# Do materials exist that absorb Visible light and emit Near InfraRed light?

I am thinking about the possibility of making a hat or other clothing item that might show up funny on cameras, as they are more sensitive to infrared light than human eyes. Therefore, I was wondering if there are any (non hazardous) materials that emit InfraRed light when they are exposed to visible light.

Do these exist, and would they be suited for this kind of application?

## 3 Answers

NIR-emitting long-persistent luminescent nanoparticles have been prepared from $\ce{Cr(III)}$- and $\ce{Pr(III)}$-doped gallogermanates. According to the authors, these materials exhibit very long afterglows up to 360h (= 15 days).

Source: DOI1, DOI2

• interesting! Is this a case of NIR phosphorescence? – G M Apr 2 '14 at 19:16
• @GM Yes! Isn't ist amazing? For the answer to this question, I I thought in commercially available organic fluorescent dyes (Cy7 Cy7.5, and the long-wave Alexa dyes) first, but their Stokes shifts are in the 20-30 nm range only. This inorganic stuff is weird :) – Klaus-Dieter Warzecha Apr 2 '14 at 19:22
• I've worked in NIR luminescence imaging a little bit, if I find where I've the file I'll add an image of $CdS$ luminescence but I've never seen NIR phosphorescence! Thanks for the articles! – G M Apr 2 '14 at 19:30

CCD cameras typically have an IR-absorbing filter. Short wavelength scattering (varies as inverse fourth power of wavelength) renders white cloth opaque, cellulose versus air refractive index. If the scattering is quenched by wetting or avoided by IR-viewing, transparency results. The champion visible-absorber, IR-fluorescent mateiral is chlorophyll. Military camouflage extends into the IR.

IR-fluorescent dyes are common as biochemistry tags,

• +1 I like those dyes, however the scattering in this case doesn't matter because the light is absorbed in the visible, NIR transparent materials are still okay. – G M Apr 2 '14 at 19:13

Yes, there are some minerals that shows VIS-NIR luminescence here a couple:

• Synthetic ruby $\ce{Al2O3:Cr}$
• Kyanite ($\ce{Al2SiO5}$)

A more sophisticated material is $\ce{LiSrPO4:Eu2+, Pr3+}$ you can read the full article in 11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3161.

There are also some pigments like egyptian blue or cadmium yellow (CI pigment yellow 37, that is cadmium sulfide) that were also used in the past. Egyptian blue (calcium copper silicate ($\ce{CaCuSi4O10}$)) is safe, other pigments with cadmium are more toxic.

One of the most spread pigment shows VIS-NIR luminescence: Chlorophyll!

Have a look to this site for a nice overview.

All the luminesce processes have not a high quantum yield, so to record them, most of the times, you have to avoid reflection. Tungsten, halogen lamps and sun emit a lot of NIR radiation so is possible that reflected light overhang the luminescence. When I performed NIR luminescence I used VIS LEDs and a long pass filter at 850 nm. If you don't have result, first thing you have to try is to remove the ICF (IR-Cut off-Filter) filter from your camera so NIR light can reach the sensor more easily.