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.
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.
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.
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.
End Part 1. Continued part 2.