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Relative Brightness as Explained by Biology

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It seems to be a well known fact on LPF that green lasers appear to be brighter than any other color. The question however has left many users wondering “Why is that?!”
I poked around the forum for a while and it seems that no one has given a good explanation as to “Why?” So I will attempt to answer that clearly here.

I will start with a brief introduction to the mechanism of human vision:
Vertebrate retinas (including human) contain two types of light sensitive cells: RODS and CONES.

ROD CELLS:
Rod cells are extremely sensitive to light between 440nm and 700nm with a peak at 500nm (Photo 1). This is because rod cells contain a chemical pigment called 11-cis-retinal.
The retinal in rod cells absorbs light in this spectrum into a specific chemical bond. When the energy from the light is absorbed, the molecule changes shape.
The retinal is part of a protein found in rod cells. When the shape-change occurs, the protein begins a signaling pathway that causes a nerve cell to fire carrying the signal to the brain.
The retinal in rod cells is extremely sensitive to a wide spectrum, but cannot distinguish between different wavelengths. Rod cells are able to react to single photons allowing us to perceive VERY low levels of light. The light absorbed can only act to turn signaling “on” or “off”, but rod cells are unable to distinguish between wavelengths, so they are unable to perceive color.

Photo 1:


CONE CELLS:
Cone cells are responsible for color vision.
There are 3 different pigments found in cone cells which are similar to the retinal of rod cells, but they are sensitive to a much narrower spectrum. Peak absorbance of the respective cone cell pigments are 426nm, 530nm, and 552-557nm (Photo 2).
Each cone cell contains only one type of pigment, and therefore is sensitive to a narrow spectrum. When the pigment within a cone cell absorbs light, a signal is produced in the same manner as in the rod cells. When this signal is relayed to the brain, it is interpreted as “blue”, “green” or “red” depending on which pigment is present.
While cones are able to distinguish color, they are much less sensitive than rods, and only function when enough light is present. The blue cones are less sensitive at their respective peak when compared to either the red or green cones.

Photo 2:


MAKING SENSE OF THE MECHANISM:
Our retinas are composed of a very fine mosaic of rods, “blue cones”, “green cones”, and “red cones”. There is a higher density of green and red cones, than of blue cones.
As light is focused onto this mosaic, our brain receives millions of signals and interprets them as a color image. (I find this ability astonishing!)
Since cones are not as sensitive as rods, they only really function when there is enough light to activate them. When light becomes dim, humans lose most, if not all, ability to perceive color.
When we walk into a dark area or turn off the lights in a room, we notice that it takes us a few minutes to adjust to the darkness. This adjustment is partially due to dilation of our pupils, but mostly due to an adjustment in the interpretation of the signal by our brain. Since rods are incredibly sensitive to light, they are being activated most of the time when we are in lighted situations. The brain conserves resources by filtering out (ignoring) the signal produced by the rods when the signals are too numerous. When we turn off the lights, it takes the brain a moment to start paying attention to the rod-signals, but once it has made the transition, our night vision improves dramatically.

WHERE DO LASERS FIT IN?:
The most common lasers currently available are:
405nm (violet)
445-450nm (blue)
532nm (green)
635-642nm (red)
650-660nm (red)
808+nm (infrared).
If you line these wavelengths up with the absorption spectra of the pigments in our retinas, you can begin to understand exactly how our eyes will respond to different wavelengths.

CONCLUSION (Finally!):
Since our eyes are receiving light in two very different ways (rods vs. cones), we need to consider two types of relative brightness: The amount of light absorbed by rods (brightness) and the amount of light absorbed by cones (relative color brightness).

Lasers at the upper and lower ends of the visible spectrum (<405nm and >660nm respectively) are very difficult to see at all because they fall outside of the range of wavelengths absorbed by the pigments in our retinas. Of the commonly available visible lasers, 405nm and 660nm are the least absorbed by both rods and by all of three cones resulting in less perceived “brightness”, and less perceived “relative color brightness”.

445nm lasers are perceived fairly well by rods and by green and blue cones, but very poorly by red cones. The absorption however, is not at a maximum by any of them. The brightness and relative color brightness are high, but not at a maximum.

635-642nm light is absorbed poorly by rods, red cones and green cones and not at all by blue cones. 650-660nm light is absorbed even more poorly. As a result, 635-642nm lasers have a higher brightness and higher relative color brightness when compared to 650-660nm. Because we have more red and green cones then we do blue cones, and because the red and green cones are relatively more sensitive than the blue cones, red lasers are perceived to have higher relative color brightness than 445nm.

532nm lasers are absorbed very well by rods and red cones, but poorly by blue cones. They are also absorbed very nearly maximally by the green cones. Green lasers have a high perceived brightness and a VERY high perceived relative color brightness. As a result, 532nm green lasers appear much brighter than any of the other common wavelengths. In fact, 532nm is very close to the maximum brightness of any possible pure wavelength.


Here is a link to the "Relative Brightness Calculator"; created by our own RHD and often used here on LPF.
Just created this as a follow-up to my NM to RGB tool:

Here it is - Calculate Relative Perceived Brightness/



I hope this explains the phenomenon of perceived relative brightness. Of course there is a LOT more information that should go in here, but if anyone reads this far they should get the picture. Let me know if you think I missed anything important, or if I made any mistakes.
Cheers! :beer:
 
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ZRaffleticket

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Whoa thats really neat, I always wondered this!
Thanks for the info +1

PS: upload the images as 800(w)x600(h) or scale it to a 15" monitor. (depends where you upload it to)
 
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You can use Paint, then upload it with tinypic. If you're on a Mac you can resize with tinypic.
 
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you graph don't show it but blue should have a peak that is more narrow as well, or as to the peaks are not the same for all colors, green would have a flat top on a more resolute graph, here I will show. Please be aware I did not make this but found it to explain what I was saying, I take no credit for making this at all. Thanks for the info though good read.
 

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Meatball

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Rod cells are extremely sensitive to light between 440nm and 700nm with a peak at 500nm (Photo 1). This is because rod cells contain a chemical pigment called 11-cis-retinal.
The retinal in rod cells absorbs light in this spectrum into a specific chemical bond. When the energy from the light is absorbed, the molecule changes shape.
The retinal is part of a protein found in rod cells. When the shape-change
Retinal aka a form of Vitamin A.

That kids, is why you need to take your vitamins.

Great post!

-Tyler
 
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you graph don't show it but blue should have a peak that is more narrow as well, or as to the peaks are not the same for all colors, green would have a flat top on a more resolute graph, here I will show. Please be aware I did not make this but found it to explain what I was saying, I take no credit for making this at all. Thanks for the info though good read.
Thanks for posting that.
Yes, it should be noted that the graph I used represents the relative locations of the absorption peaks of each pigment. It does not show the exact amount of absorption, or all of the intricacies of a complete graph.

I also did not create the graphs I used. They were the easiest to read of all the graphs i found. As noted in the corner of the figures, they came from Stryer's "Biochemistry" (7th ed).
 
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Spectral Sensitivity of the Eye a good read and where I stole the graph :p It is funny to notice our peak red is more um yellow than red :p
Great link!

Yea, that is very interesting! 550nm is even closer to green than it is to yellow!
The "green" and "red" pigments are actually evolutionary homologs (they evolved from a common predecessor), and they are very similar. The only reason that we can use them to see different colors is that the upper end of the absorption band for the "red" pigment is slightly higher than that of the "green" pigment.
 
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I think it is also very important to note that, even though the pigments in our eyes cannot absorb anything outside of the visible spectrum (~400-700nm), the energy of the light outside of this range is still present. The absorption bands DO NOT represent relative amounts of energy present in those wavelengths.

808nm light from IR lasers, for example, is not absorbed by the necessary (11-cis) chemical bond the pigments which our retinas use for sight. It IS however absorbed by other parts of those pigments, as well as every other protein and non-protein molecule found in our eyes.

Just because we can't see it does not mean that there is no energy there. Goggles are a MUST for ALL high powered lasers, especially those which emit light outside of the visible spectrum.
 
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Great link!

Yea, that is very interesting! 550nm is even closer to green than it is to yellow!
The "green" and "red" pigments are actually evolutionary homologs (they evolved from a common predecessor), and they are very similar. The only reason that we can use them to see different colors is that the upper end of the absorption band for the "red" pigment is slightly higher than that of the "green" pigment.
yep I was wanting to to laser safety test on pigs eyes, not the pig, just the eyes. for that very reason, as far as I know our eyes and pigs eyes are a very close match in the way they work, not common for most animals. Not that I studied this in school, I am pretty sure I saw it on a documentary, or read it in a book while :toilet:.
 
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yep I was wanting to to laser safety test on pigs eyes, not the pig, just the eyes. for that very reason, as far as I know our eyes and pigs eyes are a very close match in the way they work, not common for most animals. Not that I studied this in school, I am pretty sure I saw it on a documentary, or read it in a book while :toilet:.
I like that idea! I try to do as much studying as possible here---> :toilet:

I'm not sure how you would go about studying this without some type of retinal scanner to measure physical damage thresholds, or by purifying proteins after exposure to see a difference.

Obviously any type of physical damage that you could see would be FAR beyond the lower limits of what would cause blindness.
 
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I like that idea! I try to do as much studying as possible here---> :toilet:

I'm not sure how you wold go about studying this without some type of retinal scanner to measure physical damage thresholds, or by purifying proteins after exposure to see a difference.

Obviously any type of physical damage that you could see would be FAR beyond the lower limits of what would cause blindness.
not completely true not, far beyond, a laser will leave a mark if it blinds you, I could not prove it could not harm your vision, but blindness would be a retinal burn other noticeable events,,, is myth-busters still on the air? also allot of us love to view our lasers with no protection at night, me too, and can you say blue light hazard so I am hoping on cyborg eyes in the next 20 years, cause I wont be blind but I bet I wont see too much. Also a 405 shot to the eye seems the worse to me, higher energy photons and more chemical reactions, than other common lasers we use, and a very nice dot.

You are right though, I don't have the resources to pull the testing off.
 
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not completely true not, far beyond, a laser will leave a mark if it blinds you, I could not prove it could not harm your vision, but blindness would be a retinal burn other noticeable events,,, is myth-busters still on the air? also allot of us love to view our lasers with no protection at night, me too, and can you say blue light hazard so I am hoping on cyborg eyes in the next 20 years, cause I wont be blind but I bet I wont see too much. Also a 405 shot to the eye seems the worse to me, higher energy photons and more chemical reactions, than other common lasers we use, and a very nice dot.

You are right though, I don't have the resources to pull the testing off.
I'll concede that. I was speaking more along the lines of frying the pigments and opsin proteins that carry them on a chemical level. This might be invisible, but could potentially cause complete blindness.

I agree, it would be a great idea to try this and get as much information as you can without an ideal setup...

Where do you get pig eyes?:thinking:
 
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