Can there be black light?
Questions about light that is black (as opposed to the misnomer blacklight) are frequent enough, some duplicates come here. Examples: Is it possible to project black light? or Why does projecting black light from a screen mask white light shining through it?
I'm going to focus on this question as it is written.
Light has an intensity or energy and frequency or wavelength, absence of those would result in black and an absence of light (not "black light"). When we see darkness it's because the intensity is very low. Human eyes consist of rods, cones, and ipRGCs; it is the rods that are responsible for non color vision under very dark conditions.
In the complete absence of light, above absolute zero, objects still have black-body radiation, much of which is outside the spectrum (frequencies) of human vision, a black-body outputs a curve of intensities, at a range of frequencies, based on its temperature.
I mean is it possible to devise a machine that outputs darkness?
Using a modern version of lamp black I introduce you to: SWNT forests.
See: "A black body absorber from vertically aligned single-walled carbon nanotubes" (October 19, 2008) by Kohei Mizunoa, Juntaro Ishiib, Hideo Kishidac, Yuhei Hayamizua, Satoshi Yasudaa, Don N. Futabaa, Motoo Yumuraa,
and Kenji Hata.
"Among all known materials, we found that a forest of vertically
aligned single-walled carbon nanotubes behaves most similarly to
a black body, a theoretical material that absorbs all incident light.
A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition.
Here, we report that among all known materials, a forest of vertically aligned single-walled carbon nanotubes (SWNTs) behaves most similarly to a black body. Specifically, from optical studies, we revealed that a SWNT forest possesses a nearly constant and near-unity emissivity (absorptivity) of 0.98–0.99 across a wide spectral range from UV (200 nm) to far infrared
(200 $\mu$m). We speculate that this important black body behavior originates from the homogeneous sparseness and alignment of the SWNTs within the forest.
Microscopic structure of SWNT forest. (A) SWNT forest grown on an
8-in silicon wafer. (B) SEM image of SWNT forest vertically standing on a silicon substrate. (Scale bar, 0.5 mm.) (C) SEM image showing top surface of SWNT forest. (Scale bar, 0.5$\mu$m.) (D) SEM image showing side surface of SWNT forest. (Scale bar, 5 $\mu$m.)
Reflectance and transmittance spectra of SWNT forest. (A) Reflectance in the UV-to-near IR region (spectral range of 0.2–2.0 $\mu$m). The Inset illustrates the configuration of the reflectance measurements in the UV-to-near IR and the mid-to-far IR regions. (B) Reflectance in the near-to-mid IR region (2–20 $\mu$m). (C) Reflectance in the mid-to-far IR region (25–200 $\mu$m, red line) and transmittance of the substrate (black) and forest + substrate (blue).
Relationship between incident light and SWNT forest. (A and B)
Schematic diagrams illustrating the interaction between incident light and
SWNT forest (A) and individual SWNT (B). RI and R denote refractive index and reflectance, respectively. (C) Specular reflectance as a function of incident angle. Measurement with nonpolarized light with the wavelength $\approx$ 5 $\mu$m.
In conclusion, we have shown that the emissivity was 0.98–0.99
over a spectral range of 5–12 $\mu$m and the reflectance was 0.01–0.02 over a 0.2- to 200-$\mu$m range. These results highlight that a vertically aligned SWNT forest is a material most similar to a black body. This black body behavior originates from the unique structure of the forest as an assembly of nanotubes both sparsely distributed and vertically aligned.
Synthesis of SWNT Forest. Vertically aligned SWNTs (forests) were synthesized by water-assisted CVD ‘‘SuperGrowth’’ on silicon substrates at 750 °C with ethylene as a carbon source and water as a catalyst enhancer and preserver.
Emissivity Analysis. Normal spectral emissivity was evaluated in a home-built FT-IR spectrometer equipped with a Michelson interferometer and a photovoltaic HgCdTe detector at the National Metrology Institute of Japan. All optical components, the SWNT forest sample, and 2 reference black body furnaces (set at -20 °C and 100 °C) were placed in vacuum to reduce absorption by air, and the estimated standard relative uncertainty was $\lt$ 1%.
Reflectance and Transmittance Analyses. Optical reflectance of the SWNT forests was evaluated by 3 independent optical systems [see also Table S1]. For the UV-to-near IR (0.2–2 $\mu$m)/near-to-mid IR (2–20 $\mu$m) spectral ranges, a forest sample was set at the inner periphery of an integrating sphere (Fig. 4A Inset), and the light was incident near-normal to the sample with grating monochromator/FT-IR. The reflected light was collected by a detector with the integrating sphere (i.e., hemispherical-directional reflection) and subsequently normalized to a white reflectance standard (Spectralon) / gold mirror reference sample. For the mid-to-far IR region (25–200 $\mu$m), the specular reflectance and transmittance of a forest sample was evaluated by a FT-IR spectrometer and normalized to an aluminum mirror reference sample.".
Normally a thin silicon wafer would both transmit and reflect light, with a coating applied by the CVD machine an object can be turned black. Light shone on the object would be almost completely absorbed.
Instead of passing and reflecting light the coated wafer would behave like this: