A very dim visible laser can solve this problem. For example, under most conditions, a few microwatts in the visible high power laser pointer beam are almost invisible. However, if you are looking at a light beam scattered in the atmosphere rather than a light beam directly reflected from the surface, it may be difficult to see the brighter laser light. Or, for example, the reflection of the visible laser beam from Vantablack is almost invisible in a normally illuminated room.
Under the following conditions, the human eye can see the laser beam: the beam directly enters the eye. Not recommended, it may be dangerous for class 3 or 4 lasers. In fact, for Class 3B and Class 4 lasers, it is well known that it basically causes immediate damage to the eyes.
The laser beam is reflected from the “diffuse” surface. I put it in quotes because what most people call diffuse reflection is actually subsurface volume scattering. The level 4 green laser is very bright, even the “diffuse reflection” of a piece of paper or a white painted wall can cause damage to the eyes. The laser beam is scattered by fog or aerosol in the air. We used to use “fog” to make such photos, but despite the Hollywood “fog maker” swears to us, it still leaves residue on the optics.
We eventually started taking long-exposure photos like this in a dark room, where the technician passed a piece of paper through the entire path of the beam at the end of the fishing rod. You can see the light beam scattered in the air from the scattering. Green is the best laser color because the eyes are most sensitive to green, and the shorter wavelengths scatter much more than the longer wavelengths with Rayleigh scattering.
This is why the sky looks a bit like light green instead of white or purple. For the past 35 years, this photo has been on my desk. The ultraviolet laser beam can be seen indirectly, because it can stimulate the faint fluorescence in the air and even in the eyes. If you don’t know exactly what you are doing, it is quite dangerous. I have experienced it twice. I was taken aback for the first time and almost lost what I was doing.
You can also see infrared blue laser pointer beams or bright LEDs through the multiphoton absorption process. About 30 years ago, I actually observed this with a camera and a very high-power infrared laser. This has been a mystery for several days, because we know that the camera is not sensitive to the laser wavelength (the band gap does not allow electrons to enter the conduction band). After several days of investigation, we determined that multiphoton absorption was the cause. Since this is a confidential laser and project, we cannot publish our findings like many other things, we can only watch other people rediscover the same thing years later and publish their results.