linear lighting design notes

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Optical Simulation Details

Linear Lighting Design Notes

Extruded Reflectors and Diffusers


Optical Simulation DetailsLED Choice and Spacing Forms Basis for Linear Luminaire Design

§ Design goal is produce direct view linear cove lighting module§ Choice is to use many low power LEDs

§ C8WT803 at a CCT of 3000K

§ “Dense array” spacing of 3.858 mm produces a plane of uniform illuminance very close to LEDs

§ Allows for luminaire with very thin profile § With typical diffusion lumen density could be as high as 250

Lum/ft

C8WT803 array radiates into testing detector

Rayfile data allowsfor accurate characterization

To model “real life” set output luminous flux = 4.5 Lum/LED for T(junction) ~ 85 C


Optical Simulation DetailsCharacterize Baseline Linear Array for Optical Performance

Illuminance

Luminous Intensity


Optical Simulation DetailsPlane of Uniform Illuminance at 3.8 mm From Emitter

§ Output from the linear array of LEDs shows uniform illuminance at a distance of 3.8mm from the tops of the LEDs forming the strip

§ This means that if a diffuser were placed at 3.8 mm height the light distribution on the diffuser surface would look uniform when viewed directly

§ The detector width is ~ 1.5” - length is 13.8”§ Set output flux density = 351 Lum/ft§ Measured output flux density is 348 Lum/ft§ 3 lumens are lost due to edge effects

The dense LED packing which produced this illuminance distribution would allow for a very thin luminaire!


Optical Simulation DetailsOutput Luminous Intensity Distribution of Baseline Array is Lambertian

§ Luminous intensity is a “far field” characterization§ The component C8WT803 LEDs all have Lambertian luminous intensity profiles§ The output of the baseline linear array must also have a circularly symmetrical Lambertian outputs

By adding diffusers to the linear strip one can modify the circular symmetry of the output


Optical Simulation DetailsUse Bi-Directional Scatter Function Data for Accurate Results

§ The real world manner in which optical materials reflect and transmit light can be reproduced by using bi-directional scatter functions obtained from direct measurement of the materials used in the manufacture of diffusers and reflectors

§ These files can frequently be obtained directly from manufacturers of optical materials

BTDF files were used to obtain highly accurate data for the diffuser characterization in this Note

Black background illustrates how a

diffuser can broaden a tight

input light distribution


Optical Simulation DetailsLuminous Intensity Output Modified by “Satin Ice” Bulk Diffuser

§ “Satin Ice” is a common form of bulk diffuser produced by mixing tiny optical spheres into acrylic (PMMA) – output looks white/”icy”

§ Here we simulate the use of a Satin Ice diffuser 1/8” thick – a very typical form

§ For clarity, performance results for diffusion will be normalized to the Lambertian output of the LED strip without diffusion

False color LI of Linear Strip False color LI of Linear Strip with Satin Ice Diffuser Over Top

Linear Strip348 Lum/ft

Satin Ice255 Lum/ft

Satin Ice diffuser results in 73% transmission efficiency but narrows

distribution and increases CBCP by 5%

Linear StripFWHM = 120 Deg

w/ Satin IceFWHM = 90 Deg


Optical Simulation DetailsResults from Holographic Surface Diffuser

False color LI of Linear Strip False color LI of Linear Strip Luminit 20 Diffuser Over Top

§ The Luminit 20 is a 20 deg surface diffuser produced by replicating a non-periodic holographic pattern on to a thin sheet of polycarbonate

§ This holographic diffuser works better with collimated light but is also used for general diffusion


Satin Ice255 Lum/ft

Luminit 20 deg258 Lum/ft

Luminit 20 deg diffuser results in 74% transmission efficiency but broadens

distribution and decreases CBCP by 13%w/ Luminit 20

FWHM = 96 Deg


Optical Simulation DetailsAn Extruded Reflector is Used to Generate Asymmetric Outputs

Use reflector to preferentially make vertical rays “tighter”

Horizontal

Vertical

Angled view shows LED array at base of extruded reflector 1” deep

Extruded housings are common for LED boards

Reflector design for optimum optical performance differentiates products

§ Asymmetric outputs are a common design requirement for cove lighting applications

§ These types of directed outputs can only be achieved by adding elements to linear lighting that can direct light preferentially

The extruded reflector pictured above was optimized to collimate light along the vertical

output direction defined above

Reflectivity of walls set to 85%


Optical Simulation Details1st Optimized Reflector Curve Increases CBCP ~ 6X

False color chart shows vertically compressedbut brighter output

§ Optimized extruded reflector design increases Center Beam Candle Power by 6X when normalized to output of strip alone!

§ Vertical FWHM is transformed from 120 deg to 6 deg§ Note how spillbeam tracks output of original Lambertian

where reflector opening lets rays throughOutput from mirror

hits “rides” spill beam

FWHM = 6 degs

FWHM = 110 degs


Reflector327 Lum/ft

The extruded reflector produces a highly asymmetric 6 deg x 110 deg output with a

transmission efficiency of 94%

Horizontal


Optical Simulation DetailsAdd Satin Ice Diffuser Strip Over Reflector Opening

False color LI of Satin Ice Diffuser Over Reflector

FWHM = 48 degs264 Lum/ft


§ The addition of the Satin Ice Bulk Diffuser over the top of the extruded reflector transforms the 6 x 110 deg output to 48 x 64 degs

§ Note that adding the Satin Ice diffuser (or any diffuser) will abruptly broaden the shape of the vertical output

The Satin Ice Diffuser over the reflector produces a beam 1.7X brighter then the LED

with a transmission efficiency of 76%


Optical Simulation DetailsAdd Luminit 20 Surface Diffuser to Comparison

§ The addition of the Luminit 20 Surface Diffuser over the top of the extruded reflector now transforms the 6 x 110 deg output to 40 x 84 degs

§ Key observation is that now the Luminit 20 is more efficient than the Satin Ice due to the light from the extruded reflector being collimated

False color LI of Luminit 20 Diffuser Over Reflector


FWHM = 40 degs281 Lum/ftThe Luminit 20/reflector combination

produces a beam 2X brighter then the LED array alone with a transmission efficiency of

81%


Optical Simulation DetailsBeware - Peak Illuminance from Reflectors Could be an Issue!

§ Illuminance at diffuser plane shows lines that could translate into possible objectionable hot regions for direct view product (or be a “different design feature”)

§ Note that for simulation only 85% reflectivity of common aluminum was assumed – all reflective bounces are specular – no BSDF data was used

Peak points

Output Illuminance of Reflector Extrusion at Top of Reflector

False Color image shows “hot” lux lines

“hot” lux peaks created

by reflector walls

All reflective optics show regions of high illuminance at the opening in front of the reflector and may provide regions “too

hot” for direct view products




with no Scatter


with Known Scatter

Reflector Material with Known Diffusion Solves the Hot Spot Problem

Same reflector curves w/ same set of rays

Bottom curve has known diffusion

Smoother illuminance shown in red curve results from choosing a reflector with desired scattering properties

Choosing a reflective material with known diffusion in conjunction with intelligent diffuser choice allows for linear lighting design which will hit asymmetric luminous

intensity targets while still satisfying direct view requirements



Total Internal Reflection

TIR Walls

CentralLens

TIR Extrusion Design Removes Spillbeam and Increases Brightness

Reflectors only direct light which hits the reflector’s wall. Light that doesn’t is spillbeam and does not contribute to the

vertical output of interest.

In a TIR lens light also hits a reflective wall but light from the

LED’s center emission is directed by a central lens into the vertical region of interest.

The brightness is greater due to the addition of the flux which

would ordinarily be in spillbeam TIR Lens

Output of TIR lens is brighterand base shows no spillbeam

Reflectors Produce Spillbeam



Total Internal Reflection

TIR Walls

CentralLens

Extruded TIR Optic Would Provide Optimal Vertical Beam Control

Extrusion is lipped and can be inserted into a holding

channel

Extrusion can provide excellent vertical output control but cannot

control horizontal luminous intensity output


Optical Simulation DetailsSummary and Discussion

§ The very dense linear array of low power LEDs was able to produce a light engine capable of “fast” uniform illuminance:§ Uniform illuminance was achieved at only 3.8 mm above the plane of the top of the LED packages§ Output flux density was calculated at a realistic 351 Lum/W

§ For the linear light engine diffusers were placed at 3.8 mm distance and characterized for transmission efficiency and luminous intensity output distribution§ A bulk Satin Ice Diffuser produced an output with a FWHM = 90 deg at 73% transmission efficiency which was 5%

brighter than the strip output alone § The Luminit 20 surface diffuser yielded a FWHM = 96 deg with 74% transmission efficiency with a CPBP 13 %

dimmer than the strip output§ The tighter FWHM of the Satin Ice Diffuser accounts for this difference in brightness given similar transmission

efficiencies § To create asymmetric distributions an extruded reflector was designed to optimize vertical output asymmetry

and brightness and was then used with the same two diffusers outlined above§ The extrusion itself produced an output of 6 x 110 degrees with a transmission efficiency of 94%§ With a satin ice bulk diffuser cover the output changed to 48 x 64 degrees with a brightness peak 1.7X brighter than

the light engine Lambertian output alone (transmission efficiency = 76%)§ The Luminit 20 surface diffuser produced an output of 40 x 84 degrees which was 2X as bright as the baseline

Lambertian (with a transmission efficiency of 81%)§ Note that because of the collimated output coming from the vertical part of the distribution the holographic Luminit 20

diffuser bested the Satin Ice bulk diffuser

§ An extruded TIR lens can be used to redirect spillbeam and provide even greater vertical beam control and brightness


Output Redirecting Optics


Optical Simulation DetailsThe Ledil “Sophie” Single-Sided TIR Lens Gives Asymmetric Output


Optical Simulation DetailsOptical Characterization of Sophie Using IES File

Polar Detector Centered on IES file


Optical Simulation DetailsLuminous Intensity Slice Shows Output Directed Up at 20 Deg

Optic also spillslight down and

forward


Optical Simulation DetailsLuminit Redirecting Film Based on Beam Shifting Prisms

§ Since redirection is achieved by using beam shifting prisms the effect is optimum for directed light beams

§ Best use is in front of a spotlight

§ Light with wider beam angles may show peculiar, “dispersive” type of results since tighter beam angles will be redirected better than wider beam angles

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