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I'd like to verify if my concluded statements regarding different emissivities are correct or not. And if not, why.

1

Two objects, one with an emissivity of 1 (object A) and one with a very low emissivity (object B), sitting in a room, everything at the same temperature T. Object A has the same T because it’s absorbing and emitting an equal high energy. Object B has the same T because it’s absorbing and emitting an equal low energy. Object A is therefore emitting more energy than object B. However, because object B has a low emissivity, it reflects the rest of the radiation coming from the surroundings. This means that the energy that object B is emitting + reflecting is equal to the energy that object A is emitting. A detector would therefore (falsely) measure that they both are giving off an equal amount of energy.

2

Now objects A and B have the same T but are in a room that is hotter (thus not thermal equilibrium). Object A would therefore emit less energy than it absorbs. Object B would do the same albeit the energy that object B absorbs and emits is less than that of A. Object A would therefore emit more energy than object B. However, object B directly reflects the non-absorbed radiation coming from the hotter surrounding. A detector would therefore (falsely) show that object B is giving off more energy than object A.

3

Now objects A and B have the same T but in a colder room. Object A is emitting more energy than it absorbs. Object B is doing the same albeit the energy that it absorbs and emits is lower than that of object A. Object A is therefore emitting more energy than object B. Since B is a good reflector, it also reflects the low energy (that B didn’t absorb) coming from the colder surrounding. Therefore, B is emitting a low energy based on its T + reflecting a low energy from the cold surrounding and A is emitting a high energy based on the same T but doesn’t reflect energy. This doesn’t necessarily mean that B is giving off more energy than A according to a detector and depends on the emissivity of B and on the emissivity and temperature of the surroundings.

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I have tested case 3 with a silver dollar (object B) and a hand (object A), both at 98.6F, in a 72F room, in front of a FLIR. The sliver dollar was invisible: it looked just like the background, while the hand appeared hot. Some tweaking proved that the silver was acting like a mirror--it was just reflecting the image of something else at 72F into the FLIR.

Whether the FLIR was falsely reporting the temperature of the silver, or whether it was correctly reporting the temperature of the image reflected by the silver may just be a matter of taste.

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  • $\begingroup$ Thanks for trying this out. I think your finding might be the cause of one or a combination of the following; 1) the silver dollar has an extremely low emissivity compared to the surrounding radiation 2) the surroundings that emit radiation has an emissivity of a bit less than 1. In both of these cases this would give a total energy from the dollar that is similar to the energy that it reflects from the surroundings. Also, does your FLIR has an option to set the emissivity? And final question: are my first 2 statements correct or not? $\endgroup$ – JohnnyGui Jun 22 '17 at 22:55

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