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While playing with a blue/violet laser (5 mW, 405 nm) tonight, I noticed that on dark, non-fluorescing surfaces, the beam reflection was trailed by what looked like a dimmer reflection of the beam. The effect was most easily observed when I moved the laser in a circular motion. The apparent 2nd reflection was not produced by a side lobe of the laser diode, as it always trailed the main reflection, regardless of clockwise or counterclockwise rotation. Also when I stop moving the beam, the second 'reflection' quickly merged with the main beam reflection.

I doubt it is caused by phosphorescence of the material (cloth) I was lasering, as it also showed up when I pointed the laser at the floor, other sorts of cloth and painted walls.

So, do rods, cones, visual purple or other eye components phosphoresce when hit with deep blue light?

Wayfaring Stranger
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Instead of phosphorescence, a more parsimonious explanation is the perception of afterimages. Afterimages regularly appear after viewing a bright stimulus. They occur in the same location in the visual field as the original stimulus, but lack clarity. Afterimages depend on the intensity and contrast of the original stimulus (i.e., they are more pronounced with bright stimuli in a dark environment), the time of fixation (i.e., longer fixation generate more persistent afterimages), and retinal adaptation (i.e., dark environments enhance afterimages).

After images are almost always the complementary color (negative afterimage) to the original stimulus, but can very briefly be the same color (positive afterimage) when viewing an exceptionally bright stimulus. A stimulus consistently produces the same afterimage, which varies in size based on the distance between the person and the background. After images are often revived by blinking.

Afterimages are thought to derive mainly from photobleaching of the retina, although cerebral processes may contribute too.

Phosphorescence in the retina has to be induced by injecting phosphorescent markers (Wanek et al., 2013). As far as I know, the retina is not autophosphorescent.

References
- Gersztenkorn & Lee, Survey Ophthalmol, (2015): 1-35
- Wanek et al., Curr Eye Res (2012); 37(2): 132–137

AliceD
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  • The main reflection of the laser off a dark surface doesn't appear all that bright. Your ref doesn't say whether after image intensity is wavelength dependent, but red and green lasers at the same rate of laser movement, both produce continuous light trails; persistence of vision. There's some funny kinetics going on with blue to generate a distinct trailing spot. – Wayfaring Stranger Jul 15 '15 at 04:19
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    Blue cone pigment is coded by a gene less closely related to red and green pigments. Could be different adaptation kinetics. – AliceD Jul 15 '15 at 04:24
  • So not phosphorescence, which is what I asked. Blue cone is far less sensitive than red or green: http://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response (It'd be nice to see units on that Y axis). But that doesn't explain why I see a trailing dot rather than a trailing streak as with other colors. Watching that dot merge with the main beam reflection after I stop moving the beam is truly an odd sight. Guess I'll go with funny kinetics for now. – Wayfaring Stranger Jul 16 '15 at 13:06
  • @WayfaringStranger - generally the laser beam is only visible when there are scattering particles. Only the point of impact is seen on the ground, a beamer screen etc. But no, pretty definitely not phosphorescence. Thanks for accepting! – AliceD Jul 16 '15 at 13:08
  • AliceD: Yes, this is all about the behavior of spots at the point of beam impact, and slightly trailing it depending how fast I move the beam, not smoke or dust in the air. – Wayfaring Stranger Jul 16 '15 at 13:13
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    Did you try recording this on camera and stop the video at one point? if the trailing dot is visible there, it must be some light, emitted by the material. if not, its an effect, caused by your eye. – Mystery Jul 16 '15 at 20:13