Flashlight Physics (Or Psychology?)

[milkyspit]

I was just shining my newly-arrived ARC AAA seconds at a shelf in my walk-in closet, and for the first time I was able to theorize something that I'd intuitively felt for some time. When I hold a flashlight just over my head and aim at an object, the illumination looks brighter than when held at waist level. Note that this is the same light, I'm the same distance away, the light hits the object equally well, etc.

My theory is that this occurs because the light from near-eye level hits the object and bounces straight back, directly toward my eyes, whereas from waist level the light hits the object at an angle, and more of it bounces in different directions, away from my eyes.

But that might not be the whole answer either! It's also possible that I can see both well, but somehow the light coming from the same location (basically) as my eyes and returning along a similar path psychologically causes my brain to "prioritize" that incoming light over more varied arrivals?

For a long time I've noticed this phenomenon with both near and far targets; I'm just wondering how it works. /ubbthreads/images/graemlins/confused.gif

 

[Lagged2Death]

A light source held close to your eyes will cast fewer visible shadows than one that's farther away from your eyes.

And if you're looking at a shelf that's near eye-level, then holding the flashlight near your head probably makes for a shorter light-to-target distance than holding at your waist, of course.

 

[jtice]

Yes, all of your theories are correct.
When held at waste level, the light bonces to the eyes at an angle, and is not as direct. When held at eye level, the light is bounced straight back to your eyes.

Lagged, also has a point about the shadows. Shadows can be decieving, and make things nool alot darker than they really are.

As far as your brain prioritizing that light?... hmmmm.. I'm not sure on that one. I think it has to do more with the fact that the beam of light is bounced directly back.

I have often thought about things like this,,, heres one for you.

Look at you ceiling,, chances are, its "white". Well, not really, you precieve it as white. The wall itself is not actually white. The colors of light being refected back to you combine to make white.
A black wall, looks black, because it does not reflect any light back to your eyes.
Now, lets take this a step further....
Take a light bulb,,, if you think about it, its letting off a wide variety of colors. The "whiter" the light, the more colors it is emmiting.
I wonder if an alien from another world, would "precieve" colors differently. For example: would he see the light bulb putting out "black" light? Would he see the "white" wall for the color it really is?

 

[Ginseng]

This is the same reason fog lamps are mounted as low as possible on the fenders of cars far away from the eye level of the driver. With the source (bulb) at the same angle as the detector (eye), the backscatter is maximized. Thus more light, more glare.

Lagged is also correct in that the more shadows in the field of vision, the lower the overall luminosity of the scene.

Wilkey

 

[Charles Bradshaw]

2 things:

Angle of Incidence

Angle of Reflection (or refraction)

Optical Physics.

Third thing is Distance. The greater the distance, the less light is returned.

 

[Xrunner]

A little off topics but something interesting to do to is to shine a light at something that has the 3M reflective material (scotchlight?) on it from different angels.

-Mike

 

[Ginseng]

The 3M and Reflexite materials are not reflective but retroreflective. That is, they return light in the direction from which it came irrespective of the angle of incidence. So angle of incidence and angle of reflection are both zero with respect to a plane normal to the source/viewer. This is because retroreflective materials make use of silvered sphere or micro-corner cube technology. Corner cubes are basically the 90 degree corner of a cube cut off and silvered. This simple geometry has the curious nature of directing light out the same path it came in.

This is why road signs seem so freakin bright when you drive up to it with your headlamps on but almost invisible when illuminated from a source at an angle from the driver, such as a guy on a porch with a flashlight.

Wilkey

 

[Xrunner]

Thanks for the info, I had no idea. Maybe it was a different material I was thinking of.

-Mike

 

[PlayboyJoeShmoe]

I bet a lot of people see it when I blast a big green Highway sign with a bright flashlight! It's pretty incredible!

What you sense is the same thing as shining a light at a mirror. Off center you see the light. Directly you see Dalmation Spots!!!

Or shine a light at something dark and dull. It soaks up the light and can make a Space Needle II seem dim!

Let's keep it LIGHT, and be NICE to each other, eh?

 

[milkyspit]

[ QUOTE ]
Charles Bradshaw said:
2 things:

Angle of Incidence

Angle of Reflection (or refraction)

Optical Physics.

Third thing is Distance. The greater the distance, the less light is returned.

[/ QUOTE ]

I guess small changes in the angles are *much* more critical when using light from a point source like a flashlight, where you've got a limited supply of photons and they're all headed in basically the same direction, rather than all over the place...

 

[milkyspit]

I live out in the country. The main road conveniently runs in a straight line in front of my home, and perhaps 50 yards down that road there's a "reflective" (or retroreflective?) sign. Late at night it's interesting to take a selection of flashlights out front and shine them at the sign. The Space Needle II makes it look positively radioactive, of course /ubbthreads/images/graemlins/grinser2.gif , but even the lowest-powered lights, like my NewBeam, can evoke at least a bit of reflection. It kind of overestimates the effective range of the lights, and because of that I know that any light NOT bouncing back from that sign has absolutely no chance of illuminating anything else at that range.

And because there are virtually no cars on the road from midnight through 5am, I haven't gotten tread marks run up my back yet. /ubbthreads/images/graemlins/grin.gif

Just across the road is also a large field running up a gradual hill to a wooded hedgerow at the top. For me, this is the "dark and dull" object PlayboyJoe mentions; more like a black hole, really, because NOTHING I presently own -- not the Space Needle II, not the SL Scorpion, not even a tightly focused MagCharger -- reaches the trees at the top. /ubbthreads/images/graemlins/ooo.gif If the extreme lack of reflectivity doesn't get 'em, the occasional mist or fog (or even pollen!) in the air does.