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# Can the eye differenciate @ night between a 532nm: 100mw vs 250mw vs 500mw vs 1.2w?

#### cptroost

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Get back to me! Thanx!!:can::can::can:

#### Cyparagon

##### 0

Higher power makes the laser brighter.

Yes!!

#### cptroost

##### 0
Gotcha!!

I read on another thread, that between 500 to 1500 the human eye could NOT see the difference... only when it reached above 2000 PLUS...

hmmm

#### zyxwv99

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Perceived brightness is proportional to the cube root of the intensity (in this case measured in mW), except for a point source, where it's the square root.

#### IsaacT

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If you have all of those lasers in person at the same time you would absolutely be able to tell. Not only due to intensity, but the divergence of the 1.2W would likely be much worse. So beam characteristics all around.

However, if you had a 300mW 532nm laser and a 500mW 532nm laser and you viewed them on separate nights you might not be able to tell which one was which just from remembering the intensity of the previously viewed laser.

#### Cyparagon

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Perceived brightness is proportional to the cube root of the intensity (in this case measured in mW).

How do you figure. Source please.

#### ARG

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Perceived brightness is proportional to the cube root of the intensity (in this case measured in mW), except for a point source, where it's the square root.

Source? Been trying to find information to verify the saying everyone uses - "4x the power is 2x the brightness"

#### IsaacT

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I think people are referencing(or propagating a reference made by another) of the Weber-Fechner Law and the Steven's Power Law. The Weber-Fechner Law states that human response to stimuli increases less and less at a logarithmic rate. And the Steven's Law seems to be based off Weber-Fechner but is geared towards a wide variety of subjects.

I do not have the inclination to follow the math trail on those two laws but if someone wants to engage in such and report back I would love to see if this saying is true.

Stevens' power law - Wikipedia, the free encyclopedia
Weber?Fechner law - Wikipedia, the free encyclopedia

A quote from someone on a flashlight forum. Which may end up being problematic.

According to Stevens' Power Law (http://en.wikipedia.org/wiki/Stevens'_Power_Law) the *visible* brightness difference is proportional to the quotient of the cube roots of each lumen output. That assumes a 5 degree-wide beam against a flat target in the dark.

No lasers I know of have a 5 degree beam. So I am interested to know if this law is even applicable in the nature it was originally written in.

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#### firelaser

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i have a 100 mw green a 200mw green a 600mw green and a 1.2 w blue and i can easily tell the difference ,,, especially at night

and if you were saying 1.2W green well that would just be crazy bright
the green 600mw is still brighter than the 1200mw blue btw.. to me at least

#### zyxwv99

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How do you figure. Source please.

Brightness function: Effect of area and duration
R. J. W. Mansfield
JOSA, Vol. 63, Issue 8, pp. 913-920 (1973)
Optics InfoBase: Journal of the Optical Society of America - Brightness function: Effect of area and duration
"fractional exponents: -(1/2) for point sources and -(1/3) for extended sources"

Functional Relation between Stimulus Intensity and Photically Evoked Cerebral Responses in Man
HERBERT G. VAUGHAN JUN.* & RICHARD C. HULL
Letters to Nature
Nature 206, 720 - 722 (15 May 1965); doi:10.1038/206720a0
Functional Relation between Stimulus Intensity and Photically Evoked Cerebral Responses in Man

Individual brightness functions
L. E. Marks, J. C. Stevens
Perception & Psychophysics
January 1966, Volume 1, Issue 1, pp 17-24
Individual brightness functions - Springer

As I understand it, the underlying formulas are more complicated and only represent a "best fit." The square root and cube root versions are just convenient approximations.

#### Cyparagon

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First time I've seen those - Thanks! +6 to both later...

I'm pretty sure a laser dot counts as a point light source, yes?

#### zyxwv99

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First time I've seen those - Thanks! +6 to both later...

I'm pretty sure a laser dot counts as a point light source, yes?

It depends on how expensive the laser is, and what kind of optics it has. A point source is so small that you can't see the size or shape.

One way to find out is to shine the laser on an eye chart, the Snellen chart. Somewhere at the bottom is a line of letters labelled 20/20. On that line is a capital E. When viewed from the correct distance (20 feet for the full-size version) the entire E should be 5 arcminutes from top to bottom. Each arm of the E should be 1 arcminute high, with the spaces in between also 1 arcminute high. If your laser dot can just barely fit in one of the spaces between the arms of the E, then it's right on the borderline of being a point source.

The theoretical limit should be 30 arcseconds based on the geometry of the cones in the fovea.

Stars are point sources to the naked eye (and to nearly all telescopes).

In digital imaging, a point source is defined as less than one pixel in size (although such a source may light up more than one pixel if it's size and position are borderline).

Take The Snellen Eye Test Online

#### Cyparagon

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A point source is so small that you can't see the size or shape.

Yep. I'm going with square root then, not cube.

#### Hemlock_Mike

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Maybe a guess on my part --
The reason you can see the beam is due to crap in the air. A more powerful beam will likely cause more reflections making for a perceived brighter path.
HMike