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Light
Emitting Diodes (LEDs)
Q:
How are LEDs modeled in Photopia?
A:
There are several types of LEDs on the market and different
ways to model the different types. Some LEDs use a surface
mounted chip sealed under a protective material that
diffuses the light from the chip. These are generally
referred to as “chip type” LEDs. Other LEDs
encase the chip and sometimes a surrounding reflector
inside of a clear bulb. These are referred to as LEDs
with integrated optics. Standard Photopia lamp models
can be made for the chip type LEDs and they accurately
model the true behavior of the physical LED. However,
if a standard Photopia lamp model is made for a LED
with integrated optics then there are compromises in
accuracy. Note that a “standard Photopia lamp
model” is a model that utilizes the measured luminous
intensity distribution in combination with luminous
surfaces from which the rays emanate. In the case of
a LED with integrated optics, the luminous surfaces
in the model are those of the chip. Thus, all rays leave
the chip with the measured intensity distribution. This
model type does reproduce the proper far-field intensity
distribution for the LED, but it does not accurately
reproduce the spatial distribution of the rays emanating
from the bulb, so it does not provide a good near-field
distribution. This is because the bulb in the physical
LED significantly distorts the luminous view of the
chip. The consequences of this inaccuracy are only apparent
when secondary optics such as lenses are used in very
close proximity to the LED. In these cases, the improper
distribution of rays as they emanate from the LED bulb
causes an improper distribution of rays onto the secondary
optic, and therefore an inaccurate prediction of how
the light interacts with the secondary optic. In order
to increase the accuracy of models for LEDs with integrated
optics, special lamp models are required that include
extra reflective and refractive components that interact
with the light emanated from the chip to create the
final distribution of the entire package. These LEDs
are noted in the lamp library list as those that include
integrated optics.
See
the following link for more details on an LED model
that includes integrated optics.
http://www.lighting-technologies.com/Products/Photopia/Photopia_LED.htm
See
the following link for more details on lamp modeling
techniques. Using the terminology in the paper, our
standard lamp models are Type 2 and the LEDs with integrated
optics are Type 3. Section 2.2 in the paper discusses
an LED example.
http://www.lighting-technologies.com/Downloads/Photopia/SPIE%204775-08.pdf
Q:
How do I use an LED lamp model that includes integrated
optics?
A:
If you own the General Refractor Module, then you can
import this lamp model and assign the appropriate materials
to its parts. See the "Using LED Models With Integrated
Optics.txt" document in the \LIB sub-folder of
your installation for or below
for detailed instructions. If you do not own the General
Refractor Module, then you can use the projects in your
“\LED Projects” sub-folder as a start to
a new project using the LED. These projects have the
LED model already imported with all of its materials
pre-assigned. You can import your other optical parts
around it and move, copy or re-orient the LED in these
projects as needed using Photopia’s internal CAD
system. See the "Using LED Projects with Pre-Loaded
Lamps.txt" document in your “\LED Projects”
sub-folder or below for more
details about using these LED projects.
Modeling LEDs with the Refractor
module:
To
import the complete LED model, follow these steps (Note
1: The following steps assume the LED model is imported
into Photopia on its own. To merge the LED model with
a complete luminaire drawing, import the DXF file listed
below into the luminaire drawing and then follow the
steps below using the complete luminaire model drawing
file instead of the LED DXF file. Note 2: The default
lamp model DXF file is scaled to inches, the standard
for all lamp models. For an LED model scaled to mm,
use the DXF filename listed below with "mm"
at the end of its name (e.g. LXHL-MW1Dmm.DXF)):
1.
Copy the LXHL-MW1D.DXF file (or other model) from the
\Photopia2\Lib folder to the folder in which your project
will be stored.
2. Select File / Import Luminaire Wizard -> DXF/DWG
from the main menu and select the LXHL-MW1D.DXF file
(or other model) from your project folder.
3. Be sure to have your units set to match the particular
version of the LED model used, inches or mm.
4. Click the Next button on the Import Model screen
to go to the Orient screen.
5. In the Orient Surfaces screen, be sure that Use SAIL
Propagation option is selected. Set the tolerance to
0.00001 if in inches or 0.0001 if in mm. Then click
the Apply button. Click Next.
6. At the Add Lamp screen confirm that the proper LED
lamp was found, then click the Finish button.
7. Assign materials to the reflector layers in the model.
Click on the first layer in the Reflective tab, REFL-LEDBLKBASE...
Then select the following material: Generic LEDBLKBASE.
Click OK.
8. Then assign a material to the second layer, REFL-LEDSTARBASE...
For this layer use: Generic LEDSTARBASE.
9. Go to the Refractive tab and assign materials to
the various refractor layers using the same process
as with the reflective layers. Pick the Generic material
that matches the layername.
10. If multiple LED models are needed in the project,
then copy and LEDs as needed. You can also move or re-orient
the LED model as needed within Photopia's CAD system
or your own CAD program.
11. In the Analysis / Specify Raytrace Settings dialog,
we recommend you use at least 15 reflections to get
all of the light out of the LED itself.
12. The output of the LED will be 100% of the rated
lumens when run by itself. When running it by itself
some light will be reported as absorbed on the various
optical layers. This is expected and the chip is setup
to produce more than its rated lumens initially, so
that after light interacts with the various components,
the proper rated lumens are produced.
Modeling
LEDs without the Refractor module:
If
you do not own the General Refractor Module but want
to use an LED module that requires it, then you can
do so by following these instructions:
1.
Check the Lamps.xls file in the \LTI\Photopia2\Lib folder
for the name of the lamp model for the LED you are using.
2. Open the .ray file for this LED.
3. The LED will already be loaded into this file in
its default position, which has 0,0,0 at the center
of the LED chip.
4. Select File / SaveAs from the main menu to give this
project a new name and folder location. You should not
overwrite the original LED projects so that you have
them available in their default condition for future
projects.
5. The LED models that use the refractor module require
that materals be assigned to their reflector and refractor
layers. This has already been done in these projects.
6. If you want to move, rotate, copy or array the LED
model within Photopia's CAD system, then be sure that
all of the LED layers are turned on and select all of
the LED geometry when you perform these actions.
7. If you have a complex LED layout that is difficult
to configure within Photopia's CAD system, then you
can use your own CAD program to layout the LEDs as long
as your CAD program and read and write DXF files. To
do this, import the appropriate LED lamp model DXF file
into your CAD program and create the layout with the
advice just given in regards to ensuring all of the
LED geometry is included. Then open the appropriate
.ray file for your particular LED type. Choose File
/ Import After clicking Finish in the last step of the
Luminaire Import Wizard, you will get a message that
says:
"Duplicate
layers were imported by the Wizard. Merge imported geomery
onto existing layers?"
You
should answer "NO" to this question.
8. You can import your luminaire geometry into the LED
project as needed via the Luminaire Import Wizard. If
you get the message about duplicate layers and you don't
have the LED model in your CAD file, then Photopia is
probably refering to layer "0" being found
in your project. Since this layer is empty, you can
choose "YES" in the message box.
9. The LED projects have been generally left in their
default state. The only thing that has been done is
to import the LED model and assign its materials. Therefore,
before you begin your analysis you should ensure that
the output settings and raytrace settings are appropriately
set.
Q:
What is the “core” LED lamp model and can
I use it?
A:
LED lamp models with integrated optics are a combination
of standard reflector and refractor layers along with
a lamp model. The lamp model itself is limited to the
LED chip, referred to as the "core." All other
surrounding optics are standard reflector and refractor
layers. In order to model the proper behavior of the
complete LED, you need to use the model that includes
the core chip model as well as the surrounding reflector
and refractor layers. To use this type of LED model,
see the "Using LED Models With Integrated Optics.txt"
document in the \LIB sub-folder of your installation
or see the FAQ topic related to this.
LED
Arrays
Q:
What is the best way to model a large array of LEDs?
A:
For arrays of 20 LEDs or fewer, you can array the standard
LED lamp models in your project as needed. For larger
arrays, consider making a single lamp model from the
entire array. A large array of LEDs can be modeled as
a single lamp model if the following conditions are
met:
1.
The design does not employ lens optics specific to and
in close proximity to each individual LED.
2. The specific interaction of the light (refraction)
between LEDs is not critical to the performance of the
overall design.
3. The design may have reflector and/or lens optics,
but they are designed to work on the LEDs as a group
rather than on each individual source.
In
order to make a lamp model for a LED array, start with
the “simple” version of the LED lamp model
of interest. This will be designated by the standard
lamp model name with the addition of “-SI”
to the end. For example, for the Nichia 20 degree 5mm
LED:
NSPW500BS.DXF
-> Standard refractor based LED model
NSPW500BS-SI.DXF -> Simple lamp model
If
the simple version of the lamp model is not available,
then contact LTI about availability.
Limitations
of “Simple” LED Models
The
simple lamp model has the same luminous intensity distribution
as the refractor based lamp model, however all of the
rays emanate from the LED chip and DO NOT interact with
the bulb as they exit the LED. The difference between
the simple and refractor based lamp models is therefore
the spatial resolution of the rays emanating from the
LED. The refractor based model more accurately models
the true ray emanation points from the surface of the
LED bulb. In other words, it models the effects of the
bulb as it distorts the view of the chip as seen from
various view angles. The simple model does not model
the “spatial” effects of the distorted view
of the chip, although it does model the effects of the
bulb on the luminous intensity distribution. For far
field distributions or in designs that do not have a
critical dependency on the exact location of the rays
as they emanate from the LED bulb, the simple model
is sufficient.
For
more information on the issues affecting the accuracy
of LED analyses, see section 2.2 on page 6 of the technical
paper on lamp modeling at the following link:
http://www.lighting-technologies.com/Downloads/Photopia/SPIE%204775-08.pdf
Instructions
for Creating an LED Array Lamp Model
1.
Import the simple version of the LED model into AutoCAD.
2. Turn off the Lamp Axis layer. This is the layer that
ends with the “A” character and includes
the X,Y,Z axes as well as the text indicating the lamp
model name.
3. Array the geometry on all other lamp model layers
into the required configuration. Caution: If the original
simple lamp model geometry contains several hundred
3DFACEs, then the model of the LED array can become
quite large. The most important surfaces to the lamp
model are those that emanate the light. In the case
of LEDs, this is the chip. The surrounding bulb and
internal lead structure has only a secondary effect
of interacting with light that is redirected back into
the LED from a reflector or refractor. Therefore, if
the original simple lamp model will result in more than
20,000 3DFACEs, consider reducing the number of 3DFACEs
by decreasing the resolution of the model.
4. Rename the layers so that they have a unique name
from the original layer names. Use the number of LEDs
in the array as part of the layer names. For example,
if the array has 381 LEDs, then use the following layername:
LAMP-NSPW500BS-SI1
-> LAMP-NSPW500BS381-SI1
5.
Change the text string on the Lamp Axis layer that indicates
the lamp model name. Use a new name with the same logic
as the layernames:
NSPW500BS-SI1
-> NSPW500BS381-SI1
6.
Save the DXF file for the new LED array using the name
specified in step 5. In this example, the filename is
NSPW500BS381-SI1.DXF.
7. Make a copy of the simple lamp model IES and LDF
files. These files are found in the \LTI\Photopia2\Lib
folder. Copy the files so that they share the same filename
prefix as the DXF file, i.e. NSPW500BS381-SI1.IES &
NSPW500BS381-SI1.LDF.
8. Edit the LDF file in Notepad (it is an ASCII file).
Change the following items:
a. Name – indicate the model is an array and list
the number.
b. Library Name – change to the new DXF filename
prefix.
c. Lamp Watts – multiply the original value by
the number of LEDs in the array.
d. Initial Rated Lumens – multiply the original
value by the number of LEDs in the array.
e. Layer – for each line that indicates a layername,
change the name to match the layernames used in the
DXF file (i.e. add the reference to the number of LEDs
in the array).
9. Make sure all lamp model files are in the \LTI\Photopia2\Lib
folder. This includes the new DXF, IES and LDF files.
The lamp model is now ready for use.
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