Watts The Point

[leon-1]

Leon,

I've heard of this but not sure how it works. Is it purely for night tracking or is it used to emphasise the track print?

What torch do you use for this? Does it have to be high-powered?

Thanks

Pablo

Between Johan and Demographic they have pretty much nailed it.

Green light is supposed to be the brightest light in nature, the cones of the human eye are more receptive to green light than they are to other wavelengths, thus shadows are more apparent. Blue light annihilates shadow and red light doesn't show it at all well as the parts of the eye that recieve the specific wavelengths require other things. Green is close to the centre of the visual spectrum, as such other lights can do the job as well (Woody showed me amber light and that is pretty imprsseive to)

A common misconception is that black light is towards the high end of the light spectrum, but to be honest I would say it is more in the infra rather than the ultra side of life (the military use of black light in the form of infra-red). Ultra violet will make a lot of things stand out, infra-red does not unless you are using viewing aids.

 

[Andy]

Blue light is used for penetrating smoke or fog. Useful up hills at night!

Are you sure, I always thought it was red light that would deal with the fog better, red light has longer wavelength so is less affected by particles between the sorce and the object, blue on the other hand has very short wavelength so has more energy (why it's more blinding) but is affect most. The sunrise/sunset is red as the light is passing through more of our atmosphere

 

[Montivagus]

Are you sure, I always thought it was red light that would deal with the fog better, red light has longer wavelength so is less affected by particles between the sorce and the object, blue on the other hand has very short wavelength so has more energy (why it's more blinding) but is affect most. The sunrise/sunset is red as the light is passing through more of our atmosphere

Uhhh....YES!

Try it.

 

[Pablo]

Thanks for the responses on green light. I've got a gree filter for the Tikka XP. I'll have to try it out.

Pablo

 

[ESpy]

Are you sure, I always thought it was red light that would deal with the fog better, red light has longer wavelength so is less affected by particles ...

When diving in murky waters, my blue-white HID (rated at 10W, but claimed to be equivalent to a 25W halogen) cuts through the crud far more effectively than the more yellow halogens.

 

[demographic]

A common misconception is that black light is towards the high end of the light spectrum, but to be honest I would say it is more in the infra rather than the ultra side of life (the military use of black light in the form of infra-red). Ultra violet will make a lot of things stand out, infra-red does not unless you are using viewing aids.

Err, not as far as my dye penetrant testing text books are concerned.

Black light is indeed at the ultra violet end of the spectrum but higher frequency than is visible to the naked eye (well to be accurate some wavelengths of black light is only just visible which is why you can see the tubes in an eerie violet light).

The thing about UV light is that it has higher energy (higher frequency and shorter wavelength) than visible light and although you can't see it, many things fluoresce (they absorb photons of UV light but re-emit visible light so appear brighter than they would under normal visible light.

Infra red has a lower frequency and longer wavelength than visible light and UV (black light) has a higher frequency and shorter wavelength than visible light.

That's what I was taught in my non destructive testing courses anyway.

 

[Doc]

Watts are a unit of power -ie equivalent to one joule of energy used per second. So there is no need to call it watts per second.

(I remember a newspaper report described the QE2 entering the harbour approaches at '6 knots per hour'. As a knot is 1 nautical mile per hour, knots per hour is a unit of acceleration. After a ten day voyage, accelerating as described, she'd be doing about Mach 2.......)

A 40 w incandescent light bulb will indeed consume 40 watts of electrical energy but most of this is converted to heat, not light. As you say, LEDs are more efficient and given that batteries are heavy and difficult to charge/improvise in the wilds, I really believe that incandescent bulb torches are verging on obsolete for the lightweight backpacker on anything other than short trips.

I have an Inova 24/7 which gives the choice of variable power, a discrete red light, and various signal modes (it even sends 'SOS' in rather poorly spaced morse). If it has a fault its brightest setting is sometimes not bright enough. It is very light and practical though.

Lithium cells give a lot of power for light weight. The high price tag in the high street puts people off needlessly. You can source lithium batteries from Hong Kong on ebay -a pack of ten, delivered, costs about the same as 1 or 2 from the High street.

 

[AJB]

I apologise if I’m having a little rant here, but there is some dodgy terminology knocking around here.

Black light – no such thing, it’s a phrase used by DJ’s - you are talking about ultraviolet light.

The visible portion of the electromagnetic spectrum extends from around 400 nm (nanometres) (the blue end) to around 700 nm (the red end).

Infra-red light has a wavelength longer than 700 nm, ultraviolet light has a wavelength shorter than 400 nm.

• 280 nm — near ultraviolet wavelength
• 380-420 nm — wavelength of violet light
• 420-440 nm — wavelength of indigo light
• 440-500 nm — wavelength of blue light
• 500-520 nm — wavelength of cyan light
• 520-565 nm — wavelength of green light
• 565-590 nm — wavelength of yellow light
• 590-625 nm — wavelength of orange light
• 625-740 nm — wavelength of red light

The whole rods and cones thing is a bit of a misnomer. It is an explanation of observed TYPICAL responses of the eye. BUT there are a number of simple practical demonstrations whose results are unexplainable using the rods and cones argument. Their working is not yet fully understood.

Finally, then I’ll wind my neck in, night vision.

Night vision is not about using red or green light. Night Vision or SCOTOPIC vision if a physiological effect which is triggered when ambient luminance levels drop below around 10-2 to 10-6 cd/m². (Candela per metre squared)
It is an evolutionary effect, at night, when we are illuminated by light reflected from the moon, the light we receive has been partially absorbed by the moons surface and has been further diffused and therefore less of the lower energy red end of the spectrum arrives at the earth’s surface. This is a similar reason why things underwater get progressively bluer the deeper you go, the red light is absorbed.

We have evolved to become more blue sensitive when it gets dark. It’s triggered by light levels not its colour. If you illuminate your night scene with high intensity red light, you will loose your night vision because the ambient illuminations levels will retrigger your photopic vision. Because the eye is more blue sensitive, if we illuminated our scene with white light, the blue component of it would fool the Mk1 eyeball into thinking it was brighter than it really was and we would loose the scotopic effect. Illuminating with a small amount of red light is relatively ignored by the eye’s ‘metering’ ability so does not impact on our ‘night vision’ but does illuminate the scene and is sensed by the red sensitive receptors in the eye. Things are not monochrome at night because that’s the way the eye works, it’s because you are viewing something illuminated by monochrome (in this case blue) light.

Rant over – I like torches – they’re gadgets

 

[Montivagus]

Just some more info.

The "blacklight" was invented in 1903 a bit before DJs. It is a colloquial term but does refer to a light that emits UV and crucially, some visible light. You have to call that something.

Efficiency of a light source is measured in Lumens/Candelas per watt.

 

[Montivagus]

Are you sure, I always thought it was red light that would deal with the fog better, red light has longer wavelength so is less affected by particles between the sorce and the object, blue on the other hand has very short wavelength so has more energy (why it's more blinding) but is affect most. The sunrise/sunset is red as the light is passing through more of our atmosphere

The sky appears blue precisely because that’s the wavelength that gets through all the crud in the atmosphere.

 

[Emma]

As far as I know, the reason the sky is blue is because of refraction. You know that experiment with a prism splitting white light into a rainbow? Well the atmosphere does that, and during daytime we happen to be in the blue section. During sunrises and sunsets we happen to be more in the orange-red section. Despite red light being the most easily absorbed by the particles in the atmosphere, sunrises and sunsets are reddish because that's the part of the spectrum we're in. The light is dimmer then because of the absorption.

And the reason we don't see yellow and green skies? According to my old secondary school teacher that's to do with the workings of the colour receptors in the eyes. I haven't got a clue how it woks, but somehow we pick up the blue or orange, the yellow and green are swamped or somesuch. Of course, she may have been making it up just so she'd answered the awkward question and I'd shut up...

 

[Montivagus]

As far as I know, the reason the sky is blue is because of refraction. You know that experiment with a prism splitting white light into a rainbow? Well the atmosphere does that, and during daytime we happen to be in the blue section. During sunrises and sunsets we happen to be more in the orange-red section. Despite red light being the most easily absorbed by the particles in the atmosphere, sunrises and sunsets are reddish because that's the part of the spectrum we're in. The light is dimmer then because of the absorption.

And the reason we don't see yellow and green skies? According to my old secondary school teacher that's to do with the workings of the colour receptors in the eyes. I haven't got a clue how it woks, but somehow we pick up the blue or orange, the yellow and green are swamped or somesuch. Of course, she may have been making it up just so she'd answered the awkward question and I'd shut up...

Here you go....pulled this off the web.
"There is a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (color) of the light. Sunlight is made up of all different colors of light, but because of the elements in the atmosphere the color blue is scattered much more efficiently than the other colors.

So when you look at the sky on a clear day, you can see the sun as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you. (Because red light, yellow light, green light and the other colors aren't scattered nearly as well, you see the sky as blue.) "