Invisible Fire

[GaryR]

While burning some yard material in a fire pit, I was curious as to what UV and IR captures would look like, using a B+W 403 filter. For UV, I stacked the 403 with a 2.5mm BG39, and for the IR captures, I simply removed the BG39, to make use of the major IR leak in the 403. The IR captures are green, because CWB was set for the UV stack.

 

The first shot with the BG39 removed, shows a steady fire.

 

In #2, with the 403/BG39 stack, smoke is visible, but the flames appear to be nonexistent, to the point where you can see details in the blazing fire pit.

 

Taken seconds apart, lots of flames showing in #3, with no signs of fire in #4.

 

In #6, details in and through the fire pit can be seen, even though a steady blaze is shown in #5.

 

What I find fascinating is seeing no indication of flame or fire on the a7R's LCD screen, with the ability to see through the fire, while visually observing a blazing fire in real-time.

 

Sony a7R FS-conversion, Nikkor 50mm/f1.8 Ai lens, CWB with 403+BG39

 

#1 - 403 filter, f/1.8 for 1/400s @ ISO-400

 

#2 - 403+BG39 stack, f/1.8 for 1/8s @ ISO-400

 

#3 - 403 filter, f/1.8 for 1/640s @ ISO-400

 

#4 - 403+BG39 stack, f/1.8 for 1/10s @ ISO-400

 

#5 - 403 filter, f/1.8 for 1/500s @ ISO-400

 

#6 - 403+BG39 stack, f/1.8 for 1/8s @ ISO-400

[Andy Perrin]

Huh, very interesting. I did think flames put out some UV, but perhaps it depends on what you are burning and how much UV background light there is relative to the light put out by the flames. In this case I can clearly see the horizon, indicating there is quite a bit of ambient UV to compete with what the fire is producing.

[GaryR]

Andy,

You may be right about ambient sunlight, overpowering the UV output of the flames. This same stack is able to capture a candle flame in a darkened room, but strange to see flames cancelled out completely in the outdoor shots.

[Andy Perrin]

Yeah, if you do this again at night, it would be interesting to see if it changes the situation.

[Cadmium]

Quite interesting. Andy is right, try this at night, and note exposure times.

[GaryR]

Yup, some night fire pit shots are on my 'to do' list. I'm expecting long exposures for UV, and short exposures for IR.

[GaryR]

Same setup as above, shot at night.

As expected the 403/BG39 stack resulted in diminished flames, some hot spots, with smoke and visible details within the flames.

 

What I gather from this experiment, is that hotter parts of the flames emit some UV wavelengths, but largely dependent on how hot a particular branch or brush ignites,

 

I suppose if I threw a piece of magnesium into the fire, I'd get increased UV wavelengths, but as it is, wood and dry brush doesn't generate a lot of UV. As Andy mentioned, the flames were likely cancelled out, by the ambient sunlight UV in the previous shots.

 

#1 - 403 filter, f/4 for 1/10s @ ISO-400

 

#2 - 403+BG39 stack, f/4 for 30s @ ISO-400

 

#3 - 403 filter, f/4 for 1/100s @ ISO-400

 

#4 - 403+BG39 stack, f/4 for 10s @ ISO-400

 

#5 - 403 filter, f/4 for 1/400s @ ISO-400

 

#6 - 403+BG39 stack, f/4 for 8s @ ISO-400

 

#7 - 403 filter, f/4 for 1/125s @ ISO-400

 

#8 - 403+BG39 stack, f/4 for 30s @ ISO-400

 

bonus shot

no filter, f/4 for 20s @ ISO-400, CWB setting, processed with tungsten setting

[Andy Perrin]

Yeah, it all makes sense.

[dabateman]

Gary,

Magnesium will not help you. Not many lines and your fire is already going. Would be better to place a cast iron pot or kettle in the fire. Many more UV lines come off Iron.

[Cadmium]

For what it's worth, back in December of 2017 there was a topic about UV candle light.

http://www.ultraviol...age__hl__candle

 

[GaryR]

Gary,

Magnesium will not help you. Not many lines and your fire is already going. Would be better to place a cast iron pot or kettle in the fire. Many more UV lines come off Iron.

 

David, Magnesium burns at much higher temps than a wood fire. I definitely don't intend to try it, but wouldn't that that generate more UV, than a common backyard fire?

[GaryR]

For what it's worth, back in December of 2017 there was a topic about UV candle light.

http://www.ultraviol...age__hl__candle

 

 

Thanks Steve, Interesting topic!

[Andy Perrin]

David, Magnesium burns at much higher temps than a wood fire. I definitely don't intend to try it, but wouldn't that that generate more UV, than a common backyard fire?

It would, because the magnesium is solid so it will emit a blackbody spectrum (which emits more UV as you go to higher temps).

 

David, you only see a line spectrum for gases and plasmas, not liquids or solids. While some of the magnesium might vaporize and give a few lines, I think most of the UV output will be from blackbody.

[dabateman]

I guess it depends on the amount of magnesium you have. I have a survival bar. You scrape off small fragments to place on wet wood, than ignite it. It burns quickly, generating lots of heat to catch the wet wood. But will not stick around long. Burns up quite fast.

Now if you threw a large bar into the fire, than maybe.

Iron will stick around longer going to black body. Especially if you have a cast iron kettle. The problem with that though is they rust inside like mine has. But thats a different problem.

[GaryR]

I guess it depends on the amount of magnesium you have. I have a survival bar. You scrape off small fragments to place on wet wood, than ignite it. It burns quickly, generating lots of heat to catch the wet wood. But will not stick around long. Burns up quite fast.

Now if you threw a large bar into the fire, than maybe.

Iron will stick around longer going to black body. Especially if you have a cast iron kettle. The problem with that though is they rust inside like mine has. But thats a different problem.

 

Yup, I have one of those survival bars too. I agree that the energy released from small amount of magnesium doesn't last long, but in that brief moment, a lot more UV wavelengths are emitted, than a wood fire. I'm afraid my wife would never forgive me, if I made her use a cast iron kettle. :D

[Andy Perrin]

I was imagining the classic chemistry experiment where you take a strip of magnesium metal and light it. Lasts for 30 seconds or so (depending how much you use of course). My chemistry teacher in high school was doing this demo, and when the magnesium went out, he tossed the hot tongs he had been holding it with into the sink. Big mistake. The class before had apparently poured some organic solvents down the drain, and the resulting flames went nearly to the ceiling and took my teacher’s eyebrows with them.

 

It made a strong impression on me, especially since I was an eager beaver chem student sitting in the front row, right in front of the sink.