Fifty Shades of Infrared

    I don’t know about you, but I was always wondering: what would the world look like if the RGB color channels in the human eye were sensitive to a different wavelength range? Rummaging through the guts, I found infrared flashlights (850 and 940nm), a set of IR filters (680-1050nm), a black and white digital camera (no filters at all), 3 lenses (4mm, 6mm and 50mm) designed for photography in IR light. Well, let's try to see.

    On a subject of IR photography with removal of the IR filter on a hub already wrote - this time we will have more opportunities. Also, photos with other wavelengths in the RGB channels (most often with the capture of the infrared region) can be seen in posts from Mars and about space in general.


    These are flashlights with IR diodes: 2 left at 850nm, right - at 940nm. The eye sees a faint glow at 840nm, the right - only in complete darkness. For the IR camera, they are dazzling. The eye seems to retain microscopic sensitivity to near IR + LED radiation comes with lower intensity and at shorter (= more visible) wavelengths. Naturally, with powerful IR LEDs you need to be careful - with luck you can quietly get a retina burn (like from IR lasers) - it only saves that the eye cannot focus the radiation to a point.

    Black and white 5-megapixel noname USB camera - on the Aptina Mt9p031 sensor. They shook the Chinese for a long time on the topic of black and white cameras - and one seller finally found what I needed. There are no filters in the camera at all - you can see from 350nm to ~ 1050nm.


    Lenses: this one is at 4mm, there is still at 6 and 50mm. At 4 and 6 mm - designed to work in the IR range - without this, for the IR range without refocusing, the images would be out of focus (the example will be lower with a conventional camera and IR radiation of 940 nm). It turned out that the C mount (and CS with a 5mm working distance) - we got another 16mm movie camera from the beginning of the century. Lenses are still being actively produced - but already for video surveillance systems, including by well-known companies like Tamron (a 4mm lens is just from them: 13FM04IR ).


    Filters: I again found among the Chinese a set of IR filters from 680 to 1050nm. However, the IR transmission test yielded unexpected results — it’s not like band-pass filters (as I imagined it), but a different “density” of color — which changes the minimum wavelength of transmitted light. Filters after 850nm turned out to be very dense, and require long exposures. IR-Cut filter - on the contrary, it passes only visible light, we will need it when shooting money.

    Filters in visible light:


    IR filters: red and green channels - in the light of a 940nm flashlight, blue - 850nm. IR-Cut filter - reflects IR radiation, therefore it has such a cheerful color.


    Let's start shooting

    Panorama in the afternoon in IR: red channel - with a filter at 1050nm, green - 850nm, blue - 760nm. We see that the trees reflect particularly well the very near IR. Colored clouds and colored spots on the ground - turned out due to the movement of clouds between frames. Individual frames were combined (if there could have been a random camera shift) and stitched into 1 color image in CCDStack2 - a program for processing astronomical photographs, where color photographs are often taken from several frames with different filters.




    Panorama at night: the difference in color of different light sources is visible: “energy-efficient” - blue, visible only in the nearest infrared. Incandescent lamps - white, shine in the entire range.




    Bookshelf: almost all ordinary objects are almost colorless in IR. Either black or white. Only some paints have a pronounced "blue" (short-wave IR - 760nm) shade. LCD screen of the game “Wait!” - in the IR range does not show anything (although it works on reflection).




    A cell phone with an AMOLED screen: absolutely nothing is visible on it in the IR, as well as the blue indicator LED on the stand. In the background - nothing is visible on the LCD screen either. The blue paint on the metro ticket is transparent in IR - and the antenna for the RFID chip inside the ticket is visible .




    At 400 degrees, the soldering iron and hairdryer glow quite brightly:


    Stars

    It is known that the sky is blue due to Rayleigh scattering - accordingly, in the infrared range it has a much lower brightness. Is it possible to see stars in the evening or even in the afternoon against the sky?

    Photo of the first star in the evening with a conventional camera:


    IR camera without filter:


    IR camera with IR filter: it looks like the signal-to-noise ratio is at least an order of magnitude better, with statistical processing of hundreds and thousands of frames it can be possible to find stars in the afternoon. Filming in IR can be a salvation for “urban astronomy” - parasitic illumination of the sky by the city is also much less.


    Another example of the first star in the background of the city:


    Money

    The first thing that comes to mind to verify the authenticity of money is UV radiation. However, banknotes have a lot of special elements that appear in the IR range, including those visible with the eye. We already briefly wrote about this at the hub - now let's see for yourself:

    1000 rubles with filters 760, 850 and 1050nm: only certain elements are printed with ink that absorbs IR radiation:


    5000 rubles:


    5000 rubles without filters, but with lighting with different wavelengths:
    red = 940nm , green - 850nm, blue - 625nm (= red light):


    However, infrared money tricks do not end there. There are anti-Stokes marks on the bills - when illuminated with 940nm IR light, they glow in the visible range. Photography with an ordinary camera - as you can see, IR light passes through the built-in IR-Cut filter a bit - but the lens is not optimized for IR - the image does not get into focus. Infrared light looks light purple because Bayer’s RGB filters are transparent to IR .


    Now, if we add an IR-Cut filter, we will see only glowing anti-Stokes tags. The element above "5000" - glows the brightest, it can be seen even with not bright room lighting and backlight 4W 940nm diode / flashlight. This element also has a red phosphor - it glows for several seconds after irradiation with white light (or IR-> green from the anti-Stokes phosphor of the same label).

    The element slightly to the right of "5000" is a phosphor that glows green for some time after exposure to white light (it does not require infrared radiation).


    Summary

    Money in the IR range turned out to be extremely tricky, and you can check it in the field not only with UV, but also with a 940nm IR flashlight. The results of shooting the sky in IR - give rise to hope for amateur astrophotography without traveling far beyond the city limits.

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