Thread: OCP with LED

Have you tried a Green LED... The human eye is most sensitive to green light... Jaycar sell a green CREE 5mm LED that is very very bright... I put one on my auto collimator..

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Yes green was the first colour I tried but it didn't have the same effect. Also tried blue, yellow and white, none of these had the same effect. I'm wondering if the brass colour in the base had something to do with it?

Originally Posted by sacc51

Yes green was the first colour I tried but it didn't have the same effect. Also tried blue, yellow and white, none of these had the same effect. I'm wondering if the brass colour in the base had something to do with it?

More likely something to do with the plastic that the rod is made from.

As a side note, I got a 50 mm length of 30 mm hex brass bar yesterday with the intention of making a new one.

Originally Posted by .RC.

The human eye is most sensitive to green light.

I'm not so sure that applies in this case.. sure at low light levels its correct...... either way it looks great

I wonder if dying the crosshairs would help? (red in the case of red LEDs I think)

Stuart

I bought a heap of LEDs a couple of days ago with as close to the same values as I could get, some green, some red and some white, again red showed up the best. I also turned another base the other day from stainless and the effect was no where near as noticeable, now I'm confused!.

Originally Posted by Stustoys

I'm not so sure that applies in this case.. sure at low light levels its correct...... either way it looks great

I wonder if dying the crosshairs would help? (red in the case of red LEDs I think)

Stuart

We were taught that your eye is most sensitive to red light, in particular because it has the longest wavelength and gets deeper in the "receptor" and builds more charge or has a bigger effect. Certainly with camera's CCDs this is true but your eye is definitely more sensitive to red light. On the original TV CCD cameras, if there was a spotlight in one part of the picture, you would see a red stripe from top to bottom of the screen because the CCD's charge overflowed with red light first and the stripe was created by a drain at the side of the rows of CCDs, used to clock the charge out for reading, overflowing. They fixed the problem of seeing a red stripe by simply making it white, which looked like a specteral your eye sees if looking towards a spotlight. You still got the stripe but it was white and looked normal.
Your eye's sensitivity to red its why its harder to match red paint on cars than other colours.
I had a quick look on the internet and I think you get rods and cones in your eye, with one doing daylight situations and the other more or less appearing black and white for night vision. The thing I read said that a ships captain will have red instruments because the red light has least effect on night vision. I think it said cones were for night vision and most sensitive to green.
There you are, I used to know something in the old days, which surprises me at least!

Short version. As I understand it, red controls the size of the Iris and also rods dont see red. The Iris responses far faster than the other changes that the eye makes to low light. So red light it used for red eye reduction in cameras and ships. Rods for night..but you have almost no rods "straight ahead" so at night you sometimes need to look "not at" something to see it lol

This covers it better.
http://hyperphysics.phy-astr.gsu.edu...n/rodcone.html

Though as I said I doubt most of our sheds are that dark

Stuart

Rods allow for night vision or low light levels whereas cones allow for colour perception. For example nocturnal animals have more rods than cones, and being nocturnal (in the wild at least) good night vision is very important, colour vision less so. This then renders the animal to a state of almost colour blindness, at best they may be able to see shades of purple.

That's interstingaboutr the eyes response to different colours. I had always been taught that our eyes spectral response was similar to a bell shaped curve. Sloping away at near infra red and the violet end with the maximum response being pretty much smack bang in the middle at 555nm which is green. Just so happens that this is the middle of the suns spetrum too.

Didn't know about the nodes and rods. I used to be into astronomy and always wondered why when looking at a faint object in the sky I had to look next to it, not at it. Now I know, thanks Stuart!!

Oh, I had better give credit where it's due. Nice OCP!

Simon

Originally Posted by simonl

T I used to be into astronomy and always wondered why when looking at a faint object in the sky I had to look next to it, not at it. Now I know, thanks Stuart!!

I always thought my eyes were damaged when I could see an object in the dark better when I was not looking direct at it... Now I know it is normal...

Completely off topic, but....

While navigating our yacht at night I (and many other people) use red light because it doesn't impair night vision. When using white light only for a brief moment it can take up to 30 minutes for nightvision to become optimal again. This translates to the iris not reacting to red light as being light. (more light-smaller hole, less light bigger hole -> only red light-big hole. Is pupil a proper word for this hole in english?). As for the relation between colour blindness and night vision: this is not only true in animals. Colourblind people (=mostly men, but that is a whole other story) usually have better night vision, so colourblindness is not only a handicap.
As to seeing better in the dark when not focussing on a object: that is true although our night vision will never be particularly sharp. The part of our retina we use for focussed vieuw (using both eyes: depth vision) almost exclusively contains cones (not only colour view, but also better contrast. Need more light though), so no night vision. There is one other special spot on our retinas: the blind spot. This is where the optical nerve is connected to the retina: neither cones nor staves, so no vision at all

(in a previous life I was a biologist specialised in senses)

On second thought, it might not be as off topic as I thought: it explains why good light inside the optical centre punch is important. We need a sharp and focused view with good contrast of the mark we want to centre-punch. Therefore we want to use our cones. Hence the need for ample light in our field of view!

Regards,
Peter