Is there a way to see light frequencies invisible to the human eye without the use of electronic sensors? I wonder if it's possible to be able to see light frequencies that cannot be normally seen by human eye, without using sensors designed specifically for detecting a certain spectrum range, but rather using some combination of lenses or something else that could transform the light without causing a noticeable latency.
 A: It's possible to see the wavelengths beyond the red using non-linear optics: Second harmonic generation.
Basically two photons combine to form a twice more energetic photon.
Green laser works this way.
The opposite of this is the Spontaneous parametric down-conversion, to see beyond the blue. This involves non-linear optics again. In this process one photon splits into two (entangled) photons.
Efficiency of this process is very low. (But probably usable to see the UV from the Sun.)
A more effective way is using interference of light of different frequencies. The interference can form a third wave whose frequency can be in the visible light range: $\omega_1 - \omega_2 = \omega_3$. This is the principle of the the optical down conversion. This way you can shift the visible range towards the higher frequencies.
Anyway electronic sensors remain the easiest ways to see the invisible.
A: I have built two pairs of these.  They work as advertised, and experimentation with prisms and near-infrared lasers has convinced me that I'm truly seeing near-infrared.
Edit:
The linked page gives instructions on making goggles that block out what is traditionally considered 'visible' light (400nm-700nm).  Once you block that, you are left with the near-infrared, which your eyes can see, but with only something like 1/10000 of the sensitivity they have for visible light.  However, on a bright sunny day, even .01% of the ambient light is enough; the final effect is substantially brighter than moonlight, which is about 1/40000 as bright as sunlight.
