve real-world building safety? These are things we currently work on.

If you look for reading, check the paper on the extinction coefficient by the German colleagues:  https://arxiv.org/abs/2306.16182

If you strive for more podcast episodes:

The research was funded by the National Science Centre, Poland, based on a contract for the implementation and financing of a research project OPUS LAP No 2020/39/I/ST8/03159 and by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the project number 465392452, for the joint project: “Visibility Prediction Framework – a next-generation model for visibility in smoke in built environment”. 


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Wojciech Wegrzynski:

Hello everybody, welcome to the Fire Science Show. Today we are talking visibility in smoke and if you follow the podcast, you know that this topic is very dear to my heart. It is something that I research on my own and that has been a very important part of my scientific career thus far and probably will be for ongoing years. Many years ago I've done an episode with Lucas Arnold about visibility prediction framework. Our new back then grant that we were just starting at that point that we hope that will allow us to revolutionize the way how visibility is assessed in fire safety engineering. This project is actually ongoing, but I finally have some findings that I can share with you and that makes me super happy. Have some findings that I can share with you and that makes me super happy.

Wojciech Wegrzynski:

A year ago I have recorded an episode in the series I've called the experiments that changed fire science and in that episode I've covered experiments by Japanese scientist Jin, which are the basis of this model, and I said back then that Jin's model needs urgent repeat, that we really need to do it, and I knew we were going to repeat it because we've built the rig to do it, but shortly after that episode was published a very happy thing to me happened. I got a student from Hong Kong, Wai-Kit Wilson-Chung, from Hong Kong Polytechnic University, from Xinyan Huang's Wai-Kit, stayed with me for six months and he was really focused on doing those experiments. So, together with Waikid, we've actually redone the Jin's experiments. We've redone them, we've processed the data and today I am happy to show you some findings, because while I think I understood the Jin experiment and you can listen to that in the episode about Gene's experiments, why I think I knew what Gene done actually repeating those experiments, you know, doing that research on your own really opens your eyes on what is important in those experiments. And this is a broader reflection the papers don't tell you the full story. You need to talk to the scientists to tell you the full story. You need to talk to the scientist to really learn the full story what was hard, what was easy, where the challenges were, and I think we've narrowed down where the challenges of the visibility smoke model lie really, and this is what's gonna be said in this episode. I've presented this at the sfp, edinburgh. So finally, a time.

Wojciech Wegrzynski:

This reaches also my dear fire science show audience. So let's spin the intro and jump into the episode. Welcome to the fire science show. My name is vojtěj věkřínski and I will be your host. The FireSense show is into its third year of continued support from its sponsor of our Consultants, who are an independent, multi-award winning fire engineering consultancy with a reputation for delivering innovative safety-driven solutions. As the UK leading independent fire risk consultancy, ofar's globally established team have developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and the planet. Established in the UK in 2016 as a startup business by two highly experienced fire engineering consultants, the business continues to grow at a phenomenal rate, with offices across the country in eight locations, from Edinburgh to Bath, and plans for future expansions. If you're keen to find out more or join OFR Consultants during this exciting period of growth, visit their website at ofrconsultantscom. And now back to the episode.

Wojciech Wegrzynski:

So when I talk about visibility in smoke and recently I have a lot of chances to talk about this model I like to start with very difficult questions. Easy questions are always the most difficult to answer. What is visibility in smoke Like? What are we really talking about? Because, on one hand, you could consider it being a measure of a distance at which something can be observed. I think that would be the simplest, easiest definition of what visibility in smoke is. And this definition probably is right, but it has a lot of details in it, like what does it mean you can see something? Is it enough that you see a glimpse of light? Is it enough that you see a shape, a blurred out shape? Or you have to be able to process data that's shown on the thing that you're observing. How certain you have to be that the thing you observe is the thing that you observe, and those things change when the visibility changes.

Wojciech Wegrzynski:

If you're a physicist, then perhaps this is ill-defined for you. Perhaps it's a distance at which the light can still pass through obscuring medium. So if you point a ray of light through smoke, fog, whatever aerosol that disturbs it, the light will start to decay, it will scatter to the sides, it will be absorbed by the particles. So with every meter you will have less and less light passing through, and if you capture the point at which the light has decayed completely, that's the point where you cannot see the light anymore. So perhaps this distance is visibility in smoke In some way. This is how we measure it. This is how we measure it. In laboratories we have densitometer, optical densitometers which emit light, which measure light, and they measure how much light is lost in between the emitter and the target. So a physicist could define visibility as such a distance.

Wojciech Wegrzynski:

If you think about genes experiments the ones that brought us the visibility in smoke model that we use in fire science, visibility was actually the initial condition or actually the assumption of the study, the distance itself, the meters. So Jin built his facility in such a way that you could observe the science from either 5, 10, or 15 meters and in all honesty, we still have not found how he measured the 10 meter distance. But anyway, the rig is reported to be able to measure at 5, 10 and 15 meter distance. So it's discrete points in space rather than continuous spectrum. And in Gene's experiment the participant was placed at this distance. They were observing a light source through a smoke and they had a little widget that they could turn around and this widget would make the light dimmer and dimmer and they would just find the dimmest light they could see through the smoke and then Jin would save that data and use that in the further research. So in Jin's research the visibility is actually a fixed number. It's either 5, 10, or 15. There's no intermediate values for that. It's quite interesting when you think about it. If you're a firefighter, the visibility range would be something that's very natural to you, because that's the distance you can see in your fires, how far into the building, how far into the field you can see. So, perhaps closest to the first, most simple definition that I've brought up and allegedly, the observations of firefighters the distances they were saying are 10 meters is probably enough. Those were the background for creating the discrete values of visibility that we use today for engineering. But it does not mean the same thing for an engineer.

Wojciech Wegrzynski:

If you're practicing fire safety engineering, you are not really assessing the ability to observe things through smoke. You're not assessing light decay in smoke and you're definitely not assessing the critical brightness of a widget that you can observe through smoke. No, you are applying a very simple mathematical correlation. In one part of the correlation you put smoke density in. How much smoke do you have in your space? And this is something you know from your CFD analysis, from your zone model analysis. You know your soot yields. You know your yields of combustion. You can calculate how much soot gets emitted to your room. You know the flows in your room, you can calculate how much smoke is there in any given part of my building while performing fire safety engineering.

Wojciech Wegrzynski:

And when you do this fire safety engineering, you have to show those results to someone and you usually choose to present them as visibility in smoke. So I'll just give a quick recap. How does the smoke density turn into visibility in our modeling? Because it's a very simple correlation. You basically have a factor we call it large K usually and it takes values of 3 for light reflecting signs, 8 for light emitting signs I'll come back to this at the end of the episode and you basically subdivide it by your smoke density multiplied by specific extinction coefficient of smoke. The extinction coefficient is something we know from experimental work from Mulholland's and I'll also come back to this at the end of the episode.

Wojciech Wegrzynski:

The logmock density you know from your CFD, and so you have all the things that you need to calculate visibility. Well, does this mean that you will be able to see for 10 meters in your building if you've calculated 10 meters visibility? Of course not. It just means that if an entire space was filled with a smoke of density, like you have in this one point of space that you've just measured, then probably in the whole room you would have visibility distance of something like 10 meters. That's pretty much what it says, but it doesn't tell you anything about what you can see, what you cannot see. It doesn't tell you anything about where the light will stop in your room and it definitely does not tell you at what kind of brightness you can observe things in your compartment. It just allows you to translate value that's perhaps a little less understandable the density of smoke into a value that people can understand visibility. That's the trick that has been used as the backbone of fire safety engineering for five decades now, I guess, and I cannot say we've been pretty successful with it. It's just that it's so profound, so impactful in our engineering that I really find it not great that we have such a coarse approximation that we use for very significant engineering decisions every day.

Wojciech Wegrzynski:

So now let's talk a little bit about the story of repeating genes experiments, because it was a very interesting story. First we've done I thought I'm clever, you know I'm not gonna, you know use a widget to control the brightness. That just gives me one point of data. I am a clever scientist and I figured out a way how I can get much more data from the same experiment. So in my experiment, contrary to Jin's, I have a huge LED screens at the back of my experiment. Jin certainly did not have those in 1970s and instead of projecting one sign, I can project as many as I want and I can make them dimmer and dimmer, and dimmer and control this through my software. So when Gene was able to project one sign on his frosted glass, I am capable of projecting however much the hell I want. So I definitely did that.

Wojciech Wegrzynski:

I did that with my student Pavel, and we were trying to do those signs emitted on a screen of TV and replicate Gene's experiment. And in Gene's experiment there is this one way he's presenting results and it's called the dimensionless brightness. It's not very useful but it's kind of relevant, so I'll talk you through. So dimensionless brightness is that Gene took the brightness of the sign that he was projecting. He was measuring that with pretty complicated optical measurements that I cover in the previous podcast episode and he was subdividing that by the brightness of the background. So if the dimensionless brightness is above one, that means the sign was brighter than the background. If the dimensionless brightness is less than one, it means that the sign was darker than the background and the higher the value, the brighter the sign right. So with Paavo we start to calculating dimensionless brightness for our results and we very quickly see that we're nowhere close to Gene's results, like we are nowhere close to the range of dimensionless brightness that Gene's used.

Wojciech Wegrzynski:

Our TV is not bright enough, and that was the first shocker because I felt the TVs, the setup that we've built, was imitating the building conditions fairly well. I've been in a lot of buildings. I've been in a lot of buildings in fires, actually, because we're doing those fire tests in them. So I know how evacuation routes in a building in emergency lighting in your evacuation conditions should look like. I have a pretty good idea of that and I thought my tvs are fairly well representative of that environment, whereas now we see that we're nowhere close to jeans results. That was quite a shocker to us.

Wojciech Wegrzynski:

And then wiki chunk came and the first job I gave gave to Wilson was to build me a light emitting source that could match gins and boy, that was a journey. We've 3D printed the box and the box was lined up with light reflecting foil. Then we've put a bunch of LEDs into the box, like it took us like five iterations of adding more and more and more and more and more LEDs into the box. Like it took us like five iterations of adding more and more and more and more and more LEDs to the box until we've reached the box. That's like literally, you know, a lighthouse lantern you cannot look straight into the box because it blinds you for like 30 seconds. That's how bright it is. And now we match Jin's. And now we match the Jin's frosted glass brightness.

Wojciech Wegrzynski:

And this was like a shocker, but also an eye-opener. Like Jin has used extremely bright sources in their experiments, extremely bright sources, and therefore he could observe those signs through very, very dense smoke. If you look at Jin's experiments closely, you'll notice that the ranges of extinction coefficients that Jin is working with they're reaching up to two. This is unbelievably dense smoke. This is a smoke at which you will not see your hand if you stretch it in front of you. You probably will not even see your elbow in smoke of this density. It's unbelievably dense smoke and he was carrying observations and calculating those models in such dense smoke conditions. So, ah, this is already something that moves Jin's experiments away from the space of real buildings, real engineering. The sources are too bright, the smoke is too dense. It's not something that we commonly work as fire safety engineers and again, you can read the paper as many times as you like.

Wojciech Wegrzynski:

You can, you know, spend ages on studying what has been done in Japan in 1970s, but it really took us to repeat the experiment to very quickly realize what we are dealing with with those light sources. Very interesting, and therefore I find this podcast episode of real value, because previously I was speaking about what I think the problems with genes experiments are. Now I know what they are because I've run into them. But that's not everything that we've realized while repeating Jin's experiments. Actually, we've learned a lot more with Wilson, so let's try digesting that.

Wojciech Wegrzynski:

When Jin was doing his experiments, the profound finding in his study and I think that's the biggest discovery of Jin really was that the relationship between the extinction coefficient and the distance at which you can observe the sign is fairly constant across a range of sign sizes, distances, background brightness and just the brightness of the sign, which meant that he could collapse a lot of results into straight lines and just find a linear correlation between the variables, therefore creating the model that we are currently using. I'm not sure he was creating a model. Actually I think he was just looking for a relationship that allows his engineering to be done, whereas we turned it into the model. We, as a collective fire safety engineering community, we've started using this as a model to predict visibility. He was just representing the outcomes, I think, but anyway, in his experiments he was doing them with different range of lights with different sizes. As I said, he was also using dimensionless brightness of a sign to reduce the number of variables in the studies and he was able to actually collapse those results into one very elegant line and I think you can see this plot in sfp handbook. We'll see if it's there in the next edition of handbook which is released just in a few days. But he was basically able to simplify this a lot.

Wojciech Wegrzynski:

Now we've repeated this experiment as closely as we could. We really paid a lot of attention to repeat these experiments as closely, as accurately as we could, following whatever is being said in Jun's papers and our results, while they do keep this linear relationship between brightness and extinction coefficient at which you can observe the sign. So in some way we confirm that there is this close linear relationship. That's the basis of the model. That's, I think, a very good finding that we confirmed that. Besides that we have not been able to collapse them this elegantly as Jin did. So our results just do not collapse that easy. They do not collapse that perfectly, and we were looking into why would they not collapse that perfectly? And we were looking into why. Why would they not collapse that perfectly? And we started thinking it could be related to the background conditions, to the background brightness. We've used led strips in the in the room so we had a very nice uniform brightness across the room, jim using incandescent lights which he was turning on and off, so he definitely had non-uniform light distribution. But as we were looking into that, we realized that perhaps there's a bigger story to be told about the brightness which is not told by Jin and which I think the fire safety community needs to know and understand. So what I mean by that?

Wojciech Wegrzynski:

If you look at the perhaps most influential graph of all of them in Gene's papers, there's a graph that shows you on the y-axis, the dimensionless number which is smoke density multiplied by visibility. On the x-axis, the dimensionless brightness of your sign. And in this plot this plot basically, is the basis for the values of k3 and 8, really, that's the original of those values. And why it's important? Because we've noticed that in the top left-hand of the plot there's a note external light 180 lux. Gene has carried his experiments in extremely bright conditions. 180 lux is not something you normally have in your buildings in the evacuation phase. In Poland the minimum is one lux, of course, but in normal buildings like 100 is already a lot, really really a lot. 180 is a very, very, very bright room and you normally do not evacuate through spaces with such an immense brightness. And why I say there's a story to be told about brightness.

Wojciech Wegrzynski:

As soon as you go into the laboratory, as soon as you start repeating those experiments, as soon as you start playing with that, you immediately notice how impactful the background brightness is, how important is the brightness of the environment in which the evacuation takes place and how quickly it changes the ability to see or not see the evacuation signage. We had experiments in which you would set a brightness of a sign and at some external brightness you would not see even a glimpse of the sign. And if you tune the brightness down, you would see the sign perfectly. The same smoke, the same sign, just changing the background conditions. This is how big impact the brightness of the environment can have on visibility and actually it makes sense. It makes perfect sense. So if you have a scattering medium in your space, like smoke is, you can think about it. It's an averaging filter of light that's going through the room. Basically, all the light that passes through that scattering medium bounces off particles, let's say mixes. I'm not sure if it's a good word to be used about physics of light, but let's say that the light from different sources mixes and that's why your eyes cannot tell those lights apart, because they're mixed.

Wojciech Wegrzynski:

Now you will also see the brighter thing. If you have a very bright point next to not such a bright point, you of course see the brighter point first. So the sign has to be more bright than the background, and the bigger the difference between the background and the sign is, the easier it's going to be to observe the sign. I think that that's quite reasonable. Now your signs have a very specific light intensity that they emit. It's in their characteristic. Therefore, if your background is too bright, you cannot increase the brightness of your signs anymore. You will start losing visibility of those signs. And, trust me, if you go into the experiment. If you go into the laboratory, if you start observing those signs through the glass, and if you're ever in Warsaw, you're very welcome to come to my laboratory. I'll show it to you you will notice that the change in the background brightness perhaps has the most profound impact on the outcomes of the study.

Wojciech Wegrzynski:

And now, why it's important for fire safety engineering is because of that assumption of Gin's model 180 lux. We're not really having 180 lux. So therefore, with Wilson, we've tried to create this collapse of results for lower brightnesses to see, because Gin doesn't present a graph like that We've tried to collapse the results for different brightness conditions and for a brightness level of 22 lux or 1 lux I think very reasonable for normal evacuation conditions, we go with K values up to 11. That's much higher than Jin did and we generally observe very high k values for those conditions. Therefore, reassuring me, in my everyday practice I simply use this k value of 8 as the baseline in my engineering. I very rarely engineer for k value of 3. And I reassured myself that using high k values is actually more representative of built environment and that's one finding. Probably, if you do engineering on that and you go into third party audit, you probably should not quote a podcast on that, but I promise you there's a paper to be submitted very soon and as soon as the paper is published I'll put the link in the show notes and then you will have a peer-reviewed, credible source to quote. So I hope I give it to you, my fellow FISA engineer, so you have less troubles in your work. But anyway, to summarize, the brightness. Brightness is extremely impactful and it seems that higher K values correspond to brightness levels in buildings which are more in line of what we observe in everyday life. So that's one reassuring thing and really wow. We need to study it more because it's really interesting how much the background brightness changes the outcomes of the experiment. But this is not the only obvious thing that we knew about Jin's experiment that we did not put enough emphasis on previously.

Wojciech Wegrzynski:

The next one is the sign sizing. I've mentioned it already in the Jin's experiment podcast episode and I've actually re-listened to the episode and I found ah, I've talked about it but I did not really recognize the impact of that the sign signage. So, as I told you, Gene was observing signs from 5, 10, or 15 meters, but he somehow did not want the sign's size to influence the results of the study. Therefore he also used signs of different sizes. So when he was observing sign from 5 meters, the sign dimension was 5 centimeters. When he was observing it from 10 meters, it was 10 centimeters. When he was observing it from 15 meters, it was 15 centimeters. So the further he was, the bigger the sign was and now, as you can imagine, it has profound impact on the outcomes.

Wojciech Wegrzynski:

Profound, of course, in real building, if you are further away from the thing, you observe, the thing looks smaller. The smaller it is, the harder it is to observe it through obscuring mediums such as smoke. In jinn's experiment the signs were always the same size no matter what distance you observe them. So he was really looking into ability to see the light from the source and not really observe a real evacuation signage in a real evacuation scenario. And as soon as you go into the lab and you start observing those signs, you immediately see that. So our setup has multiple mirrors. You can observe the signs from different distances at the same time, really. So you can really narrow the time at which you see the sign at 10 meters. You see the sign at 5 meters, you cannot see it at 15. Or, even better. You see it perfectly in 5 meters. You almost lose the visibility of it at 10 meters and you cannot see it at all at 15. That's probably a better description. So yeah, in our case we're able to see those differences and we've took the measurements and it's going to be another paper that's in production with Wilson, which is how much does the size of the sign change the outcomes of the experiment? But it's just important to know that in Gene's experiment that was constrained and I think this has really considerable impact on our ability to engineer. I think this has really considerable impact on our ability to engineer.

Wojciech Wegrzynski:

I always felt it a lackluster that I cannot do any engineering with my science. Like, I go into a building, I do my CFD simulations, I do some smoke, I measure the visibility, of course, and I find the reason of visibility. Let's say, 9 meters. That's the outcome and my HAJ is very unhappy because it was supposed to be above 10 meters and I don't meet my tenability criterion. So I have to put a lot of extraction in that room to increase the my capability of removing smoke, to decrease the smoke density, to improve the visibility conditions, and then my building is considered safe because I finally have more than 10 meters visibility.

Wojciech Wegrzynski:

But if, instead of placing another fan, I could just put twice the size evacuation signage, would that help In real engineering scenario? It definitely would help. It would be a difference in fire safety engineering if I could just use a larger signs in my experiments. It would be a difference in fire safety engineering if I could just use a larger science in my experiments. It would make a big, big difference.

Wojciech Wegrzynski:

But with Jin's model it does not recognize the size of the sign. Therefore it's impossible to use that, as you know, your design variable and unfortunately I do not think there is an easy way how this could be implemented in Jin's method. The reason is that the results collapse into a line because the size is constrained. As soon as you remove the size constraint from those relationships, they do not collapse into a line anymore. And if there is an empirical relationship, it's going to be very complicated. So it's going to be extreme challenge for us to have a model that would include for the sign size in it. I'm not saying we're not trying, but it's very hard and I think it's a flow of genes model that we probably will not be able to go over.

Wojciech Wegrzynski:

However, what I want to say is that I think in many documents in which fire engineer has ability to use their knowledge for their benefits, it's fairly fair to say in my building, in order to improve the evacuation conditions, I have doubled the size of the evacuation signage, therefore making them more observable from larger distances, therefore making them more observable from larger distances, and just put a claim that the existing visibility in smoke model does not account for size of the signage. Therefore, using larger signage is conservative versus the normal approach. I think that that's highly justifiable. Even though you cannot quantitatively tell how big the difference is, it's obvious that the larger size will be easier visible from larger distances. And actually, the second thing is that if you ever use Jin's model to assess visibility from distances higher than, let's say, 10-15 meters, I would say 15 is already questionable. I would not use it. Like tenability distance 10 meters, yeah, okay, fine, we can use it. But if you are interested in visibilities of 30, 50, 100 meters, let's say you're designing a runway for an airport or you're designing a traffic system or something else, perhaps using larger values of visibility that are far away from Jin's experiment, that would mean that the signs observed are absurdly large, it makes no sense and I would not extrapolate into distance for sure.

Wojciech Wegrzynski:

And the third very challenging thing that we found about Jin's experiments and this is something I do not fully understand yet is how he has considered the light reflecting signs. So, as I mentioned before, you have this dimensionless brightness how much brighter or dimmer the sign is than your background and now you have your light reflecting signs. For light reflecting signs, by definition, your value cannot be larger than one, Like your sign cannot reflect more light than the background. And the dimensionless brightness, in my opinion, will be reflectance of the sign. That's it Like. It's impossible for the signage to have a higher value than the reflectance In smoke. It could even have a lower value because the light has to reach the sign and it's already obscured on its way from the background to the sign to be reflected. Therefore, this will be probably even lower than the reflectance. But in general you cannot have value larger than one and indeed in Gene's model he does not show any results for value higher than one, but it doesn't stop the line to cross that point, you know, and there are like extrapolations for values between one and two.

Wojciech Wegrzynski:

The second thing is, when you have those light reflecting signs. You really do start losing the ability to see them very quickly and this is something that we start having a little bit different results than gin, extremely scattered results, I would say. It's space in which, especially in darker conditions, it's really challenging to observe those light-reflecting signs. And we're also looking into photoluminescent signs, which are of interest to us because it's a common technology used in my country for almost all of your evacuation signage. So we start to see different results. Perhaps we also get results that could match Jin's observations, but in this case we had five observers, so it was us the researchers, but five different people, and the differences between us are profound in this regime. So it's very, very hard to create a good universal model for that. For the light-emitting science, for the very bright science, we found good agreement between us. It was actually much more reasonable to approximate among the population.

Wojciech Wegrzynski:

But for light reflecting science, this is like way, way, way more difficult than I thought it's going to be. It's a part of the paper that we really struggled to write. And also the ability to see those signs. You lose it at very low obscuration densities of your smoke and also you lose them at very low background lighting. There are background conditions that you don't even need smoke and you still cannot see the signs. So definitely this part of the model is probably way more complicated than a simple relationship brought by Jin and probably very difficult to use in real-world engineering.

Wojciech Wegrzynski:

What, for me, is even more challenging is that Jin has made a claim that you could use this approach to distinguish the presence of solid boundaries like walls, columns, doors etc. And many engineers practice it like that, and for me this is very questionable because the size will matter a lot. Like I mentioned before, the size of the sign will really influence the outcomes of the observation, and if we're talking about a column or a door, they are enormous compared to a little placard of an evacuation sign. Therefore, I struggle to say that you can use the same rule applied to both of those and have the same results as your outcome. The other thing is the brightness of the background plays a lot of role and in this case, the brighter the background, the better the visibility of the signage. So you can see we come into some sort of competing objective.

Wojciech Wegrzynski:

If you want your backlit evacuation signs to be best visible, you would like to have as dark environment as possible, because that allows you to see the backlit, the self-illuminated signs in the best way. If you want your reflecting signs to be visible, you probably want the brighter environment, as bright as possible. Only the brightest environment will allow you to see those. And if you want to look at your photoluminescence signs, you probably want as dark as possible, because any light will overshadow them. So it's a challenging engineering environment in which you have to make choices. Should my evacuation emergency lighting conditions be very bright or should they be dim? Is there the perfect point at which you get the most of both worlds? I'm not sure. Probably there is. Maybe between 30 and 60 looks. Probably somewhere around there you get the perfect point where you still have a great visibility of your emergency lighting and still sufficient visibility of your obstacles and reflecting signs.

Wojciech Wegrzynski:

Again, don't quote me on that. I still have to write this into a peer-reviewed paper and if it comes out of a journal, then you're very welcome to quote me on that. But it's just my feelings and it's really funny that you study a thing for so many years. I studied visibility for at least 10 years. Now. I think I've started studying visibility in 2015, which would make it 10 years of research and I've read Jin's papers countless number of times and yet it took me to repeat the experiments with Wilson to really understand the challenges in that model. So to recap that, because I'm babbling for a long time already and I'm not sure if you're getting any value out of that, recap Brightness, background by brightness.

Wojciech Wegrzynski:

This is a critical condition in assessing visibility in your buildings. Gin, the relationship that you know and that you use in your engineering, assumed 180 lux in the background, which is enormously high, and usually in your building you would have less and that also means that the K factors in darker buildings are larger. So you probably are on the safe side if you're already engineering for K values around 8. You probably could even increase that number if you know your background conditions and we are working very hard to provide you a quotable item on how much you can increase that and what space you can play with. But it seems reasonable that our buildings are a little darker than Jin's experiments and the visibility in those buildings should be a little better than what's predicted. That's good news. Second news is that the sign size was constrained in Jin's model, which makes the model not as directly useful as we hope it to be. We are looking for a solution, which I do not have right now, but I think it's highly justifiable to use larger signs in your fire safety engineering as a measure to increase safety in your buildings. I am 100% convinced that if you use larger evacuation signage, you improve safety of your buildings.

Wojciech Wegrzynski:

The third thing is the reflectance of the light reflecting signs. This is something that's going to dictate their performance and the performance is lost very quickly in almost any smoke, probably even worse than in Jin's model. So beware, those signs in smoke will not really work well and you, if you have conditions in which you have smoke on your evacuation routes, you really should use the backlit signs, because those are the ones that will be visible in smoke conditions. I've also mentioned Mulholland's specific smoke extinction coefficient. There's a default value of 8.7 square meters per gram, I believe, and this value is being default used in FDS and my colleagues from Germany, the ones that I'm doing project with Christine Berger, professor Lukas Arnold, alexander Belt and Christoph Gnendiga and Tolsten Schutze. Together they found some new ways to measure extinction coefficients from aerosols that are very interesting. I'll link the paper in the show notes. They show largely different values than the ones reported by Mulholland, so it's also something we need to take in mind.

Wojciech Wegrzynski:

So where do we go ahead with this in fire safety engineering? One thing that is obvious is that the existing model whether it's wrong or not, it definitely allowed us to engineer buildings that would be considered safe, and I do not question that I've been asked. So what do we do now? Do we cancel the visibility in smoke model? No, we cannot cancel it. We use it in like 93% of our projects as the tenability criterion. It's too impactful to be canceled. We need to use it, but we need to find a smart way to use it better. Therefore, I still think it's a useful tool In my engineering.

Wojciech Wegrzynski:

A long time ago, I've moved away from visibility in smoke and I started using smoke density as the measure of my tenability using smoke density as the measure of my tenability, and I like this approach much better because it just works on raw data from CFD that I get smoke densities what I do get from my CFD. Therefore, I like to work with smoke density directly. But I think at this point the model while I feel it's a bit wrong in the spaces that I've described in this episode it plays an important role in modern engineering and still can be used. I'm really looking into possibility to modify it. Perhaps due to the sign problem it's not not possible to just simply, you know, find a new k value and be done with it. Perhaps there is more engineering to be done to have the model work. But if there is a way we will find one. And we're also working on a new generation of visibility in smoke models. Our friends at Wuppertal are having great progress in numerical modeling smoke and optical properties of smoke. I'm super excited for the work from Lucas Arnold's group on this and we're working together to find new relationships, new models and new ways to engineer fire safe buildings with visibility in mind, and visibility understood as ability to see objects through smoke. So that's for the future.

Wojciech Wegrzynski:

Think I will stop here. That would be it for today's fire science show episode. Thank you for being here with me again listening to my rambling about visibility in smoke. I hope this time I've made it remotely useful to you and brought you some opinions of my own that will help you guide your engineering. Let's call them design considerations I love how FSRI calls their recommendations to firefighters and the considerations. So I gave you some visibility in smoke considerations that, whatever you do with them, it's yours, but I hope they're useful in your engineering. Thanks for being here with me in the Fast Science Show and I'm looking forward to see you here next Wednesday. Cheers, bye, bye.