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Author Topic: Photon Blade: Tests, Measurements and Physics  (Read 3137 times)

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Offline Photonic Bladesmith

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Photon Blade: Tests, Measurements and Physics
« on: August 07, 2016, 04:15:27 AM »
Fluorescent wavelength shifting has become a standard saber blade technique since The Custom Saber Shop and Vader's Vault has started selling finished blades constructed with fluorescent dyed plastic.  The Custom Saber Shop has also added both thin and thick wall fluorescent green polycarbonate tubing with matching fluorescent green tips to their line of blade construction supplies.

The Fluorescent green blade dyed with fluorescein has attracted the most attention due to its high brightness when illuminated by blue LEDs.  There has been many questions posed on the various online saber forums on fluorescent enhanced blades that could be answered by a series of tests and measurements.

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Why Fluorescent Dyed Plastic Saber Blades?

The fluorescein dyed fluorescent green blade solves a peculiar problem that exists with current technology LEDs.

 


If the goal is to obtain the maximum visual blade brightness the best color to use is green since it is at that wavelength that a given amount of optical power emitted from the LED will appear the brightest to the human eye.   

As specific examples, you would need around 20 watts of optical output power in the deep blue or red to equal the apparent brightness of one watt emitted in the green.

Unfortunately current technology green LEDs are only about 8% efficient at converting electrical energy to photons. Blue and red LEDs are on the order of 30% or more efficient depending upon the drive current.  In particular the latest Blue LEDs such as the CREE Royal Blue XT-E developed as the pump source for white light phosphors are capable of outputting over five times the amount of optical output power when overdriven than the best green CREE LEDs.   

Fluorescein ("day glow green") dyed saber blades now commonly called Photon Blades are able to convert at high efficiency the blue light produced by these powerful Blue LEDs into these optimal high human eye sensitivity green wavelengths.

A secondary brightness benefit is that the short wavelength components of Sunlight and room lighting also acts as an optical pump to the Fluorescein dye adding to the brightness of the Photon Blade as opposed to washing out the illuminated blade color which is the case for conventional blades.

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Brightness of the Green Photon Blade

To estimate the theoretical brightness of the green Photon Blades being sold as finished blades and assembled from TCSS components we need to know the distribution of wavelengths emitted from the blade when excited by a blue LED.  It is a known fact that the emission and absorption spectra of fluorescein changes with the media that the dye is embedded in so a direct measurement of the emission and absorption of the dyed plastics is required.

 




Fluorescence spectra measurement being taken of a sawed off section of a Custom Saber Shop polycarbonate green fluorescent dyed saber blade illuminated by a Royal Blue CREE XT-E LED (460 nm).

The cut sections showed that the Photon Blade tubes are dyed in the bulk and not just surface painted.


 




Fluorescent emission spectra of Fluorescein dyed polycarbonate and acrylic when pumped by 460 nm light from a Royal Blue CREE XT-E LED. 

The lumens per watt optical power to perceived brightness conversion factor was obtained by taking the data for the fluorescent emission curve and writing a program to perform a weighted averaging with the wavelength dependent sensitivity response curve of the human eye.

 


The conversion efficiency of blue pump photons at 460 nm to green fluorescent light emission is remarkably high at over 90% for fluorescein which is one of the reasons that the green Photon Blade works so well.

As an example calculation:

An overdriven (10 amps) quad 460nm Royal Blue CREE XT-E has been measured by an optical power meter to produce 8.4 watts of optical output power and is being used to illuminate a green polycarbonate tube Photon Blade.

8.4 watts  x   0.95 (efficiency)  x   522  lumens/watt   =   4,166 lumens

As a comparative reference value, Luminara (Vader's Vault) posted data that an overdriven green TriCREE XP-E2 star, considered to be a very high brightness conventional blade illumination source, outputs 800 lumens.

This type of calculation explains why the green fluorescent photon blade is the only color with a striking increase in brightness over the pump source.   For instance, converting blue photons to red photons via fluorescence does not yield a significant increase in brightness since both blue light and red light are on the low end of the lumens/watt human eye sensitivity curve.  This particular blue to red frequency conversion via fluorescence is also unnecessary if one simply wants the brightest possible red blade since red LEDs are just as efficient in converting electrical energy to light as blue LEDs.

The important concept here is that fluorescence doesn't increase blade brightness by amplifying the amount of light, it increases brightness by wavelength shifting the existing LED light to colors where the human eye is more sensitive.

The preceding calculation assumes that all of the pump light is absorbed, and that the fluorescent light exits the blade without being trapped or self absorbed.

To estimate what these negative factors are we have to measure the spectral transmission curves of the thin and thick wall polycarbonate Photon Blade tubes.

 


Transmission spectra of fluorescein dyed polycarbonate being measured.


 




Optical transmission spectra of thin (1/16") and thick (1/8") wall TCSS Photon Blade tubing. (Linear scale)


 



Optical transmission spectra of thin (1/16") and thick (1/8") wall TCSS Photon Blade tubing. (Log scale)

The transmission curves show that the wavelength dependent absorption does not significantly self absorb the green fluorescent light output.

These measured curves indicate that the emission spectra of the blue LED pump light at 460nm is well matched to the absorption peak of the fluorescein dyed polycarbonate and is over 99% in the thick wall blade tube and about 94% in the thin wall tube.   The non-absorbed pump light is transmitted through the blade.


 




The fact that the measured residual transmission of light through the tube wall is a factor of ten less in the thick wall tube, which is twice the thickness of the thin wall tube, is consistent with the dye concentration being identical in both the thin and thick wall green Photon Blades.  This exponential dependence absorption of as a function of increased path length is due to the explicit exponential dependence of absorption with path length in absorbing media given by the Beer-Lambert equation.  This is the equation used to calculate the effect of changing the thickness, dye concentration or pump wavelength on the fraction of pump light or emitted fluorescent light absorbed.

 


The 6% transmitted blue pump light that shines through the thin wall Photon Blade is what gives the blade an off color cyan appearance where the blue pump light is most intense.  This cyan tinge is absent in the thick wall blade where the pump light absorption is measured as being greater than 99%.

If Vader's Vault or TCSS want to eliminate this cyan coloration in the thin wall green Photon Blade, requesting that their manufacturer double the fluorescein dye concentration in the thin wall blade tubing will solve this problem.

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Blade Tips

The first Photon Blades were constructed with conventional sanded finish clear polycarbonate tips.  This was unsatisfactory as the blue LED pump light would turn the tip a bright blue which looked out of place terminating a green blade.  Since then, tips machined from fluorescein dyed polycarbonate have become available and the odd looking bright blue tip on a green blade is no longer an issue.

 



The Custom Saber Shop tips feature a partial area coverage mirror.   Curious about the difference between a full area mirror and what TCSS decided was optimal, I added a full diameter mirror to one TCSS tip and assembled two otherwise identical blades driven by two identical Quad Royal Blue CREE XT-E sabers. 

 


 


Viewed from the side, the reduced area mirror as sold by TCSS does an excellent job of providing a true green color tip that is brightness matched to the blade.  The full diameter mirror produces a noticeably dark green tip.

Viewed from the front, the TCSS stock tip shows some blue LED light shine through.  The full diameter mirror tip keeps its dim green appearance with no cyan shine through.

Since saber blades are very rarely seen end on during demos and cosplay, an optimized side view appearance is far more important than the end on appearance.  Overall, The Custom Saber shop did a very good design job on their fluorescent dyed, reduced size mirror tip.
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End Part 1.   Continued part 2.
« Last Edit: March 09, 2017, 09:58:59 AM by Photonic Bladesmith »

Offline Photonic Bladesmith

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #1 on: August 07, 2016, 04:16:10 AM »
_________________

Photon Blade: Tests, Measurements and Physics          Part 2.


Sanded vs. Polished Outer Tube Wall Photon Blades

There are four constructional variants of green Photon Blades:
- Thick wall (1/8" tube wall thickness) polished wall blade

- Thin wall (1/16" tube wall thickness) polished wall blade

- Thick wall sanded tube wall blade

- Thin wall sanded tube wall blade

One obvious question is whether these variants make any difference in blade brightness or appearance.

One each of these Photon Blades variants were constructed, each having 4 feet of rolled up clear cellophane gift wrap diffuser inserted into the tubing.


 


The same 10 amp LED drive Quad Royal Blue CREE XT-E saber is used to illuminate each Photon Blade variant.

The blade brightness is measured by placing the blade starting at the hilt over a solar cell panel and measuring the current generated with a Fluke DVM.  The short circuit current output from a solar panel is a linear function of the illumination intensity.

 


 


There are three main differences observed and measured:

- Thin wall tube Photon Blades are brighter that thick wall Photon Blades.

- Thin wall Photon Blades have a distinct cyan colored core compared with the thick wall which is a solid green color.

- Sanded Photon Blades are brighter than Photon Blades that are left with the factory polished tube wall.


Explanations and Theory:

The fractional increase in brightness of the thin wall over the thick wall blades is quantitatively almost exactly the same factor as the ratio of the greater clear aperture of the thin wall tube over the thick wall tube.

 


The thin wall blade is brighter simply because it has more open area to let more of the cylinder of light produced by the saber LED array shine down the blade tube.


The cyan colored core appearance of a thin walled green Photon Blade as opposed to the solid green color of a thick walled Photon Blade when illuminated by the same blue LED saber is explained by the Photon green tubing spectral absorption measurements that showed that thin wall Photon Blades transmits through about 6% of the pump light as opposed to the thick wall Blade which has a 0.6% pump light shine through.


The observation that sanding the outside of the blade increases the blade brightness of fluorescence based Photon Blades is explained a subtle aspect of fluorescence light emission.

 



Due to the physics of fluorescence and the random orientation of fluorescein molecules in dyed plastic, the direction that the green photon will be emitted after being excited by blue light is random.

Fluorescent light that is emitted nearly parallel to the blade length can be channeled by total internal reflections from the inner and outer polished tube surfaces and ultimately absorbed and turned in to heat.

Sanding the outside of the blade eliminates the outer reflective surface and scatters and releases these shallow angle photons resulting in brightening the blade.

A quantitative analysis of this effect is complicated by the fact that the inner wall of the blade tube is in contact in a somewhat variable way with the diffuser film which can also act as a scattering surface suppressing the total internal reflection effect.

Summary:

- The brightest Photon Blade variant is the sanded thin wall blade.  The Vader's Vault Photon Blade is sold standard in this configuration.

- If the cyan tinge bothers you, the sanded thick wall blade eliminates this off-color shine through, though with about a 20% loss in brightness.

- Leaving the blade with polished walls loses about 20% of the brightness of sanded outer wall blades, but allows for some interesting visual effects (next section).

________________

Other Color Blades and Bright Core Effects



 


The spectral transmission measurements of fluorescein dyed polycarbonate used to make green photon blades when compared to the emission spectra of white LEDs suggests an interesting blade effect may be possible: that LED pump light shining through the green Photon Blades wall can be used to create a bright "hot core" effect with a simultaneously visible from all angles brightly glowing colored edge.

For the 5000K and 8300K "cool white" LEDs, there is a large emission spike in the blue region ideal for exciting the green fluorescein dye with the rest of the longer wavelengths being able to pass through the blade virtually unattenuated.

Using a 4,000 lumen white light saber to drive a polished outer wall (unsanded) thick wall green Photon Blade:

- The majority of the white light illumination from the inner cellophane wrap diffuser shines straight through the tube wall generating a very bright whitish-yellow-green core.

- The blue light portion of the white spectrum excites the wall of the tube which produces a normal colored green glowing outer wall edge since there is no brightly white light lit cellophane diffuser core behind it.

This white core with a colored glowing outer edge effect does not work with sanded blades since the separation of the outer green glowing wall at the edge and the whitish center core is washed out by scattered light.

The thick wall blade shows this effect far better than the thin wall blade.

The white light saber used for these outdoor daylight photos is described in:

30 Amp LED Sabers & Fluorescence Enhanced Saber Blades (Part 1 - Saber)      Photonic Bladesmith
30 Amp LED Sabers & Fluorescence Enhanced Saber Blades (Part 1 - Saber)


 







 


A similar effect can be achieved with an unsanded fluorescent red dyed thick wall blade.

Test Results: Fluorescent Red Saber Blade      Photonic Bladesmith
Test Results: Fluorescent Red Saber Blade


Habbena has been experimenting with other blade colors and modification of the diffuser core to optimize this effect.

"White Core" Blade Testing    Habbena
"White Core" Blade Testing

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Finally, I would like to thank Vader's Vault and The Custom Saber shop for making fluorescent blades and components for constructing them available so quickly and at such a reasonable cost after the first postings of the early experimental results.


« Last Edit: March 08, 2017, 10:44:22 PM by Photonic Bladesmith »

Offline Greywolf

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #2 on: August 23, 2016, 12:30:52 AM »
Finally had some time to read your post :wink:  Really interesting findings!

Thank you for sharing and even more for your dedication!
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Offline ShadyCanuck

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #3 on: August 23, 2016, 09:03:46 AM »
Fantastic research.  I really appreciate all the work you have put into this.
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Offline Kappy79

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #4 on: August 24, 2016, 04:44:22 PM »
Awesome info and research! Answered a few questions I had, and confirmed my hunch about the differences between sanding and not sanding the Photon Blades.  Glad I have one of each even more than I was before.

Offline Gallandro

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #5 on: January 03, 2017, 01:23:29 PM »
Question: how would the regular blue LED wavelengths compare to the royal blue, given the same brightness, in terms of being able to generate light from the phantom blade?

I found the answer to my question over on the Quad Cree XT-E Royal Blue post:

Quote
It turns out that for the thick wall blade the absorption of the pump light is over 99% for both the blue and royal blue colors.

For the thin wall blade, which is thin enough that the abosrption does not saturate out, the difference in absorption throughout the range of blue light is only on the order of a few percent.

A secondary issue, though it is also only a small few percent factor, is that shorter wavelengths have been measured to have higher quantum yield (number of photons emitted vs. number of photon absorbed) than longer pump wavelengths.

The conclusion is that if you want to choose a blue color solely to drive a Photon Blade, optical output power (quantity of photons) is the most important factor.

However if the output power of the blue and royal blue were equal between the LEDs you are selecting between, I would choose the longest blue wavelength since it would look subjectively brighter at the same power to the human eye when used with a regular non-fluorescent blade.
« Last Edit: January 03, 2017, 06:07:00 PM by Gallandro »

Offline jsummit

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Re: Photon Blade: Tests, Measurements and Physics
« Reply #6 on: March 08, 2017, 01:32:07 PM »
This helped ton in my finding the right blade effects!