# The Visual Light part of the EM Spectrum **URL:** \ **Category:** Factual Questions\ **Created:** [August 6, 2006, 3:39am UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527 "2006-08-06T03:39:14Z")\ **Posts on this page:** 8\ **Page:** 3
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [August 13, 2006, 2:54am UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/41 "2006-08-13T02:54:08Z")
[QUOTE=scr4] [li]Infrared is difficult because any object near body temperature emits infrared. You can’t see anything if your eyeball is glowing.[/li][/QUOTE] I’ve often reflected on how useful infrared vision would be – e.g., being able to gauge something’s temperature by looking at it (no more burning your fingers on a hot surface you didn’t know was hot). I guess that’s why it wouldn’t work. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 13, 2006, 6:27am UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/42 "2006-08-13T06:27:35Z")
[QUOTE=BrainGlutton] [Re: **scr4** ’s comment on warm eyes themselves emitting infrared and therefore being useless as detectors] I’ve often reflected on how useful infrared vision would be – e.g., being able to gauge something’s temperature by looking at it (no more burning your fingers on a hot surface you didn’t know was hot). I guess that’s why it wouldn’t work. [/QUOTE] Depends on the wavelengths/frequencies/colours of infrared emitted, doesn’t it? What is the wavelength of the peak IR emitted by human skin? Is it anywhere near these near-IR wavelengths (880, 900, 950 nanometres) that we are talking about? When I was in architecture school back in the stone age, we had a demonstration of an infrared video camera (very expensive at the time, IIRC). It was used for viewing heat leakage from buildings (easiest on cold days). We aimed it at people and put the result on a monitor. It was very odd: people would have different heat patterns on their skin depending on what they were doing; if they were embarassed, they just lit up under the arms, on the face, on the breasts… very embarassing for one poor girl. Unfortunately, I don’t recall what wavelengths the camera was sensitive to. [This](http://us.fluke.com/usen/products/specifications.htm?cs_id=35657(FlukeProducts)&category=THG(FlukeProducts)) infrared camera used for checking buildings is sensitive to IR of wavelength 8 µm to 14 µm (8000 nm to 14000 nm), far longer than the ones we are looking at. Wikipedia says of [blackbody radiation](http://en.wikipedia.org/wiki/Black_body): > [@](#): > > …black-body laws can also be applied to things not in radiative equilibrium. A great deal of a person’s energy is radiated away in the form of electromagnetic radiation. Most of that radiation is in the infrared. > > In addition to emitting energy, humans absorb energy from the surrounding environment. The net power (energy/second) of energy radiated away is the difference between what someone absorbs and what they radiate… > > People have an area of about 2 square meters, and emissivity of nearly 1. They also have a skin temperature of about 32 °C (305 K). But clothing reduces the surface temperature a few degrees, so in addition to reducing heat loss through conduction, it reduces loss of heat by radiation. So for surface temperature of people we should use 301 K. The temperature of the surrounding environment varies, but for a rough order of magnitude answer, one can use 20 °C (293 K). Plugging in these values results in a net rate emission of energy for people of about… 95 watts. > > In this scenario, people are roughly 100 watt light bulbs, except they emit all infrared and longer wavelength light. The amount of energy in a whole day turns out to be almost 9 million joules, or 2,000 (food) calories. Normal rate of metabolism is typically 100-120 watts, and a person losing more than 160 watts (with extra losses by evaporation, convection and conduction) would feel cold and need to increase activity or cover with clothes. In contrast, during physical activity the metabolism is much higher and since the emission is not large enough, the excess heat is carried by sweating. > > Also, applying Wein’s Law to humans, one finds that the peak wavelength of light emitted by a person is… 9500nm. Much longer than the wavelenths the goggles pass and our eyes are sensitive to, which appear to be shorter than 900 nm. Incidentally, if you google this, you get all kinds of articles on medical imaging in the infrared. What are they _doing_? ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 13, 2006, 6:44am UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/43 "2006-08-13T06:44:16Z")
[QUOTE=commasense] Yes, pretty much. [/quote] I started to really wonder about that. I can see it working for monochrome IR photographs at whatever wavelength, but it ocurred to me that those pictures as displayed on the monitor _cannot_ look like the infrared reality, because my monitor does not have infrared pixels! On the other hand, if the IR picture is using the tail end of my red sensitivity, maybe it would look the same. > [@commasense](#): > > The IR focus point is closer than the visible. I noticed that in a shot in which I had set the focus to about 6 feet, the background (at infinity) was in sharp focus. So keep the focus set close in. (Back in the old days, I remember seeing lenses that had IR marks on them, for use with IR film.) I vaguely-remember seeing those as well. > [@commasense](#): > > The filters are so dark that you can’t see anything through either the digital or optical viewfinders. So shooting requires long exposures and use of a tripod. Most of the shots I took were about 3 or 4 seconds (at 200 ASA). I just pointed and snapped. After the exposure I looked at the image on the LCD and made any necessary adjustments to the framing. (You could also, of course, just remove the filter for framing, and put it back for the exposure.) So, focus for visible, tweak it back a bit for IR, slap the filter on, then shoot? 🙂 > [@commasense](#): > > I’m not sure I follow you. Do you mean a gray filter for one eye and IR for the other? I was thinking that this would be an interesting idea, but the goggles I got were the the one-piece rectangular type, not the round ones. I was wondering what would happen if, instead of blocking all the light above the infrared, we let through just a trickle, so that the eye was evenly sensitive across the whole spectrum from infrared to UV. This would require filters whose blocking curve with respect to frequency was the inverse of the eyes’ sensitivity curve: the more easily the eye picked up the frequency, the more of it would be blocked. That way, the regular light wouldn’t swamp the infrared at the lower end of the sensitivity curve. And maybe we could see some new colour mixing… And I’d like to set up some filters that would let us use some of that residual sensitivity in the UV near 300 nm as well. But that would require… precautions. > [@commasense](#): > > As I mentioned before, with these filters it takes a **lot** of light to see anything. But it would be interesting to try. Let us know your results. ---
**Author:** ![Ranchoth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ranchoth/32/1066_2.png) [@Ranchoth](https://boards.straightdope.com/u/Ranchoth)\ **Post date:** [August 13, 2006, 10:26am UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/44 "2006-08-13T10:26:00Z")
By the by, there’s been some suggestion that [some of Monet’s later paintings](http://en.wikipedia.org/wiki/Claude_Monet) featured images from the UV spectrum, which he may have been able to see after cataract surgery. ---
**Author:** ![flex727](https://avatars.discourse-cdn.com/v4/letter/f/258eb7/32.png) [@flex727](https://boards.straightdope.com/u/flex727)\ **Post date:** [August 13, 2006, 1:45pm UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/45 "2006-08-13T13:45:17Z")
[QUOTE=Sunspace] When I was in architecture school back in the stone age, we had a demonstration of an infrared video camera (very expensive at the time, IIRC). It was used for viewing heat leakage from buildings (easiest on cold days). We aimed it at people and put the result on a monitor. It was very odd: people would have different heat patterns on their skin depending on what they were doing; if they were embarassed, they just lit up under the arms, on the face, on the breasts… very embarassing for one poor girl. Unfortunately, I don’t recall what wavelengths the camera was sensitive to. [This](http://us.fluke.com/usen/products/specifications.htm?cs_id=35657(FlukeProducts)&category=THG(FlukeProducts)) infrared camera used for checking buildings is sensitive to IR of wavelength 8 µm to 14 µm (8000 nm to 14000 nm), far longer than the ones we are looking at. [/QUOTE] In a previous job, I had access to a 10um video camera as well and used to observe the same phenomenon. One thing I found particularly interesting was when the camera was pointed at an old sheet of galvanized aluminum, it appeared as a perfect mirror. Mirror properties are a function of the smoothness of the surface as compared to the wavelength of the light. At 10um wavelength, that surface can be pretty rough and still provide a perfect reflection. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 13, 2006, 6:09pm UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/46 "2006-08-13T18:09:33Z")
[QUOTE=flex727] In a previous job, I had access to a 10um video camera as well and used to observe the same phenomenon. One thing I found particularly interesting was when the camera was pointed at an old sheet of galvanized aluminum, it appeared as a perfect mirror. Mirror properties are a function of the smoothness of the surface as compared to the wavelength of the light. At 10um wavelength, that surface can be pretty rough and still provide a perfect reflection. [/QUOTE] Cool. That’s something that never would have occurred to me for infrared, even though I know about the same phenomenon with radio (satellite dishes have to be mirrors at radio wavelengths, but this means that the large old ones can be made of mesh). Now I want to get one of these IR cameras. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 13, 2006, 10:24pm UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/47 "2006-08-13T22:24:54Z")
> [@](#): > > It was very odd: people would have different heat patterns on their skin depending on what they were doing; if they were embarassed, they just lit up under the arms, on the face, on the breasts… very embarassing for one poor girl. Especially if she was wearing the wrong outfit. Some synthetic fibers (I think that rayon is one of them, for instance) are completely transparent in the near IR. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [August 13, 2006, 11:26pm UTC](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527/48 "2006-08-13T23:26:44Z")
[QUOTE=Chronos] Especially if she was wearing the wrong outfit. Some synthetic fibers (I think that rayon is one of them, for instance) are completely transparent in the near IR. [/QUOTE] So all those people with _X-ray_ glasses were barking up the wrong tree, so to speak? 😃 [Previous page](https://boards.straightdope.com/t/the-visual-light-part-of-the-em-spectrum/367527.md?page=2)