[ Continued from - 30 Amp LED Sabers & Fluorescence Enhanced Saber Blades (Part 1 - Saber) ]
The big design challenge for an ultra bright saber is ultimately operation outdoors in daylight.
In order to design a daylight usable blade we have to know quantitatively what we are operating against. The upper left graph shows solar power density vs. time for a typical northern latitude location. When run through the proper calculations the result is that a sunlight illuminated white translucent blade realistically scatters and reflects on average between 500 to 1,500 lumens of white light depending on clarity of the atmosphere, time of day and angle with respect to the Sun. With a totally clear atmosphere at perpendicular solar incidence at noon the scattered and reflected light from a white translucent saber blade can be as high as 2,500 lumens. No surprise then, that a typical colored LED saber that outputs in the high tens to low hundreds of lumens gets washed out by the much brighter scattered and reflected Sunlight from the saber blade. However, sabers that have blade brightness's in the mid-thousands of lumens gets us in the range of engineering possibility for sunlight visible blade illumination.
Fluorescence Enhanced Saber Blades
To challenge the solar background, one possible scheme would involve:
- Illumination of the blade with a source brighter than this reflected and scattered white sunlight
- Making the Solar illumination of the blade work partially for us as opposed to 100% against us
- Using a time modulated illumination source to make the saber illumination look live (shimmering) and distinct from the background light
Key engineering concepts behind a fluorescence based daylight optimized blade design:
Lumens - the lumens brightness unit is a measure of the human eye perceived brightness of a light source where the absolute optical power is multiplied by a weighing factor that takes into account the different sensitivity of the human eye to different colors. (Lower left hand graph.)
Fluorescence - the emission of a photon of light of a different (almost always longer) wavelength than the photon of light that puts the material into a photon emitting excited state. Fluorescein dye ("day glow green") has a conversion efficiency of up to 92% and outputs its light in the part of the spectrum where the human eye is the most sensitive. (See lower right fluorescein dye emission and excitation curves vs. wavelength.) Additionally, a translucent "day glow green" blade does not have the near 100% white light reflecting and scattering properties of conventional saber blades.
Fluorescence based color shifting is fundamentally different from filtering which simply absorbs and removes part of the LED spectrum illuminating the blade.Fluorescence - Wikipedia, the free encyclopediaFluorescein - Wikipedia, the free encyclopedia
Using fluorescent wavelength shifting, the 600 blue lumens that the 30 amp CBT-120 diode emits can in principle be increased in brightness into the 5,000 lumen range if used to illuminate and excite a fluorescent green blade. See lower left lumens vs. color graph.
Solar spectrum - Roughly half of the visible portion of the solar spectrum is capable of being absorbed by a fluorescein dyed translucent blade which then turns this absorbed sunlight into "day glow green" emitted light. Scattered indirect sunlight from the sky on overcast days or from light entering through windows also contains the short wavelength light that can excite a fluorescein dyed saber blade. ( See upper right solar spectrum graph. )
When using a fluorescence capable blade, the Sun acts to excite the blade to emit the desired color, instead of acting solely to wash out illumination from the LED. Adding to this lighting up of the blade by solar fluorescence is the added increased of brightness from the modulated blue LED further exciting the blade to emit even brighter green light. The 20 to 30 Hz square wave modulation of the blue LED provides a shimmering overlay over the solar excited green light to make the blade seem live as opposed to being a constant intensity "glow stick."
That is the theory at least...
To test this blue LED excited fluorescent blade idea, a 36" long, 1" OD, 1/8" wall fluorescein ("day glow green") dyed acrylic saber blade was constructed. A mirrored end cap was attached to the tip, and a couple of feet of rolled up clear gift wrap diffuser was inserted into the blade. Since this is an acrylic and not a polycarbonate blade, it is unsuitable for full power free sparring.
30 Amp Blue LED Saber with Fluorescent Blade - Tests in Different Background Light Environments
Indoors - brightly lit room, all room lights on, exterior door and a 4 foot by 6 foot window open to daylight
The 600 lumen blue LED driven fluorescent green blade visually looks brighter than the 5,000 lumen rated white LED. You can tell by the green tinge to everything in the room how much brighter the fluorescent green blade is as compared to the previous saber group picture with a conventional blade on the blue LED saber. So far as functioning as an extremely bright green indoor blade powered by fluorescence, the blade definitely works. Detached from the saber with the rolled up gift wrap diffuser removed, the fluorescein dyed acrylic blade is visually a nearly transparent lightly green tinted clear tube. However, there is no trace of blue light from the LED in the blade's visual appearance when driven by the blue LED saber indicating that the fluorescent wavelength shifting is complete and efficient. The blue LED excited green fluorescent blade is so bright that looking directly at the blade under normal indoor lighting produces dark linear after images in your field of view.
Outdoors - Overcast Day
The picture on the left is the saber with the LED off. The center and the right side photos are with the saber with the blue LED on at 30 amps.LightSaber 30Amp OvercastDay - YouTube
Video of fluorescent blade tests in midday overcast skies.
Time Aliasing Video Artifacts
Since the sabers modulate their LEDs with an on-off cycle just slightly slower than the video frame capture rates of most common cameras, the blade will appear in a video to flash on and off at a rate far different (slower) than what one would visually observe in person. The technical term for this is time aliasing or popularly, "The Stroboscopic Effect" or "The Wagon Wheel Effect."Stroboscopic effect - Wikipedia, the free encyclopediaWagon-wheel effect - Wikipedia, the free encyclopedia
I tried to compensate for this by altering the flashing rate of the saber. However, I discovered that the point and shoot camera that I am using to take the videos has a frame rate that is not constant with time over the short term, or with different levels of background illumination. Just be aware when watching the videos that the rate at which the blade shimmers on the video will in general appear to be in slow motion compared with what you would see in person. The most accurate video captured shimmer is shown in the first half of the Overcast Day video.
Outdoors - Late Afternoon (an hour before sunset), Sun shining directly on Blade
The left photo has the blade off. The right photo has the LED on at 30 amps.
Outdoors - Early Afternoon Sun
The left picture has the saber off. The right photo has the saber on with the blue LED driven at 30 amps.YouTube
The saber starts out in the shade at the start of the video, and then is moved into direct sunlight.
Outdoors - Clear Sunny Day at Noon (location Albuquerque, mile high New Mexico Desert)
The left picture has the saber off. The right photo has the saber on with the blue LED driven at 30 amps.YouTube
The saber blade is exposed to direct noon time Sunlight for all saber off-on sequences in this video. The saber is moved to different locations for successive activations and shut downs in order to see the blade LED illumination being turned on and off with different backgrounds.
Visual Impressions Summary (Since camera images of illuminated blades don't equal visual appearances) :
Overall: The fluorescent green blade's in person visual color when powered up by the blue LED and/or by sunlight is various shades of light green to yellowish green. It never visually appears white when lit up brightly like in the photos and videos.
Operating indoors with normal artificial lighting: The fluorescent green blade driven by a 20-30 Hz pulsed blue LED is visually spectacular close up. The fact that the illumination of the blade is rapidly strobed and is bright enough to cause dark retinal after images makes the blade look like a shimmering fuzzy edged strip of light with random dark bands going up and down the lighted blade. The 30 amp 5,000 lumen white saber also produces these strobed retinal saturation visual illusions. This effect has to be seen in person and cannot be captured on camera. The illuminated saber blade under these lighting conditions is apparently exceeding the upper end of the brightness dynamic range where the human eye is able to accurately perceive fine detail.
Outdoors in an overcast sky, or in a shaded outdoor area on a sunny day: The blade has a bright green shimmering luminosity easily acceptable for costuming demonstrations as a realistic appearing saber blade. Under overcast skies, the fluorescent green blade driven by the 30 amp blue LED appears far brighter outdoors during the day than the Master Replicas Anakin looks indoors at night under artificial light. The blade looks otherwise like a normally lit glow tube. The strobed retinal saturation effects and blade after images are not present with eyes adjusted to daylight outdoor lighting conditions.
Outdoors in subdued sunlight: Low levels of direct sunlight on the blade such as what you get just after sunrise or just before sunset gives an illuminated blade appearance similar to operating in an overcast sky.
Outdoors with the blade directly illuminated by strong sunlight: You can tell that the blade is artificially lit up when the saber is activated, but it looks like a slightly shimmering bright green fluorescent day glow tube only slightly brighter than with the LED off. This is at noon however, in mile high desert in the American Southwest, so this may be a worst case "contesting sunlight" scenario. Higher brightness LEDs are needed!
What about the White LED Saber?
The white LED saber was constructed because the CBT-140 white LED had the highest lumen rating of any single emitter LED that I found for sale and I wanted to build this LED into a saber and drive it at full power to see what it would look like. White, however, is probably the worst color to try to use outdoors in daylight so it was not used for outdoor operation testing.
The white LED 5,000 lumen blade was unexpectedly put to the test one night during a meeting at the Quelab makerspace where I fabricated some of the saber parts on their lathe and mill. I brought along my newly finished sabers to show to the other members. Then during the meeting there was a power outage that blacked out our makespace building including the meeting room. This seemed like the ideal time to activate the new 30 amp LED white blade lightsaber. After the "what is that!?" exclamations after the saber was lit up, all of the other members present pulled out their phone cameras, posting a picture of the saber illuminating the meeting room on their Quelab twitter feed shortly after!
Meeting room lit up by a 30 amp white lightsaber during a power outage.#nmmakers hashtag on TwitterQuelab on Twitter: "When The power goes out, #NMMAKERS turn to lightsabers.
Anyway, these are my first two LED-in-hilt sabers builds. It was a lot of fun designing, building, testing them and showing them off. Many thanks to all of the forum members who created the knowledge base that I used in order to design and build my sabers. I am open to any and all suggestions for improvements from the more experienced saber builders on the forum.
Thanks for reading.